Meteoritics amp Planetary Science 38 Nr 12 1865ndash1875 (2003)Abstract available online at httpmeteoriticsorg

1865 copy Meteoritical Society 2003 Printed in USA

Mineralogy microtexture and composition of shock-induced melt pockets in theLos Angeles basaltic shergottite

Erin L WALTON and John G SPRAY

Planetary and Space Science Centre Department of Geology University of New Brunswick2 Bailey Drive Fredericton New Brunswick E3B 5A3 Canada

Corresponding author E-mail j5rngunbca

(Received 20 February 2003 revision accepted 19 December 2003)

AbstractndashAnalytical electron microscopy of shock features in the basaltic shergottite Los Angeles(stone 1) reveals 1) shock recorded in the bulk sample and 2) localized pressure and temperatureexcursions that have generated melt pockets up to 4 mm in diameter Bulk shock effects includemicrofaulting (offsets 1ndash200 mm) mosaicism deformed exsolution lamellae and planar fracturing inpyroxene undulose extinction in whitlockite mechanical twinning in titanomagnetite and ilmeniteand the transformation of plagioclase to maskelynite (pound4 remnant reduced birefringence) Thepressure estimates for bulk shock are 35ndash40 GPa Localized shock excursions have generated threetypes of discrete melt zones (007 times 13 mm to 30 times 35 mm apparent diameter) possessing glassy tomicrocrystalline groundmasses These melt pockets are differentiated on the basis of size clastvolume and degree of crystallization and vesiculation Melt veins and melt dikelets emanate from themelt pockets up to 3 mm into the host rock but do not necessarily connect with other melt pockets Themelt pockets were generated by pressure-temperature excursions of 60ndash80 GPa and 1600ndash2000degCresulting in discrete melting of adjacent host rock minerals at grain boundary margins Concentriczoning in the margins of clinopyroxenes coincides with a progressive reduction in birefringence asmelt pockets are approached This suggests that the shock excursions were focused as point sourcesin the wake of the shock front that induced bulk damage

INTRODUCTION

Los Angeles is a basaltic shergottite comprising tworelatively unweathered stones (stone 1 4526 g stone 22454 g) recovered from the private rock collection of RobertVerish in 1999 (Rubin et al 2000) Rb-Sr and Sm-Ndradiometric ages of the meteorite are 165 plusmn 11 and 172 plusmn8 Ma respectively with a preferred age of 170 plusmn 8 Ma(Nyquist et al 2001) This young radiometric age falls withinthe range of 165ndash475 Ma for other basaltic and lherzoliticshergottites interpreted as the timing of magmaticcrystallization (McSween 2002) An ejection age of ~3 Masuggests that Los Angeles was launched from the martiansurface at the same time as the basaltic shergottites QUE94201 Shergotty and Zagami (Garrison and Bogard 2000Nishiizumi et al 2000 Terribilini et al 2000)

This work describes the microtexture and composition ofshock-induced melt pockets in Los Angeles 7058 (hereafterreferred to as Los Angeles) Two polished thin sections ofstone 1 have been investigated using analytical electronmicroscopy Studies of shock metamorphism in SNC

meteorites help to constrain launch conditions in the contextof the martian impact process and furthers our understandingof the origin of extreme shock excursions

SAMPLES AND ANALYTICAL PROCEDURES

Two polished thin sections of Los Angeles stone 1 (7058-2 21 cm times 12 cm 7058-5 13 cm times 1 cm) were obtainedfrom the Smithsonian Institution for study They are part of agroup of serially sectioned slices made from a ~20 g slabthrough the approximate center of the stone which wasdonated to the Smithsonian Institution by Robert Verish Meltpockets and their host minerals in the 2 polished thin sectionsof Los Angeles were investigated using a JEOL 6400 digitalscanning electron microscope (SEM) equipped with a LinkAnalytical eXL energy dispersive spectrometer (EDS) fittedwith a Si (Li) LZ-4 Pentafet detector Count times were100 sec per analysis at beam operating conditions of 15 kVand 25 nA at a working distance of 39 mm SEMbackscattered electron imagery (BEI) was used to investigatethe microtextures and mineralogy of the meteorite

1866 E L Walton and J G Spray

accompanied by the single spot analysis of individualminerals Where alkali mobility was not deemed problematiccertain clast minerals were analyzed using a JEOL 733electron microprobe equipped with 4 wavelength dispersive(WDS) spectrometers The accelerating voltage was 15 kVwith a 30 nA beam current and the approximate beamdiameter was 1ndash2 mm Both analytical SEM and microprobedata and images were collected at the University of NewBrunswick Additionally high resolution images wereobtained at the JEOL Laboratory in Peabody MA using aJEOL 6500F field emission gun scanning electronmicroscope (FEG-SEM) This was equipped with an In-LensThermal FEG which produces a probe current sufficient forWDS and EDS Photomicrographs were taken with anaccelerating voltage of 15 kV and a 5 nA beam current at aworking distance of 10 mm

Raster scanning was used to determine the approximatebulk chemistry of the melt pockets Crystallites and normal(non-diaplectic) glasses were rastered in separate areas of20 mm by 26 mm at 5000times magnification EDS was usedinstead of WDS because it not only enables rapidsimultaneous major element analysis but also operates atlower beam currents than WDS This minimizes thermalexcitation and charge effects and hence alkali metalmigration (eg Spray and Rae 1995) Lithic and crystalfragments derived from host rock minerals adjacent to themelt pockets were excluded from bulk melt pocket analysisIdentification of lithic and crystal fragments was based on thepresence of reaction rims subroundedrounded morphologiesresulting from thermal corrosion a high density of irregularfractures within clasts (typically containing injections ofremobilized Fe-sulfides and melt material) andor thepresence of abundant dendritic crystallites that nucleated atxenocryst margins

HOST MINERALOGY

Los Angeles is igneous textured and dominated byrelatively coarse-grained (2ndash4 mm) anhedral to subhedralclinopyroxene (ferroaugite) and subhedral to euhedralplagioclase (now maskelynite) It is thus a microgabbroprobably having formed within the core of a thick (severalmeter) lava flow or as a near-surface feeder to basalticextrusives Los Angeles has a ferroan bulk composition alower bulk MgO than any other known martian meteoriteenriched concentrations of incompatible elements and a highabundance of late-stage phases As such Los Angeles is themost differentiated basaltic shergottite yet discovered (Rubinet al 2000 Warren et al 2000) A modal analysis (in vol) ofstone 1 given by Rubin et al (2000) yields maskelynitizedplagioclase (448) and pyroxene (437) with subordinateamounts of silica (24) titanomagnetite (21) fayalite(19) K-rich feldspathic glass (16) and merrillitewhitlockite (15) accessory chlorapatite (09) ilmenite(03) and pyrrhotite (06) and rare baddeleyitepentlandite and hercynitic spinel

SHOCK EFFECTS

The shock damage in Los Angeles (summarized in Table1) is manifest as 1) effects recorded in the bulk sample and2) localized temperature and pressure excursions

Bulk Shock Effects

A brief description of bulk shock effects in Los Angelesbased largely on optical observations is given by Rubin et al(2000) They note the transformation of plagioclase tomaskelynite deformed exsolution lamellae mosaicism and

Table 1 Shock effects recorded in various mineral constituents of Los Angelesa

Bulk rock constituents

Shock effect Plagioclase Pyroxenesb Whitlockite Ti-Magnetite Ilmenite Pyrrhotite Fay + aug + silc Si K glassd

Reduced birefringence plusmn (plusmn) (+) ndash ndash ndash ndash ndashDiaplectic glass +Undulose extinction plusmn plusmn ndash ndash ndash ndash ndashMosaisicm (+) ndash ndash ndash ndash ndashFracturing (plusmn) (+) (+) (+) (+) (+) (+) (+)Planar elements +Mechanical twins (plusmn) (plusmn)

Melt pocketLocalized melting + + + + + + +Present as quenchcrystallites

+ + + + plusmn +

Present as clastswithin melt pockets

plusmn plusmn + + + +

a+ = shock effects that are commonly observed plusmn = shock effects that are rarely observed ( ) = shock effects that increase in intensity approaching areas oflocalized melting ndash = shock effect doesnrsquot apply to specified mineralphase

b(Ferro) augite pigeonitecPyroxferroite breakdown material (fayalite + Fe-augite + silica symplectic intergrowth)dSilica and K-rich alkali symplectic glasses

Shock-induced melt pockets in Los Angeles 1867

planar fracturing of clinopyroxene pyrrhotite veins withinclinopyroxene polycrystallinity in some pyrrhotite grainsand a 350 mm-long microfault with ~15 mm displacementNyquist et al (2001) report peak shock equilibrationpressures for Los Angeles of 35ndash40 GPa which fall within theproposed launch window for martian meteorites (Melosh1984 1985)

In this study we also observe mechanical deformationmanifest as microfaulting (with observed offsets of 1ndash200 mm) mosaicism deformed exsolution lamellae andplanar fracturing of pyroxenes undulose extinction ofwhitlockite and mechanical twinning in ilmenite andtitanomagnetite Maskelynite grains display radiatingfractures that emanate from their outer margins intoneighboring minerals (typically pyroxene) a feature noted byChen and El Goresy (2000) for maskelynite in Zagami Dar alGani 476 and ALH 84001 Strongly reduced birefringencehas been observed in rare grains of maskelynite a feature alsonoted by Rubin et al (2000) The majority (96) ofplagioclase in Los Angeles is completely isotropicXirouchakis et al (2002) report weak plagioclase peaks in atleast one grain analyzed by synchrotron X-ray diffractionindicating that a small proportion of the crystalline structurehas survived Reduced birefringence at the edges of raregrains indicates that the plagioclase (An46-64Ab30-46) in LosAngeles is just beyond the transition from partially isotropicto fully isotropic

Localized Shock Effects

Localized regions of melting comprise lt1 of themeteorite by volume forming self-contained (within the 2dimensions of the thin section) melt pockets melt veins andmelt dikelets

Under the optical microscope melt pockets appear asirregularly-shaped dark brown to black enclaves (Figs 1aand 1b) They are commonly spatially related to large openfractures infilled with secondary terrestrial carbonates(Rubin et al 2000) Calcite is also observed lining the innercavities of vesicles within the melt pockets Color variationsat higher magnification consist of light brown dark brownand colorless flow-layered glasses in the larger melt pockets(as compared to smaller microcrystalline melt pockets thatare essentially opaque) Schlieren is used here to describe alayer in the melt pocket glass that is either texturally distinctfrom the surrounding glass andor is compositionally distinctfrom surrounding glass SEM analyses reveal that the darkbrown schlieren are due to an abundance of small (pound05 mm)inclusions of Fe-sulfides and idiomorphic titanomagnetitegrains (pound1 mm) while light-brown schlieren are devoid ofthem (both light and dark brown variants are otherwisecompositionally similar having approximate pyroxenecomposition) Colorless schlieren are normal (non-diaplectic) plagioclase glass that have incompletely mixed

homogenized with other coexisting silicate and sulfidemelts The colorless plagioclase glasses are like light brownschlieren free of minute Fe-sulfides and Ti-magnetitecrystals The composition of plagioclase schlieren lieswithin the range of those obtained for host rock maskelynite(Table 2)

Host phase ferroaugite and ferropigeonite bordering themelt pockets show reduced birefringence grading in somecases to partial isotropism (Figs 1andash1c) These effects arerestricted to the side of the grain located adjacent to the meltpocket In plane polarized light the rims typically appear darkbrown and blackened with proximity to the melt zonePyroxenes with d = 0012ndash0015 at the interior of the grainshow birefringence reduced to d = 0007ndash0000 at the marginAdditionally the zones are commonly concentricallyarranged SEM studies have revealed for 3 pyroxene grains azone of reaction with the melt zone corresponding to thosemargins showing reduced birefringence in opticalobservations (Figs 1c and 1d)

Three distinct types of melt pocket occur in the studiedthin sections (Figs 2 and 3)

1 Type 1 Microscopic (05 mm times 07 mm 007 mm times13 mm) vesiculated clast-rich with abundantcrystallites (Fig 2) X-ray compositional maps obtainedfor a 3900 mm2 area within a Type 1 melt pocket showthat crystallites are rich in Al Si and Ca (plagioclase)Entrained clasts are subangular and consist of isotropicand partially birefringent whitlockite and clastscomprising a symplectic intergrowth of alkali- and silica-rich glass The clast sizes range from 05 to 200 mm(average = 39 mm n = 100) Type 1 melt pockets aredevoid of spheres of immiscible Fe-sulfide (pyrrhotite)The contact with adjacent host rock minerals varies fromsmooth boundaries to intrusive stringers and veins ofmelt material Isolated wisps and stringers of meltmaterial within maskelynite have also been observed inthe immediate vicinity of the melt pocket One such meltpocket contains clasts that display interior quenchtextures (Fig 2f) Pyroxene compositions are almost

Table 2 Range in chemical composition (wt oxide) for maskelynite and plagioclase schlieren

Maskelynite SchlierenWt avg min max avg min max

SiO2 558 535 588 570 544 595Al2O3 271 256 287 254 236 285FeOa

aTotal Fe reported as FeO

07 05 08 11 02 19CaO 101 79 118 88 68 118Na2O 54 43 65 57 46 66K2O 03 01 06 09 01 12Total 992 988nb

bNumber of grains analyzed

32 14

1868 E L Walton and J G Spray

exclusively manifest as normal glass within the smallermelt pockets however rare clasts have been observed atthe melt pocket margins

2 Type 2 Microscopic (003 times 04 mm) vesiculatedclast-poor with abundant crystallites of idiomorphic tosubidiomorphic silica crystals (with accessory Al2O3)embedded in an unresolvable glassy groundmass rich inFe Ca and Na (Figs 3andash3c) The groundmass isrelatively pristine and shows no signs of devitrificationor alteration to aggregates of new phases (eg claysiddingsite) The interior of the melt pocket is coarsergrained fining toward the margin (Fig 3d)Maskelynite adjacent to the melt pocket hasrecrystallized in a ~5 mm-wide rim surrounding the

melt pocket As with Type 1 melt pockets Type 2 arealso characterized by an absence of immiscible blebs ofFe-sulfides

3 Type 3 Macroscopic (35 mm times 30 mm) dominantlyvesicular relatively clast-poor with flow textures andabundant spheresblebs of immiscible Fe-sulfides (Figs2andash2e) Sulfide spheres range from pound01 to 20 mm indiameter and comprise mono- and polycrystallinepyrrhotite Most vesicles are spherical indicative ofquenching at low confining pressure and low shearstrain Some are elongated with their c-axes parallel tothe direction of flow Vesicles range from sup31 mm to05 mm apparent diameter Individual schlieren can betraced for several mm yet their length and width are

Fig 1 a) and b) Optical photomicrographs of the contact between a host rock clinopyroxene grain (cpx) with a melt pocket (mp) in LosAngeles Zoning in the pyroxene (obtained from microprobe point analyses) is complex but overall changes from a Mg-rich core to a Fe-richrim In plane polarized light (a) the margin adjacent to the melt pocket is ldquostainedrdquo a light-brown color arranged in concentric zones b) crossedpolars view of the same area shown in (a) revealing the marked change in birefringence from dmax = 0020ndash0000 at the melt pocket marginDeformed exsolution lamellae in the pyroxene are indicated by the black arrow c) and d) backscattered electron images (BEI) of the samepyroxene as in (a) in contact with the melt pocket rotated 90deg c) abundant fragments of whitlockite (wi) are located within the melt pocketadjacent to the reaction zone The arrow indicates plagioclase schlieren d) reaction zone at the margin of cpx adjacent to the melt pocketshowing the distribution of silica pyrrhotite titanomagneite and pyroxene The location is highlighted as a white box in (c)

Shock-induced melt pockets in Los Angeles 1869

Fig 2 Backscattered electron images of Type 3 (andashe) and Type 1 (fndashg) melt pockets in context with host rock minerals maskelynite (mk) andclinopyroxene (cpx) a) crosscutting relationship between two coexisting silicate melts within one melt pocket (Type 3) The contact betweenthe 2 melts is diffuse indicating that the higher viscosity melt was relatively warm when the other was intruded The higher viscosity meltcontains abundant subidiomorphic crystals with square- and diamond-shaped cross-sections that increase in abundance with distance from themain melt pocket Small crystal size prohibits quantitative analysis but qualitative microprobe analysis indicates that only iron and titaniumare present in proportions 3 to 1 suggesting titanomagnetite b) the pyroxferroite breakdown material (pbm) with typical vermicular texturein the host rock (upper right hand side) changes to skeletal and feather fayalite crystals embedded in a glassy groundmass with pyrrhotiteglobules at the contact between host rock phases and the melt pocket c) intrusive relationship between maskelynite (dark greyblack right handside) and the melt pocket The white spheres are pyrrhotite The cross-sections through quenched whitlockite needles embedded in the glassesare shown by the white arrow d) glassy groundmass of a Type 3 melt pocket with compositionally distinct flow-layering (revealed by grey-scale variations in BEI imaging) and cellular texture of glasses (upper and lower right hand side) The two black subspherical structures at thebottom of the image are vesicles connected by an open irregular fracture e) dendritic crystallites of pigeonite and orthopyroxene nucleatingat the edge of a host-rock pyroxene grain at the melt pocket margin f) clast containing quench textures within a Type 1 melt pocket g)microcrystalline groundmass typical of a Type 1 melt pocket containing abundant crystallites rich in Al Si and Ca (plagioclase)

1870 E L Walton and J G Spray

highly variable Contact relationships of individualschlieren with the surrounding glass range from sharp tohighly diffuse The schlieren show compositionalvariability attributed to melting of different proportionsof minerals adjacent to the melt pockets (Tables 2 and 3)The proportion of locally derived lithic and crystalfragments increases toward the melt pocket marginThese consist of titanomagnetite maskelynitewhitlockite and alkali- and silica-rich glasses Pyroxenehas never been observed as clasts entrained in any of thelarge melt pocketsTwo distinct types of remobilized melt material emanate

from the melt pockets (Fig 1) They include thin (005ndash10 mmapparent thickness) non-vesicular opaque andor glassyveins (referred to as melt veins) and thicker (50ndash210 mm

apparent thickness) vesicular sinuous apophyses of meltmaterial (referred to as melt dikelets) Melt veins comprisesulfide- and melt-filled fractures and microfaults with smalllateral offsets (up to 80 mm) They possess discordant contactswith the host minerals commonly form an interlockingnetwork of anastomosing branches surrounding unmoltenhost rock constituents and crosscut host rock minerals Meltveins have not been observed to penetrate more than 550 mmfrom their source melt pockets (ie they do not completelytransect the sample) A high concentration of melt veinsinduces a distinct darkening (opacity) of pyroxene grains inoptical microscopy (referred to as ldquoshock blackeningrdquo inchondrites Stoumlffler et al 1991)

Melt dikelets emanate from the main melt pocket andcommonly enlarge to form subsidiary melt pockets However

Fig 3 Backscattered electron images of melt pocket Type 2 in Los Angeles a) low magnification overview of melt pocket (center of photo)Host rock minerals adjacent the melt pocket are maskelynite (mk) clinopyroxene (cpx) an intergrowth of K-rich feldspathic glass and silica-rich glass (K Si) and pyroxferroite breakdown material (pbm) Carbonates (crb) partially infill an E-W-trending fracture that crosscuts bothhost rock minerals and the melt pocket b) higher resolution image of the interior of the melt pocket Silica (with minor amounts ofaluminium) forms subidioblastic to idioblastic crystals with roughly cubic-shaped cross sections The vesicles (v) are irregularly-shaped c)higher resolution cross-sections through the crystals depicted in (b) show unusual morphologies like primate metatarsals The interior of thecrystals are compositionally similar to the groundmass and are enriched in Fe Mg and Ca d) contact between the melt pocket (top) andmaskelynite (base of photo) is gradational fining toward the margin Maskelynite has recrystallized in a ~10 mm-wide rim in direct contactwith the melt pocket

Shock-induced melt pockets in Los Angeles 1871

some are observed to be wedge-shaped wherein they taperoff typically within 3 mm from the source melt pocket Incontrast to the melt veins they contain abundant vesicles andare confined to grain boundaries of relatively high and lowdensity minerals Contacts with adjacent minerals are diffuseTo observe stringers or blebs of melt co-mingling withadjacent host rock minerals is common

While interconnected melt pockets do occur in LosAngeles isolated pockets are more prevalent Due to thehighly irregular nature of typical shock veins (especially withincreasing peak shock pressure Stoumlffler et al 1991) thetapering melt veins of Los Angeles possibly connect toseemingly isolated melt pockets out of the 2-dimensionalplane of the thin section However melt veins in Los Angelesare morphologically distinct from typical discordantpseudotachylyte-like shock veins observed in other SNCmeteorites (eg Shergotty Stoumlffler et al 1986) and ordinarychondrites (Stoumlffler et al 1991) that commonly transect thewhole meteorite

In Los Angeles the contact between the melt pockets andadjacent host rock minerals is irregular and commonlytransitional (Fig 1c) Host rock minerals adjacent to the meltpockets have been partially melted or recrystallized The meltpockets and melt veins and dikelets emanating from meltpocket margins commonly crosscut both primary igneoustextures and bulk shock effects

Microscopic areas of alkali- and silica-rich glass (336ndash548 wt K2O 159ndash337 wt Na2O 879ndash975 wt SiO2and 22ndash42 wt Al2O3 respectively) are associated withmaskelynite and silica crystals in the bulk sample They rangein apparent diameter from ~20ndash350 mm and exhibit asymplectic intergrowth texture between the two phasesMicrofaults emanating from melt pocket margins displace theadjacent alkali- and silica-rich glasses (up to 15 mm)However the alkali- and silica-rich glasses at the melt pocketmargins contain entrained schlieren and globules of meltmaterial that are detached from the melt pocket The fluidnature of the material is indicated from the presence ofpyrrhotite spheres and vesicles

Regions of localized melting indicate a minimummelting temperature for nonporous basalts of 1600degC(Stoumlffler 1984) Additionally melting of ilmenite indicatestemperatures in excess of 1800degC (El Goresy et al 1968)Localized pressure and temperature excursions within meltpockets based on experimental data (Keiffer et al 1976Stoumlffler 1984 Schmitt 2000) are estimated at 60ndash80 GPa and1600ndash2000degC respectively Schaal and Houmlrz (1977) observedmelting of pyroxene in experimentally shocked basalts at aminimum pressure limit of 80 GPa

Quench CrystalsOne feature common to all melt pockets in Los Angeles

is the presence of quench crystals The crystals have avariety of distinctive shapes (skeletal dendritic feathery)

and they comprise a variety of minerals (olivinewhitlockite titanomagnetite plagioclase) Quench crystalsare invariably microscopic Severely fractured melt-intruded clinopyroxenes adjacent to the melt pockets allowfor preferential nucleation of dendritic crystals that extendfrom the contact into glass These consist of intimateintergrowths of ferropigeonite (Fs73En18) and orthopyroxene(Fs40En52) with dendritic textures (Fig 2e) In additioncrystal fragments within melt pockets locally derived fromadjacent host minerals serve as nucleation sites for dendriticgrowth (Fig 4) These textures are consistent with rapiddiffusion-controlled crystal growth from the melt(Kirkpatrick 1975)

Two melt pockets in Los Angeles show the symplecticintergrowth fayalite + ferroaugite + silica a late-stage mineralassemblage resulting from the breakdown of pyroxferroite

Fig 4 Backscattered electron images of Los Angeles a) a largetitanomagnetite clast (lower right) within a melt pocket has seededpreferential nucleation of titanomagnetite crystallites with dendriticmorphologies attesting to rapid cooling b) higher magnification ofthe titanomagnetite crystallites are depicted in (a)

1872 E L Walton and J G Spray

(Rubin et al 2000) in direct contact with the melt pocket Atthe melt pocket margin the vermicular to microgranulitictexture typical of the symplectites changes to one of skeletalfayalite (Fa96) embedded in a glassy matrix rich in Fe Mg andCa and Fe-sulfide spherules (Fig 2b) Numerous fractures inthe vicinity of the melt pocket radiate from the contact withthe melt into the fayalite + ferroaugite + silica intergrowthcausing microfaulting

MELT POCKET COMPOSITIONS

The bulk chemistries of four melt pockets in LosAngeles are presented in Table 3 The compositions arevariable from one pocket to another CIPW norm calculationsreveal that the melt pockets can be derived by fusingdifferent mixtures of the host minerals dominated byplagioclase and pyroxene Melt pocket 1 (Type 1) isdominated by normative quartz orthoclase plagioclase andhypersthene melt pocket 2 (Type 2) by quartz orthoclaseplagioclase and hypersthene (Type 3) by mainlyplagioclase diopside and hypersthene melt pocket 4 (Type3) by plagioclase hypersthene and apatite (whitlockite)Melt pocket compositions depend on which bordering hostminerals contribute to melting and the proportion of meltingof each phase (eg some melt pockets occur at the grainboundary of pyroxene and maskelynite thus these are the

main components contributing to the melt) The occurrenceof at least two coexisting silicate melts within one Type 3melt pocket is revealed by crosscutting relationships(Fig 2a) The more siliceous melt of the two (average65 wt SiO2 obtained by raster scans n = 8) is located inproximity to the melt pocket margin This melt contains lithicand crystal fragments (~50) of locally-derived silica andmaskelynite The less siliceous (average 43 wt SiO2

obtained by raster scans n = 6) lower viscosity meltcrosscuts the former tapering off to thin stringers (pound1 mm inapparent thickness) The shape of the overall melt pocket isroughly spherical (Fig 2) ie no textural evidence exists tosuggest that these crosscutting relationships are the result oftwo independently formed melt pockets Although the twosilicate melts were most probably subject to the same coolingrates the abundance of lithic and crystal fragments wouldhave increased the amount of heterogeneous nucleation sitesfavoring more rapid crystallization concomitant with anincrease in viscosity This is also consistent with rapidcrystallization in Type 1 melt pockets containing abundantclasts of host rock minerals (Figs 2f and 2g)

DISCUSSION

Melt pockets within Los Angeles can be divided into 3types based on size clast volume and degree of vesiculation

Table 3 Bulk compositions of shock melt pockets (in wt) for Los Angeles stone 1Melt pocket 1 (Type 1) Melt pocket 2 (Type 2) Melt pocket 3 (Type 3) Melt pocket 4 (Type 3)

wt avg min max avg min max avg min max avg min max 5a

SiO2 589 511 691 627 585 731 457 365 535 328 255 467 491Al2O3 95 120 06 144 119 183 42 17 91 33 15 46 112TiO2 05 03 06 03 02 05 15 04 46 11 05 11 130FeOb 219 128 329 113 6 189 305 197 404 299 234 441 212MnO 05 07 03 03 ndc 05 07 04 12 07 05 13 045MgO 09 05 26 03 06 52 42 16 110 08 06 13 353CaO 37 57 13 58 58 73 96 70 128 159 59 164 995Na2O 18 10 23 24 12 3 02 nd 11 05 01 09 222K2O 21 02 27 13 08 18 03 nd 05 04 05 06 024P2O5 ndash nd 05 ndash nd 01 ndash nd 49 111 89 265 066SO3 ndash nd 08 ndash nd nd nd 04 27 ndash nd 19 ndashTotal 1001 988 996 970 9986nd 32 30 38 24

CIPW normQuartz 144 226 19 0Orthoclase 124 78 18 25Albite 152 206 17 44Anorthite 117 25 98 58Diopside 54 37 249 07Hypersthene 398 199 534 572Olivine 0 0 0 06Ilmenite 1 06 29 22Apatite 02 0 36 272

aData from Rubin et al (2000) obtained by Instrumental Neutron Activation AnalysisbTotal Fe reported as FeOcnd = element oxides below the detection limit of the electron microprobedn = number of raster scans per melt pocket

Shock-induced melt pockets in Los Angeles 1873

Smaller (lt1 mm) pockets (Types 1 and 2) tend to becrystallite-rich while larger (gt1 mm) pockets (Type 3) tend tobe clast-poor and exhibit flow-structured glasses Opticallyshock effects are manifest as dark brown to black glassy andor microcrystalline enclaves of melt material melt veins anddeposits of sulfides and melt injected into fractures withinsilicates In total these localized regions of melting compriselt1 vol of Los Angeles and they are heterogeneouslydistributed throughout the sample

Melt pockets invariably develop at the grain boundariesof minerals with contrasting shock impedance (shock wavevelocity multiplied by the mineralrsquos density) eg plagioclaseand pyroxene This is most obvious in smaller (pound1 mm) meltpockets (Type 1 and 2) where the original grain positions andboundaries are preserved With increasing volume of melt theoriginal grains and their boundaries become obscured ordestroyed Typically minerals adjacent to the melt pocketshave smooth arcuate margins at the contact with the meltThe composition of the melt pockets corresponds to localizedmelting of bordering host rock minerals Melt veins have beenobserved to form an interlocking network of complexanastomosing branches surrounding unmolten host rockconstituents

The variable compositions of the studied melt pocketscan be attributed to the relatively coarse grain size (2ndash4 mm)of the host rock minerals compared to the diameters of themelt pockets (lt4 mm) The melt pockets are not largeenough to represent a bulk rock melt of Los AngelesInstead they represent localized mineral-scale meltingproducts Melt pocket compositions presented in Table 3differ markedly when comparing those of the smaller meltpockets Types 1 and 2 to the larger Type 3 melt pocketsTypes 1 and 2 are enriched in SiO2 Al2O3 and alkalis ascompared to Type 3 The CIPW norm calculations alsolisted in Table 3 indicate that the precursors of Types 1 and2 melt pockets were mainly quartz + alkali feldspar +plagioclase + pyroxene while those for Type 3 were mainlyplagioclase + pyroxene + whitlockite Therefore theprecursors of Types 1 and 2 melt pockets likely had a largecontribution from late-stage minerals (mesostasis) while thebulk of Type 3 are derived from the main host rock lithologyThis is not surprising considering that shock melting beginsat grain boundary margins where mesostasis is moreabundant Thus smaller (Type 1 and 2) melt pockets willposses a greater contribution of mesostasis to bulkcomposition than larger (Type 3) melt pockets that havemelted approximately the same amount of mesostasis butgreater proportions of host rock phases

Based on compositional and textural lines of evidencewe conclude that the melt pockets formed by the local in situmelting of host rock minerals as opposed to forcefulinjection of extraneous impact melt along fractures in thehost rock Remobilization and injection of opaques and meltfrom the melt region into pre-existing fractures in adjacent

host rock pyroxene could have occurred during compressionand subsequent stress relaxation The contact between themelt pockets and adjacent host minerals is intrusive inplaces The intensity of shock features is observed toincrease with proximity to the melt pockets indicating alocalized increase in shock pressure This pressure gradientfrom melt pocket (60ndash80 GPa) to bulk rock (35ndash40 GPa) iscertainly sufficient to account for forceful injection of Fe-sulfides and melt from the melt pockets into adjacent hostminerals Additionally intrusions could have formed bypressure generated by a volume increase (~10) associatedwith the melting event

The relationship between areas of alkali- and silica-richglass and the melt pockets is puzzling However cominglingand entrapment of liquid blebs from the melt pocket ispossible if the two phases were initially molten at the sametime This could be explained if the alkali- and silica-richglass was formed during the shock event but was quenchedbefore the shock pressure was completely released Melt fromthe melt pocket could then have remained in a liquid stateafter pressure release This is consistent with the presence ofabundant vesicles in the melt pocket glasses indicating lowconfining pressure Stopler and McSween (1979) observedsimilar textural associations in Shergotty and Zagamiconcluding that they may represent shock mixtures of alkali-feldspars and silica

The presence of clasts in one melt pocket with quenchtextures as well as the presence of crosscutting silicate meltsdoes not require multiple impact events That a single impactevent is capable of producing several shock metamorphicfeatures in a target rock is well-documented (eg Stoumlffler1984 Stoumlffler et al 1991) The high concentration ofrelatively cold lithic and crystal fragments as well as theproximity to the melt pocket margin may have facilitatedrapid crystallization of one melt with respect to the otherRapid crystallization could also cause a drastic increase in theviscosity of one melt with respect to the other

The development of abundant crystallites and glass inthe melt pockets attests to rapid cooling of the melt whichimplies that the studied Los Angeles samples were derivedfrom the chilled margins of a larger ejected body (severalmeters to tens of meters in diameter) or were never muchlarger than the recovered meteorite sizes (tens of cm indiameter) The high abundance of relatively cold clasts inthe smaller melt pockets (Type 1) probably contributed tothe rapid crystallization (generating abundant crystallites)and an absence of flow textures due to the presence ofcopious heterogeneous nucleation sites Vesiculationprobably occurred during shock decompression withvesicles persisting in the melt under lower pressureconditions (Kenkmann et al 2000) However this does notpreclude the development of high pressure high temperaturepolymorphs within the melt pockets produced by cavitationand bubble implosion (Spray 1999) Melt veins and melt

1874 E L Walton and J G Spray

dikelets occur in direct contact with the melt pockets and areconsidered morphologically distinct from typical ldquoshockveinsrdquo that transect meteorites and that form by acombination of shock and frictional melting The concentriczoning observed in the margins of clinopyroxenes in contactwith melt pockets coincides with a progressive reduction inbirefringence which suggests that shock excursions werefocused as point sources and that these were generated in thewake of the planar shock front that induced bulk shockeffects We infer that the larger melt pockets (Type 3) wereformed by higher energy excursions relative to the smallerpockets (Type 1 and 2)

Based on experimental data by Kieffer et al (1976)Schaal and Houmlrz (1977) and Schmitt (2000) shock pressureand temperature excursions of 60ndash80 GPa and 1600ndash 2000degCare estimated for Los Angeles 7058 Because pyroxene is notobserved as clasts in any of the melt pockets in Los Angelesthe higher pressure of ~80 GPa is favored coincident with theobserved melting of pyroxene in experimentally shockedbasalts (Schaal and Houmlrz 1977) These discrete excursionsare therefore approximately twice the magnitude of the bulkshock (35ndash40 GPa)

A young formation age 170 plusmn 8 Ma for the Los Angelesmeteorite (Nyquist et al 2001) precludes it recording multipleimpact events that would be more typical of target rocks fromthe older impact-scarred Noachian terrains of the southernhemisphere Instead Los Angeles records evidence of asingle shock event which favors its launching immediatelyfollowing hypervelocity impact into a relatively young (lateAmazonian) volcanic source terrain such as is found in theTharsis and Elysium regions

AcknowledgmentsndashThis work has been funded by the NaturalSciences and Engineering Research Council of Canada(NSERC) through research grants awarded to J G Spray andan NSERC PGS-b scholarship awarded to E L Walton Wethank Tim McCoy at the Smithsonian Institution WashingtonDC for loan of the samples Douglas Hall at UNB andMichael Coy at JEOL USA for EM support Cliff Shaw forhelpful discussions and Angel Gomez for draftingConstructive Reviews by Y Ikeda and R F Schmitt aregratefully acknowledged Planetary and Space Science CentreContribution 31

Editorial HandlingmdashDr Edward Scott

REFERENCES

Chen M and El Goresy A 2000 The nature of maskelynite inshocked meteorites Not a diaplectic glass but a glass quenchedfrom shock-induced dense melt at high pressures Earth andPlanetary Science Letters 179489ndash502

El Goresy A Fechtig H and Ottemann J 1968 The opaqueminerals in impact glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMono Book pp 531ndash553

Garrison D H and Bogard D D 2000 Cosmogenic and trappednoble gases in the Los Angeles meteorite (abstract) Meteoriticsamp Planetary Science 35A58

Houmlrz F and Quaide W L 1973 Debye-Scherrer investigations ofexperimentally shocked silicates The Moon 645ndash82

Kenkmann T Hornemann U and Stoumlffler D 2000 Experimentalgeneration of shock-induced pseudotachylytes alonglithological interfaces Meteoritics amp Planetary Science 351275ndash1290

Kieffer S W Schaal R B Gibbons R Houmlrz F Milton D J andDube A 1976 Shocked basalt from Lonar crater India andexperimental analogues Proceedings 7th Lunar and PlanetaryScience Conference pp 1391ndash1412

Kirkpatrick R J 1975 Crystal growth from the melt A reviewAmerican Mineralogist 60798ndash814

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McSween H Y Jr 2002 The rocks of Mars from far and nearMeteoritics amp Planetary Science 377ndash25

Melosh H J 1984 Impact ejection spallation and the origin ofmeteorites Icarus 59234ndash260

Melosh H J 1985 Ejection of rock fragments from planetary bodiesGeology 13144ndash148

Nishiizumi K Caffee M W and Masarik J 2000 Cosmogenicradiocunclides in Los Angeles martian meteorite (abstract)Meteoritics amp Planetary Science 35A120

Nyquist L E Reese Y D Wiesmann H Shih C Y and SchwandtC 2000 Rubidium-strontium age of the Los Angeles shergottiteMeteoritics amp Planetary Science 35A121ndashA122

Nyquist L E Bogard D D Shih C Y Greshake A Stoumlffler D andEugster O 2001 Ages and histories of martian meteoritesChronology and Evolution of Mars 96105ndash164

Rubin A E Warren P H Greenwood J P Verish R S Leshin LA Hervig R L and Clayton R N 2000 Los Angeles The mostdifferentiated basaltic martian meteorites Geology 281011ndash1014

Schaal R B and Houmlrz F 1977 Shock metamorphism of lunar andterrestrial basalts Proceedings 8th Lunar and Planetary ScienceConference pp 1679ndash1729

Schmitt R T 2000 Shock experiments with the H6 chondriteKernouveacute Pressure calibration of microscopic shock effectsMeteoritics amp Planetary Science 3545ndash560

Spray J G 1999 Shocking rocks by cavitation and bubble implosionGeology 27695ndash698

Spray J G and Rae D A 1995 Quantitative electron-microprobeanalysis of alkali silicate glasses A review and user guideCanadian Mineralogist 33323ndash332

Stoumlffler D 1984 Glasses formed by hypervelocity impact Journal ofNon-Crystalline Solids 67465ndash502

Stoumlffler D 2000 Maskelynite confirmed as diaplectic glassIndications for peak shock pressure below 45 GPa in all martianmeteorites (abstract 1170) 31st Lunar and Planetary ScienceConference CD-ROM

Stoumlffler D Ostertag R Jammes C and Pfannschmidt G 1986Shock metamorphism and petrography of the Shergottyachondrite Geochimica et Cosmochimica Acta 50889ndash903

Stoumlffler D Keil K and Scott E R D 1991 Shock metamorphismof ordinary chondrites Geochimica et Cosmochimica Acta 53845ndash3867

Stopler E and McSween H Y Jr 1979 Petrology and origin of theshergottite meteorites Geochimica et Cosmochimica Acta 431475ndash1498

Terribilini D Busemann H and Eugster O 2000 Krypton-81-

Shock-induced melt pockets in Los Angeles 1867

planar fracturing of clinopyroxene pyrrhotite veins withinclinopyroxene polycrystallinity in some pyrrhotite grainsand a 350 mm-long microfault with ~15 mm displacementNyquist et al (2001) report peak shock equilibrationpressures for Los Angeles of 35ndash40 GPa which fall within theproposed launch window for martian meteorites (Melosh1984 1985)

In this study we also observe mechanical deformationmanifest as microfaulting (with observed offsets of 1ndash200 mm) mosaicism deformed exsolution lamellae andplanar fracturing of pyroxenes undulose extinction ofwhitlockite and mechanical twinning in ilmenite andtitanomagnetite Maskelynite grains display radiatingfractures that emanate from their outer margins intoneighboring minerals (typically pyroxene) a feature noted byChen and El Goresy (2000) for maskelynite in Zagami Dar alGani 476 and ALH 84001 Strongly reduced birefringencehas been observed in rare grains of maskelynite a feature alsonoted by Rubin et al (2000) The majority (96) ofplagioclase in Los Angeles is completely isotropicXirouchakis et al (2002) report weak plagioclase peaks in atleast one grain analyzed by synchrotron X-ray diffractionindicating that a small proportion of the crystalline structurehas survived Reduced birefringence at the edges of raregrains indicates that the plagioclase (An46-64Ab30-46) in LosAngeles is just beyond the transition from partially isotropicto fully isotropic

Localized Shock Effects

Localized regions of melting comprise lt1 of themeteorite by volume forming self-contained (within the 2dimensions of the thin section) melt pockets melt veins andmelt dikelets

Under the optical microscope melt pockets appear asirregularly-shaped dark brown to black enclaves (Figs 1aand 1b) They are commonly spatially related to large openfractures infilled with secondary terrestrial carbonates(Rubin et al 2000) Calcite is also observed lining the innercavities of vesicles within the melt pockets Color variationsat higher magnification consist of light brown dark brownand colorless flow-layered glasses in the larger melt pockets(as compared to smaller microcrystalline melt pockets thatare essentially opaque) Schlieren is used here to describe alayer in the melt pocket glass that is either texturally distinctfrom the surrounding glass andor is compositionally distinctfrom surrounding glass SEM analyses reveal that the darkbrown schlieren are due to an abundance of small (pound05 mm)inclusions of Fe-sulfides and idiomorphic titanomagnetitegrains (pound1 mm) while light-brown schlieren are devoid ofthem (both light and dark brown variants are otherwisecompositionally similar having approximate pyroxenecomposition) Colorless schlieren are normal (non-diaplectic) plagioclase glass that have incompletely mixed

homogenized with other coexisting silicate and sulfidemelts The colorless plagioclase glasses are like light brownschlieren free of minute Fe-sulfides and Ti-magnetitecrystals The composition of plagioclase schlieren lieswithin the range of those obtained for host rock maskelynite(Table 2)

Host phase ferroaugite and ferropigeonite bordering themelt pockets show reduced birefringence grading in somecases to partial isotropism (Figs 1andash1c) These effects arerestricted to the side of the grain located adjacent to the meltpocket In plane polarized light the rims typically appear darkbrown and blackened with proximity to the melt zonePyroxenes with d = 0012ndash0015 at the interior of the grainshow birefringence reduced to d = 0007ndash0000 at the marginAdditionally the zones are commonly concentricallyarranged SEM studies have revealed for 3 pyroxene grains azone of reaction with the melt zone corresponding to thosemargins showing reduced birefringence in opticalobservations (Figs 1c and 1d)

Three distinct types of melt pocket occur in the studiedthin sections (Figs 2 and 3)

1 Type 1 Microscopic (05 mm times 07 mm 007 mm times13 mm) vesiculated clast-rich with abundantcrystallites (Fig 2) X-ray compositional maps obtainedfor a 3900 mm2 area within a Type 1 melt pocket showthat crystallites are rich in Al Si and Ca (plagioclase)Entrained clasts are subangular and consist of isotropicand partially birefringent whitlockite and clastscomprising a symplectic intergrowth of alkali- and silica-rich glass The clast sizes range from 05 to 200 mm(average = 39 mm n = 100) Type 1 melt pockets aredevoid of spheres of immiscible Fe-sulfide (pyrrhotite)The contact with adjacent host rock minerals varies fromsmooth boundaries to intrusive stringers and veins ofmelt material Isolated wisps and stringers of meltmaterial within maskelynite have also been observed inthe immediate vicinity of the melt pocket One such meltpocket contains clasts that display interior quenchtextures (Fig 2f) Pyroxene compositions are almost

Table 2 Range in chemical composition (wt oxide) for maskelynite and plagioclase schlieren

Maskelynite SchlierenWt avg min max avg min max

SiO2 558 535 588 570 544 595Al2O3 271 256 287 254 236 285FeOa

aTotal Fe reported as FeO

07 05 08 11 02 19CaO 101 79 118 88 68 118Na2O 54 43 65 57 46 66K2O 03 01 06 09 01 12Total 992 988nb

bNumber of grains analyzed

32 14

1868 E L Walton and J G Spray

exclusively manifest as normal glass within the smallermelt pockets however rare clasts have been observed atthe melt pocket margins

2 Type 2 Microscopic (003 times 04 mm) vesiculatedclast-poor with abundant crystallites of idiomorphic tosubidiomorphic silica crystals (with accessory Al2O3)embedded in an unresolvable glassy groundmass rich inFe Ca and Na (Figs 3andash3c) The groundmass isrelatively pristine and shows no signs of devitrificationor alteration to aggregates of new phases (eg claysiddingsite) The interior of the melt pocket is coarsergrained fining toward the margin (Fig 3d)Maskelynite adjacent to the melt pocket hasrecrystallized in a ~5 mm-wide rim surrounding the

melt pocket As with Type 1 melt pockets Type 2 arealso characterized by an absence of immiscible blebs ofFe-sulfides

3 Type 3 Macroscopic (35 mm times 30 mm) dominantlyvesicular relatively clast-poor with flow textures andabundant spheresblebs of immiscible Fe-sulfides (Figs2andash2e) Sulfide spheres range from pound01 to 20 mm indiameter and comprise mono- and polycrystallinepyrrhotite Most vesicles are spherical indicative ofquenching at low confining pressure and low shearstrain Some are elongated with their c-axes parallel tothe direction of flow Vesicles range from sup31 mm to05 mm apparent diameter Individual schlieren can betraced for several mm yet their length and width are

Fig 1 a) and b) Optical photomicrographs of the contact between a host rock clinopyroxene grain (cpx) with a melt pocket (mp) in LosAngeles Zoning in the pyroxene (obtained from microprobe point analyses) is complex but overall changes from a Mg-rich core to a Fe-richrim In plane polarized light (a) the margin adjacent to the melt pocket is ldquostainedrdquo a light-brown color arranged in concentric zones b) crossedpolars view of the same area shown in (a) revealing the marked change in birefringence from dmax = 0020ndash0000 at the melt pocket marginDeformed exsolution lamellae in the pyroxene are indicated by the black arrow c) and d) backscattered electron images (BEI) of the samepyroxene as in (a) in contact with the melt pocket rotated 90deg c) abundant fragments of whitlockite (wi) are located within the melt pocketadjacent to the reaction zone The arrow indicates plagioclase schlieren d) reaction zone at the margin of cpx adjacent to the melt pocketshowing the distribution of silica pyrrhotite titanomagneite and pyroxene The location is highlighted as a white box in (c)

Shock-induced melt pockets in Los Angeles 1869

Fig 2 Backscattered electron images of Type 3 (andashe) and Type 1 (fndashg) melt pockets in context with host rock minerals maskelynite (mk) andclinopyroxene (cpx) a) crosscutting relationship between two coexisting silicate melts within one melt pocket (Type 3) The contact betweenthe 2 melts is diffuse indicating that the higher viscosity melt was relatively warm when the other was intruded The higher viscosity meltcontains abundant subidiomorphic crystals with square- and diamond-shaped cross-sections that increase in abundance with distance from themain melt pocket Small crystal size prohibits quantitative analysis but qualitative microprobe analysis indicates that only iron and titaniumare present in proportions 3 to 1 suggesting titanomagnetite b) the pyroxferroite breakdown material (pbm) with typical vermicular texturein the host rock (upper right hand side) changes to skeletal and feather fayalite crystals embedded in a glassy groundmass with pyrrhotiteglobules at the contact between host rock phases and the melt pocket c) intrusive relationship between maskelynite (dark greyblack right handside) and the melt pocket The white spheres are pyrrhotite The cross-sections through quenched whitlockite needles embedded in the glassesare shown by the white arrow d) glassy groundmass of a Type 3 melt pocket with compositionally distinct flow-layering (revealed by grey-scale variations in BEI imaging) and cellular texture of glasses (upper and lower right hand side) The two black subspherical structures at thebottom of the image are vesicles connected by an open irregular fracture e) dendritic crystallites of pigeonite and orthopyroxene nucleatingat the edge of a host-rock pyroxene grain at the melt pocket margin f) clast containing quench textures within a Type 1 melt pocket g)microcrystalline groundmass typical of a Type 1 melt pocket containing abundant crystallites rich in Al Si and Ca (plagioclase)

1870 E L Walton and J G Spray

highly variable Contact relationships of individualschlieren with the surrounding glass range from sharp tohighly diffuse The schlieren show compositionalvariability attributed to melting of different proportionsof minerals adjacent to the melt pockets (Tables 2 and 3)The proportion of locally derived lithic and crystalfragments increases toward the melt pocket marginThese consist of titanomagnetite maskelynitewhitlockite and alkali- and silica-rich glasses Pyroxenehas never been observed as clasts entrained in any of thelarge melt pocketsTwo distinct types of remobilized melt material emanate

from the melt pockets (Fig 1) They include thin (005ndash10 mmapparent thickness) non-vesicular opaque andor glassyveins (referred to as melt veins) and thicker (50ndash210 mm

apparent thickness) vesicular sinuous apophyses of meltmaterial (referred to as melt dikelets) Melt veins comprisesulfide- and melt-filled fractures and microfaults with smalllateral offsets (up to 80 mm) They possess discordant contactswith the host minerals commonly form an interlockingnetwork of anastomosing branches surrounding unmoltenhost rock constituents and crosscut host rock minerals Meltveins have not been observed to penetrate more than 550 mmfrom their source melt pockets (ie they do not completelytransect the sample) A high concentration of melt veinsinduces a distinct darkening (opacity) of pyroxene grains inoptical microscopy (referred to as ldquoshock blackeningrdquo inchondrites Stoumlffler et al 1991)

Melt dikelets emanate from the main melt pocket andcommonly enlarge to form subsidiary melt pockets However

Fig 3 Backscattered electron images of melt pocket Type 2 in Los Angeles a) low magnification overview of melt pocket (center of photo)Host rock minerals adjacent the melt pocket are maskelynite (mk) clinopyroxene (cpx) an intergrowth of K-rich feldspathic glass and silica-rich glass (K Si) and pyroxferroite breakdown material (pbm) Carbonates (crb) partially infill an E-W-trending fracture that crosscuts bothhost rock minerals and the melt pocket b) higher resolution image of the interior of the melt pocket Silica (with minor amounts ofaluminium) forms subidioblastic to idioblastic crystals with roughly cubic-shaped cross sections The vesicles (v) are irregularly-shaped c)higher resolution cross-sections through the crystals depicted in (b) show unusual morphologies like primate metatarsals The interior of thecrystals are compositionally similar to the groundmass and are enriched in Fe Mg and Ca d) contact between the melt pocket (top) andmaskelynite (base of photo) is gradational fining toward the margin Maskelynite has recrystallized in a ~10 mm-wide rim in direct contactwith the melt pocket

Shock-induced melt pockets in Los Angeles 1871

some are observed to be wedge-shaped wherein they taperoff typically within 3 mm from the source melt pocket Incontrast to the melt veins they contain abundant vesicles andare confined to grain boundaries of relatively high and lowdensity minerals Contacts with adjacent minerals are diffuseTo observe stringers or blebs of melt co-mingling withadjacent host rock minerals is common

While interconnected melt pockets do occur in LosAngeles isolated pockets are more prevalent Due to thehighly irregular nature of typical shock veins (especially withincreasing peak shock pressure Stoumlffler et al 1991) thetapering melt veins of Los Angeles possibly connect toseemingly isolated melt pockets out of the 2-dimensionalplane of the thin section However melt veins in Los Angelesare morphologically distinct from typical discordantpseudotachylyte-like shock veins observed in other SNCmeteorites (eg Shergotty Stoumlffler et al 1986) and ordinarychondrites (Stoumlffler et al 1991) that commonly transect thewhole meteorite

In Los Angeles the contact between the melt pockets andadjacent host rock minerals is irregular and commonlytransitional (Fig 1c) Host rock minerals adjacent to the meltpockets have been partially melted or recrystallized The meltpockets and melt veins and dikelets emanating from meltpocket margins commonly crosscut both primary igneoustextures and bulk shock effects

Microscopic areas of alkali- and silica-rich glass (336ndash548 wt K2O 159ndash337 wt Na2O 879ndash975 wt SiO2and 22ndash42 wt Al2O3 respectively) are associated withmaskelynite and silica crystals in the bulk sample They rangein apparent diameter from ~20ndash350 mm and exhibit asymplectic intergrowth texture between the two phasesMicrofaults emanating from melt pocket margins displace theadjacent alkali- and silica-rich glasses (up to 15 mm)However the alkali- and silica-rich glasses at the melt pocketmargins contain entrained schlieren and globules of meltmaterial that are detached from the melt pocket The fluidnature of the material is indicated from the presence ofpyrrhotite spheres and vesicles

Regions of localized melting indicate a minimummelting temperature for nonporous basalts of 1600degC(Stoumlffler 1984) Additionally melting of ilmenite indicatestemperatures in excess of 1800degC (El Goresy et al 1968)Localized pressure and temperature excursions within meltpockets based on experimental data (Keiffer et al 1976Stoumlffler 1984 Schmitt 2000) are estimated at 60ndash80 GPa and1600ndash2000degC respectively Schaal and Houmlrz (1977) observedmelting of pyroxene in experimentally shocked basalts at aminimum pressure limit of 80 GPa

Quench CrystalsOne feature common to all melt pockets in Los Angeles

is the presence of quench crystals The crystals have avariety of distinctive shapes (skeletal dendritic feathery)

and they comprise a variety of minerals (olivinewhitlockite titanomagnetite plagioclase) Quench crystalsare invariably microscopic Severely fractured melt-intruded clinopyroxenes adjacent to the melt pockets allowfor preferential nucleation of dendritic crystals that extendfrom the contact into glass These consist of intimateintergrowths of ferropigeonite (Fs73En18) and orthopyroxene(Fs40En52) with dendritic textures (Fig 2e) In additioncrystal fragments within melt pockets locally derived fromadjacent host minerals serve as nucleation sites for dendriticgrowth (Fig 4) These textures are consistent with rapiddiffusion-controlled crystal growth from the melt(Kirkpatrick 1975)

Two melt pockets in Los Angeles show the symplecticintergrowth fayalite + ferroaugite + silica a late-stage mineralassemblage resulting from the breakdown of pyroxferroite

Fig 4 Backscattered electron images of Los Angeles a) a largetitanomagnetite clast (lower right) within a melt pocket has seededpreferential nucleation of titanomagnetite crystallites with dendriticmorphologies attesting to rapid cooling b) higher magnification ofthe titanomagnetite crystallites are depicted in (a)

1872 E L Walton and J G Spray

(Rubin et al 2000) in direct contact with the melt pocket Atthe melt pocket margin the vermicular to microgranulitictexture typical of the symplectites changes to one of skeletalfayalite (Fa96) embedded in a glassy matrix rich in Fe Mg andCa and Fe-sulfide spherules (Fig 2b) Numerous fractures inthe vicinity of the melt pocket radiate from the contact withthe melt into the fayalite + ferroaugite + silica intergrowthcausing microfaulting

MELT POCKET COMPOSITIONS

The bulk chemistries of four melt pockets in LosAngeles are presented in Table 3 The compositions arevariable from one pocket to another CIPW norm calculationsreveal that the melt pockets can be derived by fusingdifferent mixtures of the host minerals dominated byplagioclase and pyroxene Melt pocket 1 (Type 1) isdominated by normative quartz orthoclase plagioclase andhypersthene melt pocket 2 (Type 2) by quartz orthoclaseplagioclase and hypersthene (Type 3) by mainlyplagioclase diopside and hypersthene melt pocket 4 (Type3) by plagioclase hypersthene and apatite (whitlockite)Melt pocket compositions depend on which bordering hostminerals contribute to melting and the proportion of meltingof each phase (eg some melt pockets occur at the grainboundary of pyroxene and maskelynite thus these are the

main components contributing to the melt) The occurrenceof at least two coexisting silicate melts within one Type 3melt pocket is revealed by crosscutting relationships(Fig 2a) The more siliceous melt of the two (average65 wt SiO2 obtained by raster scans n = 8) is located inproximity to the melt pocket margin This melt contains lithicand crystal fragments (~50) of locally-derived silica andmaskelynite The less siliceous (average 43 wt SiO2

obtained by raster scans n = 6) lower viscosity meltcrosscuts the former tapering off to thin stringers (pound1 mm inapparent thickness) The shape of the overall melt pocket isroughly spherical (Fig 2) ie no textural evidence exists tosuggest that these crosscutting relationships are the result oftwo independently formed melt pockets Although the twosilicate melts were most probably subject to the same coolingrates the abundance of lithic and crystal fragments wouldhave increased the amount of heterogeneous nucleation sitesfavoring more rapid crystallization concomitant with anincrease in viscosity This is also consistent with rapidcrystallization in Type 1 melt pockets containing abundantclasts of host rock minerals (Figs 2f and 2g)

DISCUSSION

Melt pockets within Los Angeles can be divided into 3types based on size clast volume and degree of vesiculation

Table 3 Bulk compositions of shock melt pockets (in wt) for Los Angeles stone 1Melt pocket 1 (Type 1) Melt pocket 2 (Type 2) Melt pocket 3 (Type 3) Melt pocket 4 (Type 3)

wt avg min max avg min max avg min max avg min max 5a

SiO2 589 511 691 627 585 731 457 365 535 328 255 467 491Al2O3 95 120 06 144 119 183 42 17 91 33 15 46 112TiO2 05 03 06 03 02 05 15 04 46 11 05 11 130FeOb 219 128 329 113 6 189 305 197 404 299 234 441 212MnO 05 07 03 03 ndc 05 07 04 12 07 05 13 045MgO 09 05 26 03 06 52 42 16 110 08 06 13 353CaO 37 57 13 58 58 73 96 70 128 159 59 164 995Na2O 18 10 23 24 12 3 02 nd 11 05 01 09 222K2O 21 02 27 13 08 18 03 nd 05 04 05 06 024P2O5 ndash nd 05 ndash nd 01 ndash nd 49 111 89 265 066SO3 ndash nd 08 ndash nd nd nd 04 27 ndash nd 19 ndashTotal 1001 988 996 970 9986nd 32 30 38 24

CIPW normQuartz 144 226 19 0Orthoclase 124 78 18 25Albite 152 206 17 44Anorthite 117 25 98 58Diopside 54 37 249 07Hypersthene 398 199 534 572Olivine 0 0 0 06Ilmenite 1 06 29 22Apatite 02 0 36 272

aData from Rubin et al (2000) obtained by Instrumental Neutron Activation AnalysisbTotal Fe reported as FeOcnd = element oxides below the detection limit of the electron microprobedn = number of raster scans per melt pocket

Shock-induced melt pockets in Los Angeles 1873

Smaller (lt1 mm) pockets (Types 1 and 2) tend to becrystallite-rich while larger (gt1 mm) pockets (Type 3) tend tobe clast-poor and exhibit flow-structured glasses Opticallyshock effects are manifest as dark brown to black glassy andor microcrystalline enclaves of melt material melt veins anddeposits of sulfides and melt injected into fractures withinsilicates In total these localized regions of melting compriselt1 vol of Los Angeles and they are heterogeneouslydistributed throughout the sample

Melt pockets invariably develop at the grain boundariesof minerals with contrasting shock impedance (shock wavevelocity multiplied by the mineralrsquos density) eg plagioclaseand pyroxene This is most obvious in smaller (pound1 mm) meltpockets (Type 1 and 2) where the original grain positions andboundaries are preserved With increasing volume of melt theoriginal grains and their boundaries become obscured ordestroyed Typically minerals adjacent to the melt pocketshave smooth arcuate margins at the contact with the meltThe composition of the melt pockets corresponds to localizedmelting of bordering host rock minerals Melt veins have beenobserved to form an interlocking network of complexanastomosing branches surrounding unmolten host rockconstituents

The variable compositions of the studied melt pocketscan be attributed to the relatively coarse grain size (2ndash4 mm)of the host rock minerals compared to the diameters of themelt pockets (lt4 mm) The melt pockets are not largeenough to represent a bulk rock melt of Los AngelesInstead they represent localized mineral-scale meltingproducts Melt pocket compositions presented in Table 3differ markedly when comparing those of the smaller meltpockets Types 1 and 2 to the larger Type 3 melt pocketsTypes 1 and 2 are enriched in SiO2 Al2O3 and alkalis ascompared to Type 3 The CIPW norm calculations alsolisted in Table 3 indicate that the precursors of Types 1 and2 melt pockets were mainly quartz + alkali feldspar +plagioclase + pyroxene while those for Type 3 were mainlyplagioclase + pyroxene + whitlockite Therefore theprecursors of Types 1 and 2 melt pockets likely had a largecontribution from late-stage minerals (mesostasis) while thebulk of Type 3 are derived from the main host rock lithologyThis is not surprising considering that shock melting beginsat grain boundary margins where mesostasis is moreabundant Thus smaller (Type 1 and 2) melt pockets willposses a greater contribution of mesostasis to bulkcomposition than larger (Type 3) melt pockets that havemelted approximately the same amount of mesostasis butgreater proportions of host rock phases

Based on compositional and textural lines of evidencewe conclude that the melt pockets formed by the local in situmelting of host rock minerals as opposed to forcefulinjection of extraneous impact melt along fractures in thehost rock Remobilization and injection of opaques and meltfrom the melt region into pre-existing fractures in adjacent

host rock pyroxene could have occurred during compressionand subsequent stress relaxation The contact between themelt pockets and adjacent host minerals is intrusive inplaces The intensity of shock features is observed toincrease with proximity to the melt pockets indicating alocalized increase in shock pressure This pressure gradientfrom melt pocket (60ndash80 GPa) to bulk rock (35ndash40 GPa) iscertainly sufficient to account for forceful injection of Fe-sulfides and melt from the melt pockets into adjacent hostminerals Additionally intrusions could have formed bypressure generated by a volume increase (~10) associatedwith the melting event

The relationship between areas of alkali- and silica-richglass and the melt pockets is puzzling However cominglingand entrapment of liquid blebs from the melt pocket ispossible if the two phases were initially molten at the sametime This could be explained if the alkali- and silica-richglass was formed during the shock event but was quenchedbefore the shock pressure was completely released Melt fromthe melt pocket could then have remained in a liquid stateafter pressure release This is consistent with the presence ofabundant vesicles in the melt pocket glasses indicating lowconfining pressure Stopler and McSween (1979) observedsimilar textural associations in Shergotty and Zagamiconcluding that they may represent shock mixtures of alkali-feldspars and silica

The presence of clasts in one melt pocket with quenchtextures as well as the presence of crosscutting silicate meltsdoes not require multiple impact events That a single impactevent is capable of producing several shock metamorphicfeatures in a target rock is well-documented (eg Stoumlffler1984 Stoumlffler et al 1991) The high concentration ofrelatively cold lithic and crystal fragments as well as theproximity to the melt pocket margin may have facilitatedrapid crystallization of one melt with respect to the otherRapid crystallization could also cause a drastic increase in theviscosity of one melt with respect to the other

The development of abundant crystallites and glass inthe melt pockets attests to rapid cooling of the melt whichimplies that the studied Los Angeles samples were derivedfrom the chilled margins of a larger ejected body (severalmeters to tens of meters in diameter) or were never muchlarger than the recovered meteorite sizes (tens of cm indiameter) The high abundance of relatively cold clasts inthe smaller melt pockets (Type 1) probably contributed tothe rapid crystallization (generating abundant crystallites)and an absence of flow textures due to the presence ofcopious heterogeneous nucleation sites Vesiculationprobably occurred during shock decompression withvesicles persisting in the melt under lower pressureconditions (Kenkmann et al 2000) However this does notpreclude the development of high pressure high temperaturepolymorphs within the melt pockets produced by cavitationand bubble implosion (Spray 1999) Melt veins and melt

1874 E L Walton and J G Spray

dikelets occur in direct contact with the melt pockets and areconsidered morphologically distinct from typical ldquoshockveinsrdquo that transect meteorites and that form by acombination of shock and frictional melting The concentriczoning observed in the margins of clinopyroxenes in contactwith melt pockets coincides with a progressive reduction inbirefringence which suggests that shock excursions werefocused as point sources and that these were generated in thewake of the planar shock front that induced bulk shockeffects We infer that the larger melt pockets (Type 3) wereformed by higher energy excursions relative to the smallerpockets (Type 1 and 2)

Based on experimental data by Kieffer et al (1976)Schaal and Houmlrz (1977) and Schmitt (2000) shock pressureand temperature excursions of 60ndash80 GPa and 1600ndash 2000degCare estimated for Los Angeles 7058 Because pyroxene is notobserved as clasts in any of the melt pockets in Los Angelesthe higher pressure of ~80 GPa is favored coincident with theobserved melting of pyroxene in experimentally shockedbasalts (Schaal and Houmlrz 1977) These discrete excursionsare therefore approximately twice the magnitude of the bulkshock (35ndash40 GPa)

A young formation age 170 plusmn 8 Ma for the Los Angelesmeteorite (Nyquist et al 2001) precludes it recording multipleimpact events that would be more typical of target rocks fromthe older impact-scarred Noachian terrains of the southernhemisphere Instead Los Angeles records evidence of asingle shock event which favors its launching immediatelyfollowing hypervelocity impact into a relatively young (lateAmazonian) volcanic source terrain such as is found in theTharsis and Elysium regions

AcknowledgmentsndashThis work has been funded by the NaturalSciences and Engineering Research Council of Canada(NSERC) through research grants awarded to J G Spray andan NSERC PGS-b scholarship awarded to E L Walton Wethank Tim McCoy at the Smithsonian Institution WashingtonDC for loan of the samples Douglas Hall at UNB andMichael Coy at JEOL USA for EM support Cliff Shaw forhelpful discussions and Angel Gomez for draftingConstructive Reviews by Y Ikeda and R F Schmitt aregratefully acknowledged Planetary and Space Science CentreContribution 31

Editorial HandlingmdashDr Edward Scott

REFERENCES

Chen M and El Goresy A 2000 The nature of maskelynite inshocked meteorites Not a diaplectic glass but a glass quenchedfrom shock-induced dense melt at high pressures Earth andPlanetary Science Letters 179489ndash502

El Goresy A Fechtig H and Ottemann J 1968 The opaqueminerals in impact glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMono Book pp 531ndash553

Garrison D H and Bogard D D 2000 Cosmogenic and trappednoble gases in the Los Angeles meteorite (abstract) Meteoriticsamp Planetary Science 35A58

Houmlrz F and Quaide W L 1973 Debye-Scherrer investigations ofexperimentally shocked silicates The Moon 645ndash82

Kenkmann T Hornemann U and Stoumlffler D 2000 Experimentalgeneration of shock-induced pseudotachylytes alonglithological interfaces Meteoritics amp Planetary Science 351275ndash1290

Kieffer S W Schaal R B Gibbons R Houmlrz F Milton D J andDube A 1976 Shocked basalt from Lonar crater India andexperimental analogues Proceedings 7th Lunar and PlanetaryScience Conference pp 1391ndash1412

Kirkpatrick R J 1975 Crystal growth from the melt A reviewAmerican Mineralogist 60798ndash814

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McSween H Y Jr 2002 The rocks of Mars from far and nearMeteoritics amp Planetary Science 377ndash25

Melosh H J 1984 Impact ejection spallation and the origin ofmeteorites Icarus 59234ndash260

Melosh H J 1985 Ejection of rock fragments from planetary bodiesGeology 13144ndash148

Nishiizumi K Caffee M W and Masarik J 2000 Cosmogenicradiocunclides in Los Angeles martian meteorite (abstract)Meteoritics amp Planetary Science 35A120

Nyquist L E Reese Y D Wiesmann H Shih C Y and SchwandtC 2000 Rubidium-strontium age of the Los Angeles shergottiteMeteoritics amp Planetary Science 35A121ndashA122

Nyquist L E Bogard D D Shih C Y Greshake A Stoumlffler D andEugster O 2001 Ages and histories of martian meteoritesChronology and Evolution of Mars 96105ndash164

Rubin A E Warren P H Greenwood J P Verish R S Leshin LA Hervig R L and Clayton R N 2000 Los Angeles The mostdifferentiated basaltic martian meteorites Geology 281011ndash1014

Schaal R B and Houmlrz F 1977 Shock metamorphism of lunar andterrestrial basalts Proceedings 8th Lunar and Planetary ScienceConference pp 1679ndash1729

Schmitt R T 2000 Shock experiments with the H6 chondriteKernouveacute Pressure calibration of microscopic shock effectsMeteoritics amp Planetary Science 3545ndash560

Spray J G 1999 Shocking rocks by cavitation and bubble implosionGeology 27695ndash698

Spray J G and Rae D A 1995 Quantitative electron-microprobeanalysis of alkali silicate glasses A review and user guideCanadian Mineralogist 33323ndash332

Stoumlffler D 1984 Glasses formed by hypervelocity impact Journal ofNon-Crystalline Solids 67465ndash502

Stoumlffler D 2000 Maskelynite confirmed as diaplectic glassIndications for peak shock pressure below 45 GPa in all martianmeteorites (abstract 1170) 31st Lunar and Planetary ScienceConference CD-ROM

Stoumlffler D Ostertag R Jammes C and Pfannschmidt G 1986Shock metamorphism and petrography of the Shergottyachondrite Geochimica et Cosmochimica Acta 50889ndash903

Stoumlffler D Keil K and Scott E R D 1991 Shock metamorphismof ordinary chondrites Geochimica et Cosmochimica Acta 53845ndash3867

Stopler E and McSween H Y Jr 1979 Petrology and origin of theshergottite meteorites Geochimica et Cosmochimica Acta 431475ndash1498

Terribilini D Busemann H and Eugster O 2000 Krypton-81-

1868 E L Walton and J G Spray

exclusively manifest as normal glass within the smallermelt pockets however rare clasts have been observed atthe melt pocket margins

2 Type 2 Microscopic (003 times 04 mm) vesiculatedclast-poor with abundant crystallites of idiomorphic tosubidiomorphic silica crystals (with accessory Al2O3)embedded in an unresolvable glassy groundmass rich inFe Ca and Na (Figs 3andash3c) The groundmass isrelatively pristine and shows no signs of devitrificationor alteration to aggregates of new phases (eg claysiddingsite) The interior of the melt pocket is coarsergrained fining toward the margin (Fig 3d)Maskelynite adjacent to the melt pocket hasrecrystallized in a ~5 mm-wide rim surrounding the

melt pocket As with Type 1 melt pockets Type 2 arealso characterized by an absence of immiscible blebs ofFe-sulfides

3 Type 3 Macroscopic (35 mm times 30 mm) dominantlyvesicular relatively clast-poor with flow textures andabundant spheresblebs of immiscible Fe-sulfides (Figs2andash2e) Sulfide spheres range from pound01 to 20 mm indiameter and comprise mono- and polycrystallinepyrrhotite Most vesicles are spherical indicative ofquenching at low confining pressure and low shearstrain Some are elongated with their c-axes parallel tothe direction of flow Vesicles range from sup31 mm to05 mm apparent diameter Individual schlieren can betraced for several mm yet their length and width are

Fig 1 a) and b) Optical photomicrographs of the contact between a host rock clinopyroxene grain (cpx) with a melt pocket (mp) in LosAngeles Zoning in the pyroxene (obtained from microprobe point analyses) is complex but overall changes from a Mg-rich core to a Fe-richrim In plane polarized light (a) the margin adjacent to the melt pocket is ldquostainedrdquo a light-brown color arranged in concentric zones b) crossedpolars view of the same area shown in (a) revealing the marked change in birefringence from dmax = 0020ndash0000 at the melt pocket marginDeformed exsolution lamellae in the pyroxene are indicated by the black arrow c) and d) backscattered electron images (BEI) of the samepyroxene as in (a) in contact with the melt pocket rotated 90deg c) abundant fragments of whitlockite (wi) are located within the melt pocketadjacent to the reaction zone The arrow indicates plagioclase schlieren d) reaction zone at the margin of cpx adjacent to the melt pocketshowing the distribution of silica pyrrhotite titanomagneite and pyroxene The location is highlighted as a white box in (c)

Shock-induced melt pockets in Los Angeles 1869

Fig 2 Backscattered electron images of Type 3 (andashe) and Type 1 (fndashg) melt pockets in context with host rock minerals maskelynite (mk) andclinopyroxene (cpx) a) crosscutting relationship between two coexisting silicate melts within one melt pocket (Type 3) The contact betweenthe 2 melts is diffuse indicating that the higher viscosity melt was relatively warm when the other was intruded The higher viscosity meltcontains abundant subidiomorphic crystals with square- and diamond-shaped cross-sections that increase in abundance with distance from themain melt pocket Small crystal size prohibits quantitative analysis but qualitative microprobe analysis indicates that only iron and titaniumare present in proportions 3 to 1 suggesting titanomagnetite b) the pyroxferroite breakdown material (pbm) with typical vermicular texturein the host rock (upper right hand side) changes to skeletal and feather fayalite crystals embedded in a glassy groundmass with pyrrhotiteglobules at the contact between host rock phases and the melt pocket c) intrusive relationship between maskelynite (dark greyblack right handside) and the melt pocket The white spheres are pyrrhotite The cross-sections through quenched whitlockite needles embedded in the glassesare shown by the white arrow d) glassy groundmass of a Type 3 melt pocket with compositionally distinct flow-layering (revealed by grey-scale variations in BEI imaging) and cellular texture of glasses (upper and lower right hand side) The two black subspherical structures at thebottom of the image are vesicles connected by an open irregular fracture e) dendritic crystallites of pigeonite and orthopyroxene nucleatingat the edge of a host-rock pyroxene grain at the melt pocket margin f) clast containing quench textures within a Type 1 melt pocket g)microcrystalline groundmass typical of a Type 1 melt pocket containing abundant crystallites rich in Al Si and Ca (plagioclase)

1870 E L Walton and J G Spray

highly variable Contact relationships of individualschlieren with the surrounding glass range from sharp tohighly diffuse The schlieren show compositionalvariability attributed to melting of different proportionsof minerals adjacent to the melt pockets (Tables 2 and 3)The proportion of locally derived lithic and crystalfragments increases toward the melt pocket marginThese consist of titanomagnetite maskelynitewhitlockite and alkali- and silica-rich glasses Pyroxenehas never been observed as clasts entrained in any of thelarge melt pocketsTwo distinct types of remobilized melt material emanate

from the melt pockets (Fig 1) They include thin (005ndash10 mmapparent thickness) non-vesicular opaque andor glassyveins (referred to as melt veins) and thicker (50ndash210 mm

apparent thickness) vesicular sinuous apophyses of meltmaterial (referred to as melt dikelets) Melt veins comprisesulfide- and melt-filled fractures and microfaults with smalllateral offsets (up to 80 mm) They possess discordant contactswith the host minerals commonly form an interlockingnetwork of anastomosing branches surrounding unmoltenhost rock constituents and crosscut host rock minerals Meltveins have not been observed to penetrate more than 550 mmfrom their source melt pockets (ie they do not completelytransect the sample) A high concentration of melt veinsinduces a distinct darkening (opacity) of pyroxene grains inoptical microscopy (referred to as ldquoshock blackeningrdquo inchondrites Stoumlffler et al 1991)

Melt dikelets emanate from the main melt pocket andcommonly enlarge to form subsidiary melt pockets However

Fig 3 Backscattered electron images of melt pocket Type 2 in Los Angeles a) low magnification overview of melt pocket (center of photo)Host rock minerals adjacent the melt pocket are maskelynite (mk) clinopyroxene (cpx) an intergrowth of K-rich feldspathic glass and silica-rich glass (K Si) and pyroxferroite breakdown material (pbm) Carbonates (crb) partially infill an E-W-trending fracture that crosscuts bothhost rock minerals and the melt pocket b) higher resolution image of the interior of the melt pocket Silica (with minor amounts ofaluminium) forms subidioblastic to idioblastic crystals with roughly cubic-shaped cross sections The vesicles (v) are irregularly-shaped c)higher resolution cross-sections through the crystals depicted in (b) show unusual morphologies like primate metatarsals The interior of thecrystals are compositionally similar to the groundmass and are enriched in Fe Mg and Ca d) contact between the melt pocket (top) andmaskelynite (base of photo) is gradational fining toward the margin Maskelynite has recrystallized in a ~10 mm-wide rim in direct contactwith the melt pocket

Shock-induced melt pockets in Los Angeles 1871

some are observed to be wedge-shaped wherein they taperoff typically within 3 mm from the source melt pocket Incontrast to the melt veins they contain abundant vesicles andare confined to grain boundaries of relatively high and lowdensity minerals Contacts with adjacent minerals are diffuseTo observe stringers or blebs of melt co-mingling withadjacent host rock minerals is common

While interconnected melt pockets do occur in LosAngeles isolated pockets are more prevalent Due to thehighly irregular nature of typical shock veins (especially withincreasing peak shock pressure Stoumlffler et al 1991) thetapering melt veins of Los Angeles possibly connect toseemingly isolated melt pockets out of the 2-dimensionalplane of the thin section However melt veins in Los Angelesare morphologically distinct from typical discordantpseudotachylyte-like shock veins observed in other SNCmeteorites (eg Shergotty Stoumlffler et al 1986) and ordinarychondrites (Stoumlffler et al 1991) that commonly transect thewhole meteorite

In Los Angeles the contact between the melt pockets andadjacent host rock minerals is irregular and commonlytransitional (Fig 1c) Host rock minerals adjacent to the meltpockets have been partially melted or recrystallized The meltpockets and melt veins and dikelets emanating from meltpocket margins commonly crosscut both primary igneoustextures and bulk shock effects

Microscopic areas of alkali- and silica-rich glass (336ndash548 wt K2O 159ndash337 wt Na2O 879ndash975 wt SiO2and 22ndash42 wt Al2O3 respectively) are associated withmaskelynite and silica crystals in the bulk sample They rangein apparent diameter from ~20ndash350 mm and exhibit asymplectic intergrowth texture between the two phasesMicrofaults emanating from melt pocket margins displace theadjacent alkali- and silica-rich glasses (up to 15 mm)However the alkali- and silica-rich glasses at the melt pocketmargins contain entrained schlieren and globules of meltmaterial that are detached from the melt pocket The fluidnature of the material is indicated from the presence ofpyrrhotite spheres and vesicles

Regions of localized melting indicate a minimummelting temperature for nonporous basalts of 1600degC(Stoumlffler 1984) Additionally melting of ilmenite indicatestemperatures in excess of 1800degC (El Goresy et al 1968)Localized pressure and temperature excursions within meltpockets based on experimental data (Keiffer et al 1976Stoumlffler 1984 Schmitt 2000) are estimated at 60ndash80 GPa and1600ndash2000degC respectively Schaal and Houmlrz (1977) observedmelting of pyroxene in experimentally shocked basalts at aminimum pressure limit of 80 GPa

Quench CrystalsOne feature common to all melt pockets in Los Angeles

is the presence of quench crystals The crystals have avariety of distinctive shapes (skeletal dendritic feathery)

and they comprise a variety of minerals (olivinewhitlockite titanomagnetite plagioclase) Quench crystalsare invariably microscopic Severely fractured melt-intruded clinopyroxenes adjacent to the melt pockets allowfor preferential nucleation of dendritic crystals that extendfrom the contact into glass These consist of intimateintergrowths of ferropigeonite (Fs73En18) and orthopyroxene(Fs40En52) with dendritic textures (Fig 2e) In additioncrystal fragments within melt pockets locally derived fromadjacent host minerals serve as nucleation sites for dendriticgrowth (Fig 4) These textures are consistent with rapiddiffusion-controlled crystal growth from the melt(Kirkpatrick 1975)

Two melt pockets in Los Angeles show the symplecticintergrowth fayalite + ferroaugite + silica a late-stage mineralassemblage resulting from the breakdown of pyroxferroite

Fig 4 Backscattered electron images of Los Angeles a) a largetitanomagnetite clast (lower right) within a melt pocket has seededpreferential nucleation of titanomagnetite crystallites with dendriticmorphologies attesting to rapid cooling b) higher magnification ofthe titanomagnetite crystallites are depicted in (a)

1872 E L Walton and J G Spray

(Rubin et al 2000) in direct contact with the melt pocket Atthe melt pocket margin the vermicular to microgranulitictexture typical of the symplectites changes to one of skeletalfayalite (Fa96) embedded in a glassy matrix rich in Fe Mg andCa and Fe-sulfide spherules (Fig 2b) Numerous fractures inthe vicinity of the melt pocket radiate from the contact withthe melt into the fayalite + ferroaugite + silica intergrowthcausing microfaulting

MELT POCKET COMPOSITIONS

The bulk chemistries of four melt pockets in LosAngeles are presented in Table 3 The compositions arevariable from one pocket to another CIPW norm calculationsreveal that the melt pockets can be derived by fusingdifferent mixtures of the host minerals dominated byplagioclase and pyroxene Melt pocket 1 (Type 1) isdominated by normative quartz orthoclase plagioclase andhypersthene melt pocket 2 (Type 2) by quartz orthoclaseplagioclase and hypersthene (Type 3) by mainlyplagioclase diopside and hypersthene melt pocket 4 (Type3) by plagioclase hypersthene and apatite (whitlockite)Melt pocket compositions depend on which bordering hostminerals contribute to melting and the proportion of meltingof each phase (eg some melt pockets occur at the grainboundary of pyroxene and maskelynite thus these are the

main components contributing to the melt) The occurrenceof at least two coexisting silicate melts within one Type 3melt pocket is revealed by crosscutting relationships(Fig 2a) The more siliceous melt of the two (average65 wt SiO2 obtained by raster scans n = 8) is located inproximity to the melt pocket margin This melt contains lithicand crystal fragments (~50) of locally-derived silica andmaskelynite The less siliceous (average 43 wt SiO2

obtained by raster scans n = 6) lower viscosity meltcrosscuts the former tapering off to thin stringers (pound1 mm inapparent thickness) The shape of the overall melt pocket isroughly spherical (Fig 2) ie no textural evidence exists tosuggest that these crosscutting relationships are the result oftwo independently formed melt pockets Although the twosilicate melts were most probably subject to the same coolingrates the abundance of lithic and crystal fragments wouldhave increased the amount of heterogeneous nucleation sitesfavoring more rapid crystallization concomitant with anincrease in viscosity This is also consistent with rapidcrystallization in Type 1 melt pockets containing abundantclasts of host rock minerals (Figs 2f and 2g)

DISCUSSION

Melt pockets within Los Angeles can be divided into 3types based on size clast volume and degree of vesiculation

Table 3 Bulk compositions of shock melt pockets (in wt) for Los Angeles stone 1Melt pocket 1 (Type 1) Melt pocket 2 (Type 2) Melt pocket 3 (Type 3) Melt pocket 4 (Type 3)

wt avg min max avg min max avg min max avg min max 5a

SiO2 589 511 691 627 585 731 457 365 535 328 255 467 491Al2O3 95 120 06 144 119 183 42 17 91 33 15 46 112TiO2 05 03 06 03 02 05 15 04 46 11 05 11 130FeOb 219 128 329 113 6 189 305 197 404 299 234 441 212MnO 05 07 03 03 ndc 05 07 04 12 07 05 13 045MgO 09 05 26 03 06 52 42 16 110 08 06 13 353CaO 37 57 13 58 58 73 96 70 128 159 59 164 995Na2O 18 10 23 24 12 3 02 nd 11 05 01 09 222K2O 21 02 27 13 08 18 03 nd 05 04 05 06 024P2O5 ndash nd 05 ndash nd 01 ndash nd 49 111 89 265 066SO3 ndash nd 08 ndash nd nd nd 04 27 ndash nd 19 ndashTotal 1001 988 996 970 9986nd 32 30 38 24

CIPW normQuartz 144 226 19 0Orthoclase 124 78 18 25Albite 152 206 17 44Anorthite 117 25 98 58Diopside 54 37 249 07Hypersthene 398 199 534 572Olivine 0 0 0 06Ilmenite 1 06 29 22Apatite 02 0 36 272

aData from Rubin et al (2000) obtained by Instrumental Neutron Activation AnalysisbTotal Fe reported as FeOcnd = element oxides below the detection limit of the electron microprobedn = number of raster scans per melt pocket

Shock-induced melt pockets in Los Angeles 1873

Smaller (lt1 mm) pockets (Types 1 and 2) tend to becrystallite-rich while larger (gt1 mm) pockets (Type 3) tend tobe clast-poor and exhibit flow-structured glasses Opticallyshock effects are manifest as dark brown to black glassy andor microcrystalline enclaves of melt material melt veins anddeposits of sulfides and melt injected into fractures withinsilicates In total these localized regions of melting compriselt1 vol of Los Angeles and they are heterogeneouslydistributed throughout the sample

Melt pockets invariably develop at the grain boundariesof minerals with contrasting shock impedance (shock wavevelocity multiplied by the mineralrsquos density) eg plagioclaseand pyroxene This is most obvious in smaller (pound1 mm) meltpockets (Type 1 and 2) where the original grain positions andboundaries are preserved With increasing volume of melt theoriginal grains and their boundaries become obscured ordestroyed Typically minerals adjacent to the melt pocketshave smooth arcuate margins at the contact with the meltThe composition of the melt pockets corresponds to localizedmelting of bordering host rock minerals Melt veins have beenobserved to form an interlocking network of complexanastomosing branches surrounding unmolten host rockconstituents

The variable compositions of the studied melt pocketscan be attributed to the relatively coarse grain size (2ndash4 mm)of the host rock minerals compared to the diameters of themelt pockets (lt4 mm) The melt pockets are not largeenough to represent a bulk rock melt of Los AngelesInstead they represent localized mineral-scale meltingproducts Melt pocket compositions presented in Table 3differ markedly when comparing those of the smaller meltpockets Types 1 and 2 to the larger Type 3 melt pocketsTypes 1 and 2 are enriched in SiO2 Al2O3 and alkalis ascompared to Type 3 The CIPW norm calculations alsolisted in Table 3 indicate that the precursors of Types 1 and2 melt pockets were mainly quartz + alkali feldspar +plagioclase + pyroxene while those for Type 3 were mainlyplagioclase + pyroxene + whitlockite Therefore theprecursors of Types 1 and 2 melt pockets likely had a largecontribution from late-stage minerals (mesostasis) while thebulk of Type 3 are derived from the main host rock lithologyThis is not surprising considering that shock melting beginsat grain boundary margins where mesostasis is moreabundant Thus smaller (Type 1 and 2) melt pockets willposses a greater contribution of mesostasis to bulkcomposition than larger (Type 3) melt pockets that havemelted approximately the same amount of mesostasis butgreater proportions of host rock phases

Based on compositional and textural lines of evidencewe conclude that the melt pockets formed by the local in situmelting of host rock minerals as opposed to forcefulinjection of extraneous impact melt along fractures in thehost rock Remobilization and injection of opaques and meltfrom the melt region into pre-existing fractures in adjacent

host rock pyroxene could have occurred during compressionand subsequent stress relaxation The contact between themelt pockets and adjacent host minerals is intrusive inplaces The intensity of shock features is observed toincrease with proximity to the melt pockets indicating alocalized increase in shock pressure This pressure gradientfrom melt pocket (60ndash80 GPa) to bulk rock (35ndash40 GPa) iscertainly sufficient to account for forceful injection of Fe-sulfides and melt from the melt pockets into adjacent hostminerals Additionally intrusions could have formed bypressure generated by a volume increase (~10) associatedwith the melting event

The relationship between areas of alkali- and silica-richglass and the melt pockets is puzzling However cominglingand entrapment of liquid blebs from the melt pocket ispossible if the two phases were initially molten at the sametime This could be explained if the alkali- and silica-richglass was formed during the shock event but was quenchedbefore the shock pressure was completely released Melt fromthe melt pocket could then have remained in a liquid stateafter pressure release This is consistent with the presence ofabundant vesicles in the melt pocket glasses indicating lowconfining pressure Stopler and McSween (1979) observedsimilar textural associations in Shergotty and Zagamiconcluding that they may represent shock mixtures of alkali-feldspars and silica

The presence of clasts in one melt pocket with quenchtextures as well as the presence of crosscutting silicate meltsdoes not require multiple impact events That a single impactevent is capable of producing several shock metamorphicfeatures in a target rock is well-documented (eg Stoumlffler1984 Stoumlffler et al 1991) The high concentration ofrelatively cold lithic and crystal fragments as well as theproximity to the melt pocket margin may have facilitatedrapid crystallization of one melt with respect to the otherRapid crystallization could also cause a drastic increase in theviscosity of one melt with respect to the other

The development of abundant crystallites and glass inthe melt pockets attests to rapid cooling of the melt whichimplies that the studied Los Angeles samples were derivedfrom the chilled margins of a larger ejected body (severalmeters to tens of meters in diameter) or were never muchlarger than the recovered meteorite sizes (tens of cm indiameter) The high abundance of relatively cold clasts inthe smaller melt pockets (Type 1) probably contributed tothe rapid crystallization (generating abundant crystallites)and an absence of flow textures due to the presence ofcopious heterogeneous nucleation sites Vesiculationprobably occurred during shock decompression withvesicles persisting in the melt under lower pressureconditions (Kenkmann et al 2000) However this does notpreclude the development of high pressure high temperaturepolymorphs within the melt pockets produced by cavitationand bubble implosion (Spray 1999) Melt veins and melt

1874 E L Walton and J G Spray

dikelets occur in direct contact with the melt pockets and areconsidered morphologically distinct from typical ldquoshockveinsrdquo that transect meteorites and that form by acombination of shock and frictional melting The concentriczoning observed in the margins of clinopyroxenes in contactwith melt pockets coincides with a progressive reduction inbirefringence which suggests that shock excursions werefocused as point sources and that these were generated in thewake of the planar shock front that induced bulk shockeffects We infer that the larger melt pockets (Type 3) wereformed by higher energy excursions relative to the smallerpockets (Type 1 and 2)

Based on experimental data by Kieffer et al (1976)Schaal and Houmlrz (1977) and Schmitt (2000) shock pressureand temperature excursions of 60ndash80 GPa and 1600ndash 2000degCare estimated for Los Angeles 7058 Because pyroxene is notobserved as clasts in any of the melt pockets in Los Angelesthe higher pressure of ~80 GPa is favored coincident with theobserved melting of pyroxene in experimentally shockedbasalts (Schaal and Houmlrz 1977) These discrete excursionsare therefore approximately twice the magnitude of the bulkshock (35ndash40 GPa)

A young formation age 170 plusmn 8 Ma for the Los Angelesmeteorite (Nyquist et al 2001) precludes it recording multipleimpact events that would be more typical of target rocks fromthe older impact-scarred Noachian terrains of the southernhemisphere Instead Los Angeles records evidence of asingle shock event which favors its launching immediatelyfollowing hypervelocity impact into a relatively young (lateAmazonian) volcanic source terrain such as is found in theTharsis and Elysium regions

AcknowledgmentsndashThis work has been funded by the NaturalSciences and Engineering Research Council of Canada(NSERC) through research grants awarded to J G Spray andan NSERC PGS-b scholarship awarded to E L Walton Wethank Tim McCoy at the Smithsonian Institution WashingtonDC for loan of the samples Douglas Hall at UNB andMichael Coy at JEOL USA for EM support Cliff Shaw forhelpful discussions and Angel Gomez for draftingConstructive Reviews by Y Ikeda and R F Schmitt aregratefully acknowledged Planetary and Space Science CentreContribution 31

Editorial HandlingmdashDr Edward Scott

REFERENCES

Chen M and El Goresy A 2000 The nature of maskelynite inshocked meteorites Not a diaplectic glass but a glass quenchedfrom shock-induced dense melt at high pressures Earth andPlanetary Science Letters 179489ndash502

El Goresy A Fechtig H and Ottemann J 1968 The opaqueminerals in impact glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMono Book pp 531ndash553

Garrison D H and Bogard D D 2000 Cosmogenic and trappednoble gases in the Los Angeles meteorite (abstract) Meteoriticsamp Planetary Science 35A58

Houmlrz F and Quaide W L 1973 Debye-Scherrer investigations ofexperimentally shocked silicates The Moon 645ndash82

Kenkmann T Hornemann U and Stoumlffler D 2000 Experimentalgeneration of shock-induced pseudotachylytes alonglithological interfaces Meteoritics amp Planetary Science 351275ndash1290

Kieffer S W Schaal R B Gibbons R Houmlrz F Milton D J andDube A 1976 Shocked basalt from Lonar crater India andexperimental analogues Proceedings 7th Lunar and PlanetaryScience Conference pp 1391ndash1412

Kirkpatrick R J 1975 Crystal growth from the melt A reviewAmerican Mineralogist 60798ndash814

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McSween H Y Jr 2002 The rocks of Mars from far and nearMeteoritics amp Planetary Science 377ndash25

Melosh H J 1984 Impact ejection spallation and the origin ofmeteorites Icarus 59234ndash260

Melosh H J 1985 Ejection of rock fragments from planetary bodiesGeology 13144ndash148

Nishiizumi K Caffee M W and Masarik J 2000 Cosmogenicradiocunclides in Los Angeles martian meteorite (abstract)Meteoritics amp Planetary Science 35A120

Nyquist L E Reese Y D Wiesmann H Shih C Y and SchwandtC 2000 Rubidium-strontium age of the Los Angeles shergottiteMeteoritics amp Planetary Science 35A121ndashA122

Nyquist L E Bogard D D Shih C Y Greshake A Stoumlffler D andEugster O 2001 Ages and histories of martian meteoritesChronology and Evolution of Mars 96105ndash164

Rubin A E Warren P H Greenwood J P Verish R S Leshin LA Hervig R L and Clayton R N 2000 Los Angeles The mostdifferentiated basaltic martian meteorites Geology 281011ndash1014

Schaal R B and Houmlrz F 1977 Shock metamorphism of lunar andterrestrial basalts Proceedings 8th Lunar and Planetary ScienceConference pp 1679ndash1729

Schmitt R T 2000 Shock experiments with the H6 chondriteKernouveacute Pressure calibration of microscopic shock effectsMeteoritics amp Planetary Science 3545ndash560

Spray J G 1999 Shocking rocks by cavitation and bubble implosionGeology 27695ndash698

Spray J G and Rae D A 1995 Quantitative electron-microprobeanalysis of alkali silicate glasses A review and user guideCanadian Mineralogist 33323ndash332

Stoumlffler D 1984 Glasses formed by hypervelocity impact Journal ofNon-Crystalline Solids 67465ndash502

Stoumlffler D 2000 Maskelynite confirmed as diaplectic glassIndications for peak shock pressure below 45 GPa in all martianmeteorites (abstract 1170) 31st Lunar and Planetary ScienceConference CD-ROM

Stoumlffler D Ostertag R Jammes C and Pfannschmidt G 1986Shock metamorphism and petrography of the Shergottyachondrite Geochimica et Cosmochimica Acta 50889ndash903

Stoumlffler D Keil K and Scott E R D 1991 Shock metamorphismof ordinary chondrites Geochimica et Cosmochimica Acta 53845ndash3867

Stopler E and McSween H Y Jr 1979 Petrology and origin of theshergottite meteorites Geochimica et Cosmochimica Acta 431475ndash1498

Terribilini D Busemann H and Eugster O 2000 Krypton-81-

Shock-induced melt pockets in Los Angeles 1869

Fig 2 Backscattered electron images of Type 3 (andashe) and Type 1 (fndashg) melt pockets in context with host rock minerals maskelynite (mk) andclinopyroxene (cpx) a) crosscutting relationship between two coexisting silicate melts within one melt pocket (Type 3) The contact betweenthe 2 melts is diffuse indicating that the higher viscosity melt was relatively warm when the other was intruded The higher viscosity meltcontains abundant subidiomorphic crystals with square- and diamond-shaped cross-sections that increase in abundance with distance from themain melt pocket Small crystal size prohibits quantitative analysis but qualitative microprobe analysis indicates that only iron and titaniumare present in proportions 3 to 1 suggesting titanomagnetite b) the pyroxferroite breakdown material (pbm) with typical vermicular texturein the host rock (upper right hand side) changes to skeletal and feather fayalite crystals embedded in a glassy groundmass with pyrrhotiteglobules at the contact between host rock phases and the melt pocket c) intrusive relationship between maskelynite (dark greyblack right handside) and the melt pocket The white spheres are pyrrhotite The cross-sections through quenched whitlockite needles embedded in the glassesare shown by the white arrow d) glassy groundmass of a Type 3 melt pocket with compositionally distinct flow-layering (revealed by grey-scale variations in BEI imaging) and cellular texture of glasses (upper and lower right hand side) The two black subspherical structures at thebottom of the image are vesicles connected by an open irregular fracture e) dendritic crystallites of pigeonite and orthopyroxene nucleatingat the edge of a host-rock pyroxene grain at the melt pocket margin f) clast containing quench textures within a Type 1 melt pocket g)microcrystalline groundmass typical of a Type 1 melt pocket containing abundant crystallites rich in Al Si and Ca (plagioclase)

1870 E L Walton and J G Spray

highly variable Contact relationships of individualschlieren with the surrounding glass range from sharp tohighly diffuse The schlieren show compositionalvariability attributed to melting of different proportionsof minerals adjacent to the melt pockets (Tables 2 and 3)The proportion of locally derived lithic and crystalfragments increases toward the melt pocket marginThese consist of titanomagnetite maskelynitewhitlockite and alkali- and silica-rich glasses Pyroxenehas never been observed as clasts entrained in any of thelarge melt pocketsTwo distinct types of remobilized melt material emanate

from the melt pockets (Fig 1) They include thin (005ndash10 mmapparent thickness) non-vesicular opaque andor glassyveins (referred to as melt veins) and thicker (50ndash210 mm

apparent thickness) vesicular sinuous apophyses of meltmaterial (referred to as melt dikelets) Melt veins comprisesulfide- and melt-filled fractures and microfaults with smalllateral offsets (up to 80 mm) They possess discordant contactswith the host minerals commonly form an interlockingnetwork of anastomosing branches surrounding unmoltenhost rock constituents and crosscut host rock minerals Meltveins have not been observed to penetrate more than 550 mmfrom their source melt pockets (ie they do not completelytransect the sample) A high concentration of melt veinsinduces a distinct darkening (opacity) of pyroxene grains inoptical microscopy (referred to as ldquoshock blackeningrdquo inchondrites Stoumlffler et al 1991)

Melt dikelets emanate from the main melt pocket andcommonly enlarge to form subsidiary melt pockets However

Fig 3 Backscattered electron images of melt pocket Type 2 in Los Angeles a) low magnification overview of melt pocket (center of photo)Host rock minerals adjacent the melt pocket are maskelynite (mk) clinopyroxene (cpx) an intergrowth of K-rich feldspathic glass and silica-rich glass (K Si) and pyroxferroite breakdown material (pbm) Carbonates (crb) partially infill an E-W-trending fracture that crosscuts bothhost rock minerals and the melt pocket b) higher resolution image of the interior of the melt pocket Silica (with minor amounts ofaluminium) forms subidioblastic to idioblastic crystals with roughly cubic-shaped cross sections The vesicles (v) are irregularly-shaped c)higher resolution cross-sections through the crystals depicted in (b) show unusual morphologies like primate metatarsals The interior of thecrystals are compositionally similar to the groundmass and are enriched in Fe Mg and Ca d) contact between the melt pocket (top) andmaskelynite (base of photo) is gradational fining toward the margin Maskelynite has recrystallized in a ~10 mm-wide rim in direct contactwith the melt pocket

Shock-induced melt pockets in Los Angeles 1871

some are observed to be wedge-shaped wherein they taperoff typically within 3 mm from the source melt pocket Incontrast to the melt veins they contain abundant vesicles andare confined to grain boundaries of relatively high and lowdensity minerals Contacts with adjacent minerals are diffuseTo observe stringers or blebs of melt co-mingling withadjacent host rock minerals is common

While interconnected melt pockets do occur in LosAngeles isolated pockets are more prevalent Due to thehighly irregular nature of typical shock veins (especially withincreasing peak shock pressure Stoumlffler et al 1991) thetapering melt veins of Los Angeles possibly connect toseemingly isolated melt pockets out of the 2-dimensionalplane of the thin section However melt veins in Los Angelesare morphologically distinct from typical discordantpseudotachylyte-like shock veins observed in other SNCmeteorites (eg Shergotty Stoumlffler et al 1986) and ordinarychondrites (Stoumlffler et al 1991) that commonly transect thewhole meteorite

In Los Angeles the contact between the melt pockets andadjacent host rock minerals is irregular and commonlytransitional (Fig 1c) Host rock minerals adjacent to the meltpockets have been partially melted or recrystallized The meltpockets and melt veins and dikelets emanating from meltpocket margins commonly crosscut both primary igneoustextures and bulk shock effects

Microscopic areas of alkali- and silica-rich glass (336ndash548 wt K2O 159ndash337 wt Na2O 879ndash975 wt SiO2and 22ndash42 wt Al2O3 respectively) are associated withmaskelynite and silica crystals in the bulk sample They rangein apparent diameter from ~20ndash350 mm and exhibit asymplectic intergrowth texture between the two phasesMicrofaults emanating from melt pocket margins displace theadjacent alkali- and silica-rich glasses (up to 15 mm)However the alkali- and silica-rich glasses at the melt pocketmargins contain entrained schlieren and globules of meltmaterial that are detached from the melt pocket The fluidnature of the material is indicated from the presence ofpyrrhotite spheres and vesicles

Regions of localized melting indicate a minimummelting temperature for nonporous basalts of 1600degC(Stoumlffler 1984) Additionally melting of ilmenite indicatestemperatures in excess of 1800degC (El Goresy et al 1968)Localized pressure and temperature excursions within meltpockets based on experimental data (Keiffer et al 1976Stoumlffler 1984 Schmitt 2000) are estimated at 60ndash80 GPa and1600ndash2000degC respectively Schaal and Houmlrz (1977) observedmelting of pyroxene in experimentally shocked basalts at aminimum pressure limit of 80 GPa

Quench CrystalsOne feature common to all melt pockets in Los Angeles

is the presence of quench crystals The crystals have avariety of distinctive shapes (skeletal dendritic feathery)

and they comprise a variety of minerals (olivinewhitlockite titanomagnetite plagioclase) Quench crystalsare invariably microscopic Severely fractured melt-intruded clinopyroxenes adjacent to the melt pockets allowfor preferential nucleation of dendritic crystals that extendfrom the contact into glass These consist of intimateintergrowths of ferropigeonite (Fs73En18) and orthopyroxene(Fs40En52) with dendritic textures (Fig 2e) In additioncrystal fragments within melt pockets locally derived fromadjacent host minerals serve as nucleation sites for dendriticgrowth (Fig 4) These textures are consistent with rapiddiffusion-controlled crystal growth from the melt(Kirkpatrick 1975)

Two melt pockets in Los Angeles show the symplecticintergrowth fayalite + ferroaugite + silica a late-stage mineralassemblage resulting from the breakdown of pyroxferroite

Fig 4 Backscattered electron images of Los Angeles a) a largetitanomagnetite clast (lower right) within a melt pocket has seededpreferential nucleation of titanomagnetite crystallites with dendriticmorphologies attesting to rapid cooling b) higher magnification ofthe titanomagnetite crystallites are depicted in (a)

1872 E L Walton and J G Spray

(Rubin et al 2000) in direct contact with the melt pocket Atthe melt pocket margin the vermicular to microgranulitictexture typical of the symplectites changes to one of skeletalfayalite (Fa96) embedded in a glassy matrix rich in Fe Mg andCa and Fe-sulfide spherules (Fig 2b) Numerous fractures inthe vicinity of the melt pocket radiate from the contact withthe melt into the fayalite + ferroaugite + silica intergrowthcausing microfaulting

MELT POCKET COMPOSITIONS

The bulk chemistries of four melt pockets in LosAngeles are presented in Table 3 The compositions arevariable from one pocket to another CIPW norm calculationsreveal that the melt pockets can be derived by fusingdifferent mixtures of the host minerals dominated byplagioclase and pyroxene Melt pocket 1 (Type 1) isdominated by normative quartz orthoclase plagioclase andhypersthene melt pocket 2 (Type 2) by quartz orthoclaseplagioclase and hypersthene (Type 3) by mainlyplagioclase diopside and hypersthene melt pocket 4 (Type3) by plagioclase hypersthene and apatite (whitlockite)Melt pocket compositions depend on which bordering hostminerals contribute to melting and the proportion of meltingof each phase (eg some melt pockets occur at the grainboundary of pyroxene and maskelynite thus these are the

main components contributing to the melt) The occurrenceof at least two coexisting silicate melts within one Type 3melt pocket is revealed by crosscutting relationships(Fig 2a) The more siliceous melt of the two (average65 wt SiO2 obtained by raster scans n = 8) is located inproximity to the melt pocket margin This melt contains lithicand crystal fragments (~50) of locally-derived silica andmaskelynite The less siliceous (average 43 wt SiO2

obtained by raster scans n = 6) lower viscosity meltcrosscuts the former tapering off to thin stringers (pound1 mm inapparent thickness) The shape of the overall melt pocket isroughly spherical (Fig 2) ie no textural evidence exists tosuggest that these crosscutting relationships are the result oftwo independently formed melt pockets Although the twosilicate melts were most probably subject to the same coolingrates the abundance of lithic and crystal fragments wouldhave increased the amount of heterogeneous nucleation sitesfavoring more rapid crystallization concomitant with anincrease in viscosity This is also consistent with rapidcrystallization in Type 1 melt pockets containing abundantclasts of host rock minerals (Figs 2f and 2g)

DISCUSSION

Melt pockets within Los Angeles can be divided into 3types based on size clast volume and degree of vesiculation

Table 3 Bulk compositions of shock melt pockets (in wt) for Los Angeles stone 1Melt pocket 1 (Type 1) Melt pocket 2 (Type 2) Melt pocket 3 (Type 3) Melt pocket 4 (Type 3)

wt avg min max avg min max avg min max avg min max 5a

SiO2 589 511 691 627 585 731 457 365 535 328 255 467 491Al2O3 95 120 06 144 119 183 42 17 91 33 15 46 112TiO2 05 03 06 03 02 05 15 04 46 11 05 11 130FeOb 219 128 329 113 6 189 305 197 404 299 234 441 212MnO 05 07 03 03 ndc 05 07 04 12 07 05 13 045MgO 09 05 26 03 06 52 42 16 110 08 06 13 353CaO 37 57 13 58 58 73 96 70 128 159 59 164 995Na2O 18 10 23 24 12 3 02 nd 11 05 01 09 222K2O 21 02 27 13 08 18 03 nd 05 04 05 06 024P2O5 ndash nd 05 ndash nd 01 ndash nd 49 111 89 265 066SO3 ndash nd 08 ndash nd nd nd 04 27 ndash nd 19 ndashTotal 1001 988 996 970 9986nd 32 30 38 24

CIPW normQuartz 144 226 19 0Orthoclase 124 78 18 25Albite 152 206 17 44Anorthite 117 25 98 58Diopside 54 37 249 07Hypersthene 398 199 534 572Olivine 0 0 0 06Ilmenite 1 06 29 22Apatite 02 0 36 272

aData from Rubin et al (2000) obtained by Instrumental Neutron Activation AnalysisbTotal Fe reported as FeOcnd = element oxides below the detection limit of the electron microprobedn = number of raster scans per melt pocket

Shock-induced melt pockets in Los Angeles 1873

Smaller (lt1 mm) pockets (Types 1 and 2) tend to becrystallite-rich while larger (gt1 mm) pockets (Type 3) tend tobe clast-poor and exhibit flow-structured glasses Opticallyshock effects are manifest as dark brown to black glassy andor microcrystalline enclaves of melt material melt veins anddeposits of sulfides and melt injected into fractures withinsilicates In total these localized regions of melting compriselt1 vol of Los Angeles and they are heterogeneouslydistributed throughout the sample

Melt pockets invariably develop at the grain boundariesof minerals with contrasting shock impedance (shock wavevelocity multiplied by the mineralrsquos density) eg plagioclaseand pyroxene This is most obvious in smaller (pound1 mm) meltpockets (Type 1 and 2) where the original grain positions andboundaries are preserved With increasing volume of melt theoriginal grains and their boundaries become obscured ordestroyed Typically minerals adjacent to the melt pocketshave smooth arcuate margins at the contact with the meltThe composition of the melt pockets corresponds to localizedmelting of bordering host rock minerals Melt veins have beenobserved to form an interlocking network of complexanastomosing branches surrounding unmolten host rockconstituents

The variable compositions of the studied melt pocketscan be attributed to the relatively coarse grain size (2ndash4 mm)of the host rock minerals compared to the diameters of themelt pockets (lt4 mm) The melt pockets are not largeenough to represent a bulk rock melt of Los AngelesInstead they represent localized mineral-scale meltingproducts Melt pocket compositions presented in Table 3differ markedly when comparing those of the smaller meltpockets Types 1 and 2 to the larger Type 3 melt pocketsTypes 1 and 2 are enriched in SiO2 Al2O3 and alkalis ascompared to Type 3 The CIPW norm calculations alsolisted in Table 3 indicate that the precursors of Types 1 and2 melt pockets were mainly quartz + alkali feldspar +plagioclase + pyroxene while those for Type 3 were mainlyplagioclase + pyroxene + whitlockite Therefore theprecursors of Types 1 and 2 melt pockets likely had a largecontribution from late-stage minerals (mesostasis) while thebulk of Type 3 are derived from the main host rock lithologyThis is not surprising considering that shock melting beginsat grain boundary margins where mesostasis is moreabundant Thus smaller (Type 1 and 2) melt pockets willposses a greater contribution of mesostasis to bulkcomposition than larger (Type 3) melt pockets that havemelted approximately the same amount of mesostasis butgreater proportions of host rock phases

Based on compositional and textural lines of evidencewe conclude that the melt pockets formed by the local in situmelting of host rock minerals as opposed to forcefulinjection of extraneous impact melt along fractures in thehost rock Remobilization and injection of opaques and meltfrom the melt region into pre-existing fractures in adjacent

host rock pyroxene could have occurred during compressionand subsequent stress relaxation The contact between themelt pockets and adjacent host minerals is intrusive inplaces The intensity of shock features is observed toincrease with proximity to the melt pockets indicating alocalized increase in shock pressure This pressure gradientfrom melt pocket (60ndash80 GPa) to bulk rock (35ndash40 GPa) iscertainly sufficient to account for forceful injection of Fe-sulfides and melt from the melt pockets into adjacent hostminerals Additionally intrusions could have formed bypressure generated by a volume increase (~10) associatedwith the melting event

The relationship between areas of alkali- and silica-richglass and the melt pockets is puzzling However cominglingand entrapment of liquid blebs from the melt pocket ispossible if the two phases were initially molten at the sametime This could be explained if the alkali- and silica-richglass was formed during the shock event but was quenchedbefore the shock pressure was completely released Melt fromthe melt pocket could then have remained in a liquid stateafter pressure release This is consistent with the presence ofabundant vesicles in the melt pocket glasses indicating lowconfining pressure Stopler and McSween (1979) observedsimilar textural associations in Shergotty and Zagamiconcluding that they may represent shock mixtures of alkali-feldspars and silica

The presence of clasts in one melt pocket with quenchtextures as well as the presence of crosscutting silicate meltsdoes not require multiple impact events That a single impactevent is capable of producing several shock metamorphicfeatures in a target rock is well-documented (eg Stoumlffler1984 Stoumlffler et al 1991) The high concentration ofrelatively cold lithic and crystal fragments as well as theproximity to the melt pocket margin may have facilitatedrapid crystallization of one melt with respect to the otherRapid crystallization could also cause a drastic increase in theviscosity of one melt with respect to the other

The development of abundant crystallites and glass inthe melt pockets attests to rapid cooling of the melt whichimplies that the studied Los Angeles samples were derivedfrom the chilled margins of a larger ejected body (severalmeters to tens of meters in diameter) or were never muchlarger than the recovered meteorite sizes (tens of cm indiameter) The high abundance of relatively cold clasts inthe smaller melt pockets (Type 1) probably contributed tothe rapid crystallization (generating abundant crystallites)and an absence of flow textures due to the presence ofcopious heterogeneous nucleation sites Vesiculationprobably occurred during shock decompression withvesicles persisting in the melt under lower pressureconditions (Kenkmann et al 2000) However this does notpreclude the development of high pressure high temperaturepolymorphs within the melt pockets produced by cavitationand bubble implosion (Spray 1999) Melt veins and melt

1874 E L Walton and J G Spray

dikelets occur in direct contact with the melt pockets and areconsidered morphologically distinct from typical ldquoshockveinsrdquo that transect meteorites and that form by acombination of shock and frictional melting The concentriczoning observed in the margins of clinopyroxenes in contactwith melt pockets coincides with a progressive reduction inbirefringence which suggests that shock excursions werefocused as point sources and that these were generated in thewake of the planar shock front that induced bulk shockeffects We infer that the larger melt pockets (Type 3) wereformed by higher energy excursions relative to the smallerpockets (Type 1 and 2)

Based on experimental data by Kieffer et al (1976)Schaal and Houmlrz (1977) and Schmitt (2000) shock pressureand temperature excursions of 60ndash80 GPa and 1600ndash 2000degCare estimated for Los Angeles 7058 Because pyroxene is notobserved as clasts in any of the melt pockets in Los Angelesthe higher pressure of ~80 GPa is favored coincident with theobserved melting of pyroxene in experimentally shockedbasalts (Schaal and Houmlrz 1977) These discrete excursionsare therefore approximately twice the magnitude of the bulkshock (35ndash40 GPa)

A young formation age 170 plusmn 8 Ma for the Los Angelesmeteorite (Nyquist et al 2001) precludes it recording multipleimpact events that would be more typical of target rocks fromthe older impact-scarred Noachian terrains of the southernhemisphere Instead Los Angeles records evidence of asingle shock event which favors its launching immediatelyfollowing hypervelocity impact into a relatively young (lateAmazonian) volcanic source terrain such as is found in theTharsis and Elysium regions

AcknowledgmentsndashThis work has been funded by the NaturalSciences and Engineering Research Council of Canada(NSERC) through research grants awarded to J G Spray andan NSERC PGS-b scholarship awarded to E L Walton Wethank Tim McCoy at the Smithsonian Institution WashingtonDC for loan of the samples Douglas Hall at UNB andMichael Coy at JEOL USA for EM support Cliff Shaw forhelpful discussions and Angel Gomez for draftingConstructive Reviews by Y Ikeda and R F Schmitt aregratefully acknowledged Planetary and Space Science CentreContribution 31

Editorial HandlingmdashDr Edward Scott

REFERENCES

Chen M and El Goresy A 2000 The nature of maskelynite inshocked meteorites Not a diaplectic glass but a glass quenchedfrom shock-induced dense melt at high pressures Earth andPlanetary Science Letters 179489ndash502

El Goresy A Fechtig H and Ottemann J 1968 The opaqueminerals in impact glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMono Book pp 531ndash553

Garrison D H and Bogard D D 2000 Cosmogenic and trappednoble gases in the Los Angeles meteorite (abstract) Meteoriticsamp Planetary Science 35A58

Houmlrz F and Quaide W L 1973 Debye-Scherrer investigations ofexperimentally shocked silicates The Moon 645ndash82

Kenkmann T Hornemann U and Stoumlffler D 2000 Experimentalgeneration of shock-induced pseudotachylytes alonglithological interfaces Meteoritics amp Planetary Science 351275ndash1290

Kieffer S W Schaal R B Gibbons R Houmlrz F Milton D J andDube A 1976 Shocked basalt from Lonar crater India andexperimental analogues Proceedings 7th Lunar and PlanetaryScience Conference pp 1391ndash1412

Kirkpatrick R J 1975 Crystal growth from the melt A reviewAmerican Mineralogist 60798ndash814

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McSween H Y Jr 2002 The rocks of Mars from far and nearMeteoritics amp Planetary Science 377ndash25

Melosh H J 1984 Impact ejection spallation and the origin ofmeteorites Icarus 59234ndash260

Melosh H J 1985 Ejection of rock fragments from planetary bodiesGeology 13144ndash148

Nishiizumi K Caffee M W and Masarik J 2000 Cosmogenicradiocunclides in Los Angeles martian meteorite (abstract)Meteoritics amp Planetary Science 35A120

Nyquist L E Reese Y D Wiesmann H Shih C Y and SchwandtC 2000 Rubidium-strontium age of the Los Angeles shergottiteMeteoritics amp Planetary Science 35A121ndashA122

Nyquist L E Bogard D D Shih C Y Greshake A Stoumlffler D andEugster O 2001 Ages and histories of martian meteoritesChronology and Evolution of Mars 96105ndash164

Rubin A E Warren P H Greenwood J P Verish R S Leshin LA Hervig R L and Clayton R N 2000 Los Angeles The mostdifferentiated basaltic martian meteorites Geology 281011ndash1014

Schaal R B and Houmlrz F 1977 Shock metamorphism of lunar andterrestrial basalts Proceedings 8th Lunar and Planetary ScienceConference pp 1679ndash1729

Schmitt R T 2000 Shock experiments with the H6 chondriteKernouveacute Pressure calibration of microscopic shock effectsMeteoritics amp Planetary Science 3545ndash560

Spray J G 1999 Shocking rocks by cavitation and bubble implosionGeology 27695ndash698

Spray J G and Rae D A 1995 Quantitative electron-microprobeanalysis of alkali silicate glasses A review and user guideCanadian Mineralogist 33323ndash332

Stoumlffler D 1984 Glasses formed by hypervelocity impact Journal ofNon-Crystalline Solids 67465ndash502

Stoumlffler D 2000 Maskelynite confirmed as diaplectic glassIndications for peak shock pressure below 45 GPa in all martianmeteorites (abstract 1170) 31st Lunar and Planetary ScienceConference CD-ROM

Stoumlffler D Ostertag R Jammes C and Pfannschmidt G 1986Shock metamorphism and petrography of the Shergottyachondrite Geochimica et Cosmochimica Acta 50889ndash903

Stoumlffler D Keil K and Scott E R D 1991 Shock metamorphismof ordinary chondrites Geochimica et Cosmochimica Acta 53845ndash3867

Stopler E and McSween H Y Jr 1979 Petrology and origin of theshergottite meteorites Geochimica et Cosmochimica Acta 431475ndash1498

Terribilini D Busemann H and Eugster O 2000 Krypton-81-

1870 E L Walton and J G Spray

highly variable Contact relationships of individualschlieren with the surrounding glass range from sharp tohighly diffuse The schlieren show compositionalvariability attributed to melting of different proportionsof minerals adjacent to the melt pockets (Tables 2 and 3)The proportion of locally derived lithic and crystalfragments increases toward the melt pocket marginThese consist of titanomagnetite maskelynitewhitlockite and alkali- and silica-rich glasses Pyroxenehas never been observed as clasts entrained in any of thelarge melt pocketsTwo distinct types of remobilized melt material emanate

from the melt pockets (Fig 1) They include thin (005ndash10 mmapparent thickness) non-vesicular opaque andor glassyveins (referred to as melt veins) and thicker (50ndash210 mm

apparent thickness) vesicular sinuous apophyses of meltmaterial (referred to as melt dikelets) Melt veins comprisesulfide- and melt-filled fractures and microfaults with smalllateral offsets (up to 80 mm) They possess discordant contactswith the host minerals commonly form an interlockingnetwork of anastomosing branches surrounding unmoltenhost rock constituents and crosscut host rock minerals Meltveins have not been observed to penetrate more than 550 mmfrom their source melt pockets (ie they do not completelytransect the sample) A high concentration of melt veinsinduces a distinct darkening (opacity) of pyroxene grains inoptical microscopy (referred to as ldquoshock blackeningrdquo inchondrites Stoumlffler et al 1991)

Melt dikelets emanate from the main melt pocket andcommonly enlarge to form subsidiary melt pockets However

Fig 3 Backscattered electron images of melt pocket Type 2 in Los Angeles a) low magnification overview of melt pocket (center of photo)Host rock minerals adjacent the melt pocket are maskelynite (mk) clinopyroxene (cpx) an intergrowth of K-rich feldspathic glass and silica-rich glass (K Si) and pyroxferroite breakdown material (pbm) Carbonates (crb) partially infill an E-W-trending fracture that crosscuts bothhost rock minerals and the melt pocket b) higher resolution image of the interior of the melt pocket Silica (with minor amounts ofaluminium) forms subidioblastic to idioblastic crystals with roughly cubic-shaped cross sections The vesicles (v) are irregularly-shaped c)higher resolution cross-sections through the crystals depicted in (b) show unusual morphologies like primate metatarsals The interior of thecrystals are compositionally similar to the groundmass and are enriched in Fe Mg and Ca d) contact between the melt pocket (top) andmaskelynite (base of photo) is gradational fining toward the margin Maskelynite has recrystallized in a ~10 mm-wide rim in direct contactwith the melt pocket

Shock-induced melt pockets in Los Angeles 1871

some are observed to be wedge-shaped wherein they taperoff typically within 3 mm from the source melt pocket Incontrast to the melt veins they contain abundant vesicles andare confined to grain boundaries of relatively high and lowdensity minerals Contacts with adjacent minerals are diffuseTo observe stringers or blebs of melt co-mingling withadjacent host rock minerals is common

While interconnected melt pockets do occur in LosAngeles isolated pockets are more prevalent Due to thehighly irregular nature of typical shock veins (especially withincreasing peak shock pressure Stoumlffler et al 1991) thetapering melt veins of Los Angeles possibly connect toseemingly isolated melt pockets out of the 2-dimensionalplane of the thin section However melt veins in Los Angelesare morphologically distinct from typical discordantpseudotachylyte-like shock veins observed in other SNCmeteorites (eg Shergotty Stoumlffler et al 1986) and ordinarychondrites (Stoumlffler et al 1991) that commonly transect thewhole meteorite

In Los Angeles the contact between the melt pockets andadjacent host rock minerals is irregular and commonlytransitional (Fig 1c) Host rock minerals adjacent to the meltpockets have been partially melted or recrystallized The meltpockets and melt veins and dikelets emanating from meltpocket margins commonly crosscut both primary igneoustextures and bulk shock effects

Microscopic areas of alkali- and silica-rich glass (336ndash548 wt K2O 159ndash337 wt Na2O 879ndash975 wt SiO2and 22ndash42 wt Al2O3 respectively) are associated withmaskelynite and silica crystals in the bulk sample They rangein apparent diameter from ~20ndash350 mm and exhibit asymplectic intergrowth texture between the two phasesMicrofaults emanating from melt pocket margins displace theadjacent alkali- and silica-rich glasses (up to 15 mm)However the alkali- and silica-rich glasses at the melt pocketmargins contain entrained schlieren and globules of meltmaterial that are detached from the melt pocket The fluidnature of the material is indicated from the presence ofpyrrhotite spheres and vesicles

Regions of localized melting indicate a minimummelting temperature for nonporous basalts of 1600degC(Stoumlffler 1984) Additionally melting of ilmenite indicatestemperatures in excess of 1800degC (El Goresy et al 1968)Localized pressure and temperature excursions within meltpockets based on experimental data (Keiffer et al 1976Stoumlffler 1984 Schmitt 2000) are estimated at 60ndash80 GPa and1600ndash2000degC respectively Schaal and Houmlrz (1977) observedmelting of pyroxene in experimentally shocked basalts at aminimum pressure limit of 80 GPa

Quench CrystalsOne feature common to all melt pockets in Los Angeles

is the presence of quench crystals The crystals have avariety of distinctive shapes (skeletal dendritic feathery)

and they comprise a variety of minerals (olivinewhitlockite titanomagnetite plagioclase) Quench crystalsare invariably microscopic Severely fractured melt-intruded clinopyroxenes adjacent to the melt pockets allowfor preferential nucleation of dendritic crystals that extendfrom the contact into glass These consist of intimateintergrowths of ferropigeonite (Fs73En18) and orthopyroxene(Fs40En52) with dendritic textures (Fig 2e) In additioncrystal fragments within melt pockets locally derived fromadjacent host minerals serve as nucleation sites for dendriticgrowth (Fig 4) These textures are consistent with rapiddiffusion-controlled crystal growth from the melt(Kirkpatrick 1975)

Two melt pockets in Los Angeles show the symplecticintergrowth fayalite + ferroaugite + silica a late-stage mineralassemblage resulting from the breakdown of pyroxferroite

Fig 4 Backscattered electron images of Los Angeles a) a largetitanomagnetite clast (lower right) within a melt pocket has seededpreferential nucleation of titanomagnetite crystallites with dendriticmorphologies attesting to rapid cooling b) higher magnification ofthe titanomagnetite crystallites are depicted in (a)

1872 E L Walton and J G Spray

(Rubin et al 2000) in direct contact with the melt pocket Atthe melt pocket margin the vermicular to microgranulitictexture typical of the symplectites changes to one of skeletalfayalite (Fa96) embedded in a glassy matrix rich in Fe Mg andCa and Fe-sulfide spherules (Fig 2b) Numerous fractures inthe vicinity of the melt pocket radiate from the contact withthe melt into the fayalite + ferroaugite + silica intergrowthcausing microfaulting

MELT POCKET COMPOSITIONS

The bulk chemistries of four melt pockets in LosAngeles are presented in Table 3 The compositions arevariable from one pocket to another CIPW norm calculationsreveal that the melt pockets can be derived by fusingdifferent mixtures of the host minerals dominated byplagioclase and pyroxene Melt pocket 1 (Type 1) isdominated by normative quartz orthoclase plagioclase andhypersthene melt pocket 2 (Type 2) by quartz orthoclaseplagioclase and hypersthene (Type 3) by mainlyplagioclase diopside and hypersthene melt pocket 4 (Type3) by plagioclase hypersthene and apatite (whitlockite)Melt pocket compositions depend on which bordering hostminerals contribute to melting and the proportion of meltingof each phase (eg some melt pockets occur at the grainboundary of pyroxene and maskelynite thus these are the

main components contributing to the melt) The occurrenceof at least two coexisting silicate melts within one Type 3melt pocket is revealed by crosscutting relationships(Fig 2a) The more siliceous melt of the two (average65 wt SiO2 obtained by raster scans n = 8) is located inproximity to the melt pocket margin This melt contains lithicand crystal fragments (~50) of locally-derived silica andmaskelynite The less siliceous (average 43 wt SiO2

obtained by raster scans n = 6) lower viscosity meltcrosscuts the former tapering off to thin stringers (pound1 mm inapparent thickness) The shape of the overall melt pocket isroughly spherical (Fig 2) ie no textural evidence exists tosuggest that these crosscutting relationships are the result oftwo independently formed melt pockets Although the twosilicate melts were most probably subject to the same coolingrates the abundance of lithic and crystal fragments wouldhave increased the amount of heterogeneous nucleation sitesfavoring more rapid crystallization concomitant with anincrease in viscosity This is also consistent with rapidcrystallization in Type 1 melt pockets containing abundantclasts of host rock minerals (Figs 2f and 2g)

DISCUSSION

Melt pockets within Los Angeles can be divided into 3types based on size clast volume and degree of vesiculation

Table 3 Bulk compositions of shock melt pockets (in wt) for Los Angeles stone 1Melt pocket 1 (Type 1) Melt pocket 2 (Type 2) Melt pocket 3 (Type 3) Melt pocket 4 (Type 3)

wt avg min max avg min max avg min max avg min max 5a

SiO2 589 511 691 627 585 731 457 365 535 328 255 467 491Al2O3 95 120 06 144 119 183 42 17 91 33 15 46 112TiO2 05 03 06 03 02 05 15 04 46 11 05 11 130FeOb 219 128 329 113 6 189 305 197 404 299 234 441 212MnO 05 07 03 03 ndc 05 07 04 12 07 05 13 045MgO 09 05 26 03 06 52 42 16 110 08 06 13 353CaO 37 57 13 58 58 73 96 70 128 159 59 164 995Na2O 18 10 23 24 12 3 02 nd 11 05 01 09 222K2O 21 02 27 13 08 18 03 nd 05 04 05 06 024P2O5 ndash nd 05 ndash nd 01 ndash nd 49 111 89 265 066SO3 ndash nd 08 ndash nd nd nd 04 27 ndash nd 19 ndashTotal 1001 988 996 970 9986nd 32 30 38 24

CIPW normQuartz 144 226 19 0Orthoclase 124 78 18 25Albite 152 206 17 44Anorthite 117 25 98 58Diopside 54 37 249 07Hypersthene 398 199 534 572Olivine 0 0 0 06Ilmenite 1 06 29 22Apatite 02 0 36 272

aData from Rubin et al (2000) obtained by Instrumental Neutron Activation AnalysisbTotal Fe reported as FeOcnd = element oxides below the detection limit of the electron microprobedn = number of raster scans per melt pocket

Shock-induced melt pockets in Los Angeles 1873

Smaller (lt1 mm) pockets (Types 1 and 2) tend to becrystallite-rich while larger (gt1 mm) pockets (Type 3) tend tobe clast-poor and exhibit flow-structured glasses Opticallyshock effects are manifest as dark brown to black glassy andor microcrystalline enclaves of melt material melt veins anddeposits of sulfides and melt injected into fractures withinsilicates In total these localized regions of melting compriselt1 vol of Los Angeles and they are heterogeneouslydistributed throughout the sample

Melt pockets invariably develop at the grain boundariesof minerals with contrasting shock impedance (shock wavevelocity multiplied by the mineralrsquos density) eg plagioclaseand pyroxene This is most obvious in smaller (pound1 mm) meltpockets (Type 1 and 2) where the original grain positions andboundaries are preserved With increasing volume of melt theoriginal grains and their boundaries become obscured ordestroyed Typically minerals adjacent to the melt pocketshave smooth arcuate margins at the contact with the meltThe composition of the melt pockets corresponds to localizedmelting of bordering host rock minerals Melt veins have beenobserved to form an interlocking network of complexanastomosing branches surrounding unmolten host rockconstituents

The variable compositions of the studied melt pocketscan be attributed to the relatively coarse grain size (2ndash4 mm)of the host rock minerals compared to the diameters of themelt pockets (lt4 mm) The melt pockets are not largeenough to represent a bulk rock melt of Los AngelesInstead they represent localized mineral-scale meltingproducts Melt pocket compositions presented in Table 3differ markedly when comparing those of the smaller meltpockets Types 1 and 2 to the larger Type 3 melt pocketsTypes 1 and 2 are enriched in SiO2 Al2O3 and alkalis ascompared to Type 3 The CIPW norm calculations alsolisted in Table 3 indicate that the precursors of Types 1 and2 melt pockets were mainly quartz + alkali feldspar +plagioclase + pyroxene while those for Type 3 were mainlyplagioclase + pyroxene + whitlockite Therefore theprecursors of Types 1 and 2 melt pockets likely had a largecontribution from late-stage minerals (mesostasis) while thebulk of Type 3 are derived from the main host rock lithologyThis is not surprising considering that shock melting beginsat grain boundary margins where mesostasis is moreabundant Thus smaller (Type 1 and 2) melt pockets willposses a greater contribution of mesostasis to bulkcomposition than larger (Type 3) melt pockets that havemelted approximately the same amount of mesostasis butgreater proportions of host rock phases

Based on compositional and textural lines of evidencewe conclude that the melt pockets formed by the local in situmelting of host rock minerals as opposed to forcefulinjection of extraneous impact melt along fractures in thehost rock Remobilization and injection of opaques and meltfrom the melt region into pre-existing fractures in adjacent

host rock pyroxene could have occurred during compressionand subsequent stress relaxation The contact between themelt pockets and adjacent host minerals is intrusive inplaces The intensity of shock features is observed toincrease with proximity to the melt pockets indicating alocalized increase in shock pressure This pressure gradientfrom melt pocket (60ndash80 GPa) to bulk rock (35ndash40 GPa) iscertainly sufficient to account for forceful injection of Fe-sulfides and melt from the melt pockets into adjacent hostminerals Additionally intrusions could have formed bypressure generated by a volume increase (~10) associatedwith the melting event

The relationship between areas of alkali- and silica-richglass and the melt pockets is puzzling However cominglingand entrapment of liquid blebs from the melt pocket ispossible if the two phases were initially molten at the sametime This could be explained if the alkali- and silica-richglass was formed during the shock event but was quenchedbefore the shock pressure was completely released Melt fromthe melt pocket could then have remained in a liquid stateafter pressure release This is consistent with the presence ofabundant vesicles in the melt pocket glasses indicating lowconfining pressure Stopler and McSween (1979) observedsimilar textural associations in Shergotty and Zagamiconcluding that they may represent shock mixtures of alkali-feldspars and silica

The presence of clasts in one melt pocket with quenchtextures as well as the presence of crosscutting silicate meltsdoes not require multiple impact events That a single impactevent is capable of producing several shock metamorphicfeatures in a target rock is well-documented (eg Stoumlffler1984 Stoumlffler et al 1991) The high concentration ofrelatively cold lithic and crystal fragments as well as theproximity to the melt pocket margin may have facilitatedrapid crystallization of one melt with respect to the otherRapid crystallization could also cause a drastic increase in theviscosity of one melt with respect to the other

The development of abundant crystallites and glass inthe melt pockets attests to rapid cooling of the melt whichimplies that the studied Los Angeles samples were derivedfrom the chilled margins of a larger ejected body (severalmeters to tens of meters in diameter) or were never muchlarger than the recovered meteorite sizes (tens of cm indiameter) The high abundance of relatively cold clasts inthe smaller melt pockets (Type 1) probably contributed tothe rapid crystallization (generating abundant crystallites)and an absence of flow textures due to the presence ofcopious heterogeneous nucleation sites Vesiculationprobably occurred during shock decompression withvesicles persisting in the melt under lower pressureconditions (Kenkmann et al 2000) However this does notpreclude the development of high pressure high temperaturepolymorphs within the melt pockets produced by cavitationand bubble implosion (Spray 1999) Melt veins and melt

1874 E L Walton and J G Spray

dikelets occur in direct contact with the melt pockets and areconsidered morphologically distinct from typical ldquoshockveinsrdquo that transect meteorites and that form by acombination of shock and frictional melting The concentriczoning observed in the margins of clinopyroxenes in contactwith melt pockets coincides with a progressive reduction inbirefringence which suggests that shock excursions werefocused as point sources and that these were generated in thewake of the planar shock front that induced bulk shockeffects We infer that the larger melt pockets (Type 3) wereformed by higher energy excursions relative to the smallerpockets (Type 1 and 2)

Based on experimental data by Kieffer et al (1976)Schaal and Houmlrz (1977) and Schmitt (2000) shock pressureand temperature excursions of 60ndash80 GPa and 1600ndash 2000degCare estimated for Los Angeles 7058 Because pyroxene is notobserved as clasts in any of the melt pockets in Los Angelesthe higher pressure of ~80 GPa is favored coincident with theobserved melting of pyroxene in experimentally shockedbasalts (Schaal and Houmlrz 1977) These discrete excursionsare therefore approximately twice the magnitude of the bulkshock (35ndash40 GPa)

A young formation age 170 plusmn 8 Ma for the Los Angelesmeteorite (Nyquist et al 2001) precludes it recording multipleimpact events that would be more typical of target rocks fromthe older impact-scarred Noachian terrains of the southernhemisphere Instead Los Angeles records evidence of asingle shock event which favors its launching immediatelyfollowing hypervelocity impact into a relatively young (lateAmazonian) volcanic source terrain such as is found in theTharsis and Elysium regions

AcknowledgmentsndashThis work has been funded by the NaturalSciences and Engineering Research Council of Canada(NSERC) through research grants awarded to J G Spray andan NSERC PGS-b scholarship awarded to E L Walton Wethank Tim McCoy at the Smithsonian Institution WashingtonDC for loan of the samples Douglas Hall at UNB andMichael Coy at JEOL USA for EM support Cliff Shaw forhelpful discussions and Angel Gomez for draftingConstructive Reviews by Y Ikeda and R F Schmitt aregratefully acknowledged Planetary and Space Science CentreContribution 31

Editorial HandlingmdashDr Edward Scott

REFERENCES

Chen M and El Goresy A 2000 The nature of maskelynite inshocked meteorites Not a diaplectic glass but a glass quenchedfrom shock-induced dense melt at high pressures Earth andPlanetary Science Letters 179489ndash502

El Goresy A Fechtig H and Ottemann J 1968 The opaqueminerals in impact glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMono Book pp 531ndash553

Garrison D H and Bogard D D 2000 Cosmogenic and trappednoble gases in the Los Angeles meteorite (abstract) Meteoriticsamp Planetary Science 35A58

Houmlrz F and Quaide W L 1973 Debye-Scherrer investigations ofexperimentally shocked silicates The Moon 645ndash82

Kenkmann T Hornemann U and Stoumlffler D 2000 Experimentalgeneration of shock-induced pseudotachylytes alonglithological interfaces Meteoritics amp Planetary Science 351275ndash1290

Kieffer S W Schaal R B Gibbons R Houmlrz F Milton D J andDube A 1976 Shocked basalt from Lonar crater India andexperimental analogues Proceedings 7th Lunar and PlanetaryScience Conference pp 1391ndash1412

Kirkpatrick R J 1975 Crystal growth from the melt A reviewAmerican Mineralogist 60798ndash814

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McSween H Y Jr 2002 The rocks of Mars from far and nearMeteoritics amp Planetary Science 377ndash25

Melosh H J 1984 Impact ejection spallation and the origin ofmeteorites Icarus 59234ndash260

Melosh H J 1985 Ejection of rock fragments from planetary bodiesGeology 13144ndash148

Nishiizumi K Caffee M W and Masarik J 2000 Cosmogenicradiocunclides in Los Angeles martian meteorite (abstract)Meteoritics amp Planetary Science 35A120

Nyquist L E Reese Y D Wiesmann H Shih C Y and SchwandtC 2000 Rubidium-strontium age of the Los Angeles shergottiteMeteoritics amp Planetary Science 35A121ndashA122

Nyquist L E Bogard D D Shih C Y Greshake A Stoumlffler D andEugster O 2001 Ages and histories of martian meteoritesChronology and Evolution of Mars 96105ndash164

Rubin A E Warren P H Greenwood J P Verish R S Leshin LA Hervig R L and Clayton R N 2000 Los Angeles The mostdifferentiated basaltic martian meteorites Geology 281011ndash1014

Schaal R B and Houmlrz F 1977 Shock metamorphism of lunar andterrestrial basalts Proceedings 8th Lunar and Planetary ScienceConference pp 1679ndash1729

Schmitt R T 2000 Shock experiments with the H6 chondriteKernouveacute Pressure calibration of microscopic shock effectsMeteoritics amp Planetary Science 3545ndash560

Spray J G 1999 Shocking rocks by cavitation and bubble implosionGeology 27695ndash698

Spray J G and Rae D A 1995 Quantitative electron-microprobeanalysis of alkali silicate glasses A review and user guideCanadian Mineralogist 33323ndash332

Stoumlffler D 1984 Glasses formed by hypervelocity impact Journal ofNon-Crystalline Solids 67465ndash502

Stoumlffler D 2000 Maskelynite confirmed as diaplectic glassIndications for peak shock pressure below 45 GPa in all martianmeteorites (abstract 1170) 31st Lunar and Planetary ScienceConference CD-ROM

Stoumlffler D Ostertag R Jammes C and Pfannschmidt G 1986Shock metamorphism and petrography of the Shergottyachondrite Geochimica et Cosmochimica Acta 50889ndash903

Stoumlffler D Keil K and Scott E R D 1991 Shock metamorphismof ordinary chondrites Geochimica et Cosmochimica Acta 53845ndash3867

Stopler E and McSween H Y Jr 1979 Petrology and origin of theshergottite meteorites Geochimica et Cosmochimica Acta 431475ndash1498

Terribilini D Busemann H and Eugster O 2000 Krypton-81-

Shock-induced melt pockets in Los Angeles 1871

some are observed to be wedge-shaped wherein they taperoff typically within 3 mm from the source melt pocket Incontrast to the melt veins they contain abundant vesicles andare confined to grain boundaries of relatively high and lowdensity minerals Contacts with adjacent minerals are diffuseTo observe stringers or blebs of melt co-mingling withadjacent host rock minerals is common

While interconnected melt pockets do occur in LosAngeles isolated pockets are more prevalent Due to thehighly irregular nature of typical shock veins (especially withincreasing peak shock pressure Stoumlffler et al 1991) thetapering melt veins of Los Angeles possibly connect toseemingly isolated melt pockets out of the 2-dimensionalplane of the thin section However melt veins in Los Angelesare morphologically distinct from typical discordantpseudotachylyte-like shock veins observed in other SNCmeteorites (eg Shergotty Stoumlffler et al 1986) and ordinarychondrites (Stoumlffler et al 1991) that commonly transect thewhole meteorite

In Los Angeles the contact between the melt pockets andadjacent host rock minerals is irregular and commonlytransitional (Fig 1c) Host rock minerals adjacent to the meltpockets have been partially melted or recrystallized The meltpockets and melt veins and dikelets emanating from meltpocket margins commonly crosscut both primary igneoustextures and bulk shock effects

Microscopic areas of alkali- and silica-rich glass (336ndash548 wt K2O 159ndash337 wt Na2O 879ndash975 wt SiO2and 22ndash42 wt Al2O3 respectively) are associated withmaskelynite and silica crystals in the bulk sample They rangein apparent diameter from ~20ndash350 mm and exhibit asymplectic intergrowth texture between the two phasesMicrofaults emanating from melt pocket margins displace theadjacent alkali- and silica-rich glasses (up to 15 mm)However the alkali- and silica-rich glasses at the melt pocketmargins contain entrained schlieren and globules of meltmaterial that are detached from the melt pocket The fluidnature of the material is indicated from the presence ofpyrrhotite spheres and vesicles

Regions of localized melting indicate a minimummelting temperature for nonporous basalts of 1600degC(Stoumlffler 1984) Additionally melting of ilmenite indicatestemperatures in excess of 1800degC (El Goresy et al 1968)Localized pressure and temperature excursions within meltpockets based on experimental data (Keiffer et al 1976Stoumlffler 1984 Schmitt 2000) are estimated at 60ndash80 GPa and1600ndash2000degC respectively Schaal and Houmlrz (1977) observedmelting of pyroxene in experimentally shocked basalts at aminimum pressure limit of 80 GPa

Quench CrystalsOne feature common to all melt pockets in Los Angeles

is the presence of quench crystals The crystals have avariety of distinctive shapes (skeletal dendritic feathery)

and they comprise a variety of minerals (olivinewhitlockite titanomagnetite plagioclase) Quench crystalsare invariably microscopic Severely fractured melt-intruded clinopyroxenes adjacent to the melt pockets allowfor preferential nucleation of dendritic crystals that extendfrom the contact into glass These consist of intimateintergrowths of ferropigeonite (Fs73En18) and orthopyroxene(Fs40En52) with dendritic textures (Fig 2e) In additioncrystal fragments within melt pockets locally derived fromadjacent host minerals serve as nucleation sites for dendriticgrowth (Fig 4) These textures are consistent with rapiddiffusion-controlled crystal growth from the melt(Kirkpatrick 1975)

Two melt pockets in Los Angeles show the symplecticintergrowth fayalite + ferroaugite + silica a late-stage mineralassemblage resulting from the breakdown of pyroxferroite

Fig 4 Backscattered electron images of Los Angeles a) a largetitanomagnetite clast (lower right) within a melt pocket has seededpreferential nucleation of titanomagnetite crystallites with dendriticmorphologies attesting to rapid cooling b) higher magnification ofthe titanomagnetite crystallites are depicted in (a)

1872 E L Walton and J G Spray

(Rubin et al 2000) in direct contact with the melt pocket Atthe melt pocket margin the vermicular to microgranulitictexture typical of the symplectites changes to one of skeletalfayalite (Fa96) embedded in a glassy matrix rich in Fe Mg andCa and Fe-sulfide spherules (Fig 2b) Numerous fractures inthe vicinity of the melt pocket radiate from the contact withthe melt into the fayalite + ferroaugite + silica intergrowthcausing microfaulting

MELT POCKET COMPOSITIONS

The bulk chemistries of four melt pockets in LosAngeles are presented in Table 3 The compositions arevariable from one pocket to another CIPW norm calculationsreveal that the melt pockets can be derived by fusingdifferent mixtures of the host minerals dominated byplagioclase and pyroxene Melt pocket 1 (Type 1) isdominated by normative quartz orthoclase plagioclase andhypersthene melt pocket 2 (Type 2) by quartz orthoclaseplagioclase and hypersthene (Type 3) by mainlyplagioclase diopside and hypersthene melt pocket 4 (Type3) by plagioclase hypersthene and apatite (whitlockite)Melt pocket compositions depend on which bordering hostminerals contribute to melting and the proportion of meltingof each phase (eg some melt pockets occur at the grainboundary of pyroxene and maskelynite thus these are the

main components contributing to the melt) The occurrenceof at least two coexisting silicate melts within one Type 3melt pocket is revealed by crosscutting relationships(Fig 2a) The more siliceous melt of the two (average65 wt SiO2 obtained by raster scans n = 8) is located inproximity to the melt pocket margin This melt contains lithicand crystal fragments (~50) of locally-derived silica andmaskelynite The less siliceous (average 43 wt SiO2

obtained by raster scans n = 6) lower viscosity meltcrosscuts the former tapering off to thin stringers (pound1 mm inapparent thickness) The shape of the overall melt pocket isroughly spherical (Fig 2) ie no textural evidence exists tosuggest that these crosscutting relationships are the result oftwo independently formed melt pockets Although the twosilicate melts were most probably subject to the same coolingrates the abundance of lithic and crystal fragments wouldhave increased the amount of heterogeneous nucleation sitesfavoring more rapid crystallization concomitant with anincrease in viscosity This is also consistent with rapidcrystallization in Type 1 melt pockets containing abundantclasts of host rock minerals (Figs 2f and 2g)

DISCUSSION

Melt pockets within Los Angeles can be divided into 3types based on size clast volume and degree of vesiculation

Table 3 Bulk compositions of shock melt pockets (in wt) for Los Angeles stone 1Melt pocket 1 (Type 1) Melt pocket 2 (Type 2) Melt pocket 3 (Type 3) Melt pocket 4 (Type 3)

wt avg min max avg min max avg min max avg min max 5a

SiO2 589 511 691 627 585 731 457 365 535 328 255 467 491Al2O3 95 120 06 144 119 183 42 17 91 33 15 46 112TiO2 05 03 06 03 02 05 15 04 46 11 05 11 130FeOb 219 128 329 113 6 189 305 197 404 299 234 441 212MnO 05 07 03 03 ndc 05 07 04 12 07 05 13 045MgO 09 05 26 03 06 52 42 16 110 08 06 13 353CaO 37 57 13 58 58 73 96 70 128 159 59 164 995Na2O 18 10 23 24 12 3 02 nd 11 05 01 09 222K2O 21 02 27 13 08 18 03 nd 05 04 05 06 024P2O5 ndash nd 05 ndash nd 01 ndash nd 49 111 89 265 066SO3 ndash nd 08 ndash nd nd nd 04 27 ndash nd 19 ndashTotal 1001 988 996 970 9986nd 32 30 38 24

CIPW normQuartz 144 226 19 0Orthoclase 124 78 18 25Albite 152 206 17 44Anorthite 117 25 98 58Diopside 54 37 249 07Hypersthene 398 199 534 572Olivine 0 0 0 06Ilmenite 1 06 29 22Apatite 02 0 36 272

aData from Rubin et al (2000) obtained by Instrumental Neutron Activation AnalysisbTotal Fe reported as FeOcnd = element oxides below the detection limit of the electron microprobedn = number of raster scans per melt pocket

Shock-induced melt pockets in Los Angeles 1873

Smaller (lt1 mm) pockets (Types 1 and 2) tend to becrystallite-rich while larger (gt1 mm) pockets (Type 3) tend tobe clast-poor and exhibit flow-structured glasses Opticallyshock effects are manifest as dark brown to black glassy andor microcrystalline enclaves of melt material melt veins anddeposits of sulfides and melt injected into fractures withinsilicates In total these localized regions of melting compriselt1 vol of Los Angeles and they are heterogeneouslydistributed throughout the sample

Melt pockets invariably develop at the grain boundariesof minerals with contrasting shock impedance (shock wavevelocity multiplied by the mineralrsquos density) eg plagioclaseand pyroxene This is most obvious in smaller (pound1 mm) meltpockets (Type 1 and 2) where the original grain positions andboundaries are preserved With increasing volume of melt theoriginal grains and their boundaries become obscured ordestroyed Typically minerals adjacent to the melt pocketshave smooth arcuate margins at the contact with the meltThe composition of the melt pockets corresponds to localizedmelting of bordering host rock minerals Melt veins have beenobserved to form an interlocking network of complexanastomosing branches surrounding unmolten host rockconstituents

The variable compositions of the studied melt pocketscan be attributed to the relatively coarse grain size (2ndash4 mm)of the host rock minerals compared to the diameters of themelt pockets (lt4 mm) The melt pockets are not largeenough to represent a bulk rock melt of Los AngelesInstead they represent localized mineral-scale meltingproducts Melt pocket compositions presented in Table 3differ markedly when comparing those of the smaller meltpockets Types 1 and 2 to the larger Type 3 melt pocketsTypes 1 and 2 are enriched in SiO2 Al2O3 and alkalis ascompared to Type 3 The CIPW norm calculations alsolisted in Table 3 indicate that the precursors of Types 1 and2 melt pockets were mainly quartz + alkali feldspar +plagioclase + pyroxene while those for Type 3 were mainlyplagioclase + pyroxene + whitlockite Therefore theprecursors of Types 1 and 2 melt pockets likely had a largecontribution from late-stage minerals (mesostasis) while thebulk of Type 3 are derived from the main host rock lithologyThis is not surprising considering that shock melting beginsat grain boundary margins where mesostasis is moreabundant Thus smaller (Type 1 and 2) melt pockets willposses a greater contribution of mesostasis to bulkcomposition than larger (Type 3) melt pockets that havemelted approximately the same amount of mesostasis butgreater proportions of host rock phases

Based on compositional and textural lines of evidencewe conclude that the melt pockets formed by the local in situmelting of host rock minerals as opposed to forcefulinjection of extraneous impact melt along fractures in thehost rock Remobilization and injection of opaques and meltfrom the melt region into pre-existing fractures in adjacent

host rock pyroxene could have occurred during compressionand subsequent stress relaxation The contact between themelt pockets and adjacent host minerals is intrusive inplaces The intensity of shock features is observed toincrease with proximity to the melt pockets indicating alocalized increase in shock pressure This pressure gradientfrom melt pocket (60ndash80 GPa) to bulk rock (35ndash40 GPa) iscertainly sufficient to account for forceful injection of Fe-sulfides and melt from the melt pockets into adjacent hostminerals Additionally intrusions could have formed bypressure generated by a volume increase (~10) associatedwith the melting event

The relationship between areas of alkali- and silica-richglass and the melt pockets is puzzling However cominglingand entrapment of liquid blebs from the melt pocket ispossible if the two phases were initially molten at the sametime This could be explained if the alkali- and silica-richglass was formed during the shock event but was quenchedbefore the shock pressure was completely released Melt fromthe melt pocket could then have remained in a liquid stateafter pressure release This is consistent with the presence ofabundant vesicles in the melt pocket glasses indicating lowconfining pressure Stopler and McSween (1979) observedsimilar textural associations in Shergotty and Zagamiconcluding that they may represent shock mixtures of alkali-feldspars and silica

The presence of clasts in one melt pocket with quenchtextures as well as the presence of crosscutting silicate meltsdoes not require multiple impact events That a single impactevent is capable of producing several shock metamorphicfeatures in a target rock is well-documented (eg Stoumlffler1984 Stoumlffler et al 1991) The high concentration ofrelatively cold lithic and crystal fragments as well as theproximity to the melt pocket margin may have facilitatedrapid crystallization of one melt with respect to the otherRapid crystallization could also cause a drastic increase in theviscosity of one melt with respect to the other

The development of abundant crystallites and glass inthe melt pockets attests to rapid cooling of the melt whichimplies that the studied Los Angeles samples were derivedfrom the chilled margins of a larger ejected body (severalmeters to tens of meters in diameter) or were never muchlarger than the recovered meteorite sizes (tens of cm indiameter) The high abundance of relatively cold clasts inthe smaller melt pockets (Type 1) probably contributed tothe rapid crystallization (generating abundant crystallites)and an absence of flow textures due to the presence ofcopious heterogeneous nucleation sites Vesiculationprobably occurred during shock decompression withvesicles persisting in the melt under lower pressureconditions (Kenkmann et al 2000) However this does notpreclude the development of high pressure high temperaturepolymorphs within the melt pockets produced by cavitationand bubble implosion (Spray 1999) Melt veins and melt

1874 E L Walton and J G Spray

dikelets occur in direct contact with the melt pockets and areconsidered morphologically distinct from typical ldquoshockveinsrdquo that transect meteorites and that form by acombination of shock and frictional melting The concentriczoning observed in the margins of clinopyroxenes in contactwith melt pockets coincides with a progressive reduction inbirefringence which suggests that shock excursions werefocused as point sources and that these were generated in thewake of the planar shock front that induced bulk shockeffects We infer that the larger melt pockets (Type 3) wereformed by higher energy excursions relative to the smallerpockets (Type 1 and 2)

Based on experimental data by Kieffer et al (1976)Schaal and Houmlrz (1977) and Schmitt (2000) shock pressureand temperature excursions of 60ndash80 GPa and 1600ndash 2000degCare estimated for Los Angeles 7058 Because pyroxene is notobserved as clasts in any of the melt pockets in Los Angelesthe higher pressure of ~80 GPa is favored coincident with theobserved melting of pyroxene in experimentally shockedbasalts (Schaal and Houmlrz 1977) These discrete excursionsare therefore approximately twice the magnitude of the bulkshock (35ndash40 GPa)

A young formation age 170 plusmn 8 Ma for the Los Angelesmeteorite (Nyquist et al 2001) precludes it recording multipleimpact events that would be more typical of target rocks fromthe older impact-scarred Noachian terrains of the southernhemisphere Instead Los Angeles records evidence of asingle shock event which favors its launching immediatelyfollowing hypervelocity impact into a relatively young (lateAmazonian) volcanic source terrain such as is found in theTharsis and Elysium regions

AcknowledgmentsndashThis work has been funded by the NaturalSciences and Engineering Research Council of Canada(NSERC) through research grants awarded to J G Spray andan NSERC PGS-b scholarship awarded to E L Walton Wethank Tim McCoy at the Smithsonian Institution WashingtonDC for loan of the samples Douglas Hall at UNB andMichael Coy at JEOL USA for EM support Cliff Shaw forhelpful discussions and Angel Gomez for draftingConstructive Reviews by Y Ikeda and R F Schmitt aregratefully acknowledged Planetary and Space Science CentreContribution 31

Editorial HandlingmdashDr Edward Scott

REFERENCES

Chen M and El Goresy A 2000 The nature of maskelynite inshocked meteorites Not a diaplectic glass but a glass quenchedfrom shock-induced dense melt at high pressures Earth andPlanetary Science Letters 179489ndash502

El Goresy A Fechtig H and Ottemann J 1968 The opaqueminerals in impact glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMono Book pp 531ndash553

Garrison D H and Bogard D D 2000 Cosmogenic and trappednoble gases in the Los Angeles meteorite (abstract) Meteoriticsamp Planetary Science 35A58

Houmlrz F and Quaide W L 1973 Debye-Scherrer investigations ofexperimentally shocked silicates The Moon 645ndash82

Kenkmann T Hornemann U and Stoumlffler D 2000 Experimentalgeneration of shock-induced pseudotachylytes alonglithological interfaces Meteoritics amp Planetary Science 351275ndash1290

Kieffer S W Schaal R B Gibbons R Houmlrz F Milton D J andDube A 1976 Shocked basalt from Lonar crater India andexperimental analogues Proceedings 7th Lunar and PlanetaryScience Conference pp 1391ndash1412

Kirkpatrick R J 1975 Crystal growth from the melt A reviewAmerican Mineralogist 60798ndash814

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McSween H Y Jr 2002 The rocks of Mars from far and nearMeteoritics amp Planetary Science 377ndash25

Melosh H J 1984 Impact ejection spallation and the origin ofmeteorites Icarus 59234ndash260

Melosh H J 1985 Ejection of rock fragments from planetary bodiesGeology 13144ndash148

Nishiizumi K Caffee M W and Masarik J 2000 Cosmogenicradiocunclides in Los Angeles martian meteorite (abstract)Meteoritics amp Planetary Science 35A120

Nyquist L E Reese Y D Wiesmann H Shih C Y and SchwandtC 2000 Rubidium-strontium age of the Los Angeles shergottiteMeteoritics amp Planetary Science 35A121ndashA122

Nyquist L E Bogard D D Shih C Y Greshake A Stoumlffler D andEugster O 2001 Ages and histories of martian meteoritesChronology and Evolution of Mars 96105ndash164

Rubin A E Warren P H Greenwood J P Verish R S Leshin LA Hervig R L and Clayton R N 2000 Los Angeles The mostdifferentiated basaltic martian meteorites Geology 281011ndash1014

Schaal R B and Houmlrz F 1977 Shock metamorphism of lunar andterrestrial basalts Proceedings 8th Lunar and Planetary ScienceConference pp 1679ndash1729

Schmitt R T 2000 Shock experiments with the H6 chondriteKernouveacute Pressure calibration of microscopic shock effectsMeteoritics amp Planetary Science 3545ndash560

Spray J G 1999 Shocking rocks by cavitation and bubble implosionGeology 27695ndash698

Spray J G and Rae D A 1995 Quantitative electron-microprobeanalysis of alkali silicate glasses A review and user guideCanadian Mineralogist 33323ndash332

Stoumlffler D 1984 Glasses formed by hypervelocity impact Journal ofNon-Crystalline Solids 67465ndash502

Stoumlffler D 2000 Maskelynite confirmed as diaplectic glassIndications for peak shock pressure below 45 GPa in all martianmeteorites (abstract 1170) 31st Lunar and Planetary ScienceConference CD-ROM

Stoumlffler D Ostertag R Jammes C and Pfannschmidt G 1986Shock metamorphism and petrography of the Shergottyachondrite Geochimica et Cosmochimica Acta 50889ndash903

Stoumlffler D Keil K and Scott E R D 1991 Shock metamorphismof ordinary chondrites Geochimica et Cosmochimica Acta 53845ndash3867

Stopler E and McSween H Y Jr 1979 Petrology and origin of theshergottite meteorites Geochimica et Cosmochimica Acta 431475ndash1498

Terribilini D Busemann H and Eugster O 2000 Krypton-81-

1872 E L Walton and J G Spray

(Rubin et al 2000) in direct contact with the melt pocket Atthe melt pocket margin the vermicular to microgranulitictexture typical of the symplectites changes to one of skeletalfayalite (Fa96) embedded in a glassy matrix rich in Fe Mg andCa and Fe-sulfide spherules (Fig 2b) Numerous fractures inthe vicinity of the melt pocket radiate from the contact withthe melt into the fayalite + ferroaugite + silica intergrowthcausing microfaulting

MELT POCKET COMPOSITIONS

The bulk chemistries of four melt pockets in LosAngeles are presented in Table 3 The compositions arevariable from one pocket to another CIPW norm calculationsreveal that the melt pockets can be derived by fusingdifferent mixtures of the host minerals dominated byplagioclase and pyroxene Melt pocket 1 (Type 1) isdominated by normative quartz orthoclase plagioclase andhypersthene melt pocket 2 (Type 2) by quartz orthoclaseplagioclase and hypersthene (Type 3) by mainlyplagioclase diopside and hypersthene melt pocket 4 (Type3) by plagioclase hypersthene and apatite (whitlockite)Melt pocket compositions depend on which bordering hostminerals contribute to melting and the proportion of meltingof each phase (eg some melt pockets occur at the grainboundary of pyroxene and maskelynite thus these are the

main components contributing to the melt) The occurrenceof at least two coexisting silicate melts within one Type 3melt pocket is revealed by crosscutting relationships(Fig 2a) The more siliceous melt of the two (average65 wt SiO2 obtained by raster scans n = 8) is located inproximity to the melt pocket margin This melt contains lithicand crystal fragments (~50) of locally-derived silica andmaskelynite The less siliceous (average 43 wt SiO2

obtained by raster scans n = 6) lower viscosity meltcrosscuts the former tapering off to thin stringers (pound1 mm inapparent thickness) The shape of the overall melt pocket isroughly spherical (Fig 2) ie no textural evidence exists tosuggest that these crosscutting relationships are the result oftwo independently formed melt pockets Although the twosilicate melts were most probably subject to the same coolingrates the abundance of lithic and crystal fragments wouldhave increased the amount of heterogeneous nucleation sitesfavoring more rapid crystallization concomitant with anincrease in viscosity This is also consistent with rapidcrystallization in Type 1 melt pockets containing abundantclasts of host rock minerals (Figs 2f and 2g)

DISCUSSION

Melt pockets within Los Angeles can be divided into 3types based on size clast volume and degree of vesiculation

Table 3 Bulk compositions of shock melt pockets (in wt) for Los Angeles stone 1Melt pocket 1 (Type 1) Melt pocket 2 (Type 2) Melt pocket 3 (Type 3) Melt pocket 4 (Type 3)

wt avg min max avg min max avg min max avg min max 5a

SiO2 589 511 691 627 585 731 457 365 535 328 255 467 491Al2O3 95 120 06 144 119 183 42 17 91 33 15 46 112TiO2 05 03 06 03 02 05 15 04 46 11 05 11 130FeOb 219 128 329 113 6 189 305 197 404 299 234 441 212MnO 05 07 03 03 ndc 05 07 04 12 07 05 13 045MgO 09 05 26 03 06 52 42 16 110 08 06 13 353CaO 37 57 13 58 58 73 96 70 128 159 59 164 995Na2O 18 10 23 24 12 3 02 nd 11 05 01 09 222K2O 21 02 27 13 08 18 03 nd 05 04 05 06 024P2O5 ndash nd 05 ndash nd 01 ndash nd 49 111 89 265 066SO3 ndash nd 08 ndash nd nd nd 04 27 ndash nd 19 ndashTotal 1001 988 996 970 9986nd 32 30 38 24

CIPW normQuartz 144 226 19 0Orthoclase 124 78 18 25Albite 152 206 17 44Anorthite 117 25 98 58Diopside 54 37 249 07Hypersthene 398 199 534 572Olivine 0 0 0 06Ilmenite 1 06 29 22Apatite 02 0 36 272

aData from Rubin et al (2000) obtained by Instrumental Neutron Activation AnalysisbTotal Fe reported as FeOcnd = element oxides below the detection limit of the electron microprobedn = number of raster scans per melt pocket

Shock-induced melt pockets in Los Angeles 1873

Smaller (lt1 mm) pockets (Types 1 and 2) tend to becrystallite-rich while larger (gt1 mm) pockets (Type 3) tend tobe clast-poor and exhibit flow-structured glasses Opticallyshock effects are manifest as dark brown to black glassy andor microcrystalline enclaves of melt material melt veins anddeposits of sulfides and melt injected into fractures withinsilicates In total these localized regions of melting compriselt1 vol of Los Angeles and they are heterogeneouslydistributed throughout the sample

Melt pockets invariably develop at the grain boundariesof minerals with contrasting shock impedance (shock wavevelocity multiplied by the mineralrsquos density) eg plagioclaseand pyroxene This is most obvious in smaller (pound1 mm) meltpockets (Type 1 and 2) where the original grain positions andboundaries are preserved With increasing volume of melt theoriginal grains and their boundaries become obscured ordestroyed Typically minerals adjacent to the melt pocketshave smooth arcuate margins at the contact with the meltThe composition of the melt pockets corresponds to localizedmelting of bordering host rock minerals Melt veins have beenobserved to form an interlocking network of complexanastomosing branches surrounding unmolten host rockconstituents

The variable compositions of the studied melt pocketscan be attributed to the relatively coarse grain size (2ndash4 mm)of the host rock minerals compared to the diameters of themelt pockets (lt4 mm) The melt pockets are not largeenough to represent a bulk rock melt of Los AngelesInstead they represent localized mineral-scale meltingproducts Melt pocket compositions presented in Table 3differ markedly when comparing those of the smaller meltpockets Types 1 and 2 to the larger Type 3 melt pocketsTypes 1 and 2 are enriched in SiO2 Al2O3 and alkalis ascompared to Type 3 The CIPW norm calculations alsolisted in Table 3 indicate that the precursors of Types 1 and2 melt pockets were mainly quartz + alkali feldspar +plagioclase + pyroxene while those for Type 3 were mainlyplagioclase + pyroxene + whitlockite Therefore theprecursors of Types 1 and 2 melt pockets likely had a largecontribution from late-stage minerals (mesostasis) while thebulk of Type 3 are derived from the main host rock lithologyThis is not surprising considering that shock melting beginsat grain boundary margins where mesostasis is moreabundant Thus smaller (Type 1 and 2) melt pockets willposses a greater contribution of mesostasis to bulkcomposition than larger (Type 3) melt pockets that havemelted approximately the same amount of mesostasis butgreater proportions of host rock phases

Based on compositional and textural lines of evidencewe conclude that the melt pockets formed by the local in situmelting of host rock minerals as opposed to forcefulinjection of extraneous impact melt along fractures in thehost rock Remobilization and injection of opaques and meltfrom the melt region into pre-existing fractures in adjacent

host rock pyroxene could have occurred during compressionand subsequent stress relaxation The contact between themelt pockets and adjacent host minerals is intrusive inplaces The intensity of shock features is observed toincrease with proximity to the melt pockets indicating alocalized increase in shock pressure This pressure gradientfrom melt pocket (60ndash80 GPa) to bulk rock (35ndash40 GPa) iscertainly sufficient to account for forceful injection of Fe-sulfides and melt from the melt pockets into adjacent hostminerals Additionally intrusions could have formed bypressure generated by a volume increase (~10) associatedwith the melting event

The relationship between areas of alkali- and silica-richglass and the melt pockets is puzzling However cominglingand entrapment of liquid blebs from the melt pocket ispossible if the two phases were initially molten at the sametime This could be explained if the alkali- and silica-richglass was formed during the shock event but was quenchedbefore the shock pressure was completely released Melt fromthe melt pocket could then have remained in a liquid stateafter pressure release This is consistent with the presence ofabundant vesicles in the melt pocket glasses indicating lowconfining pressure Stopler and McSween (1979) observedsimilar textural associations in Shergotty and Zagamiconcluding that they may represent shock mixtures of alkali-feldspars and silica

The presence of clasts in one melt pocket with quenchtextures as well as the presence of crosscutting silicate meltsdoes not require multiple impact events That a single impactevent is capable of producing several shock metamorphicfeatures in a target rock is well-documented (eg Stoumlffler1984 Stoumlffler et al 1991) The high concentration ofrelatively cold lithic and crystal fragments as well as theproximity to the melt pocket margin may have facilitatedrapid crystallization of one melt with respect to the otherRapid crystallization could also cause a drastic increase in theviscosity of one melt with respect to the other

The development of abundant crystallites and glass inthe melt pockets attests to rapid cooling of the melt whichimplies that the studied Los Angeles samples were derivedfrom the chilled margins of a larger ejected body (severalmeters to tens of meters in diameter) or were never muchlarger than the recovered meteorite sizes (tens of cm indiameter) The high abundance of relatively cold clasts inthe smaller melt pockets (Type 1) probably contributed tothe rapid crystallization (generating abundant crystallites)and an absence of flow textures due to the presence ofcopious heterogeneous nucleation sites Vesiculationprobably occurred during shock decompression withvesicles persisting in the melt under lower pressureconditions (Kenkmann et al 2000) However this does notpreclude the development of high pressure high temperaturepolymorphs within the melt pockets produced by cavitationand bubble implosion (Spray 1999) Melt veins and melt

1874 E L Walton and J G Spray

dikelets occur in direct contact with the melt pockets and areconsidered morphologically distinct from typical ldquoshockveinsrdquo that transect meteorites and that form by acombination of shock and frictional melting The concentriczoning observed in the margins of clinopyroxenes in contactwith melt pockets coincides with a progressive reduction inbirefringence which suggests that shock excursions werefocused as point sources and that these were generated in thewake of the planar shock front that induced bulk shockeffects We infer that the larger melt pockets (Type 3) wereformed by higher energy excursions relative to the smallerpockets (Type 1 and 2)

Based on experimental data by Kieffer et al (1976)Schaal and Houmlrz (1977) and Schmitt (2000) shock pressureand temperature excursions of 60ndash80 GPa and 1600ndash 2000degCare estimated for Los Angeles 7058 Because pyroxene is notobserved as clasts in any of the melt pockets in Los Angelesthe higher pressure of ~80 GPa is favored coincident with theobserved melting of pyroxene in experimentally shockedbasalts (Schaal and Houmlrz 1977) These discrete excursionsare therefore approximately twice the magnitude of the bulkshock (35ndash40 GPa)

A young formation age 170 plusmn 8 Ma for the Los Angelesmeteorite (Nyquist et al 2001) precludes it recording multipleimpact events that would be more typical of target rocks fromthe older impact-scarred Noachian terrains of the southernhemisphere Instead Los Angeles records evidence of asingle shock event which favors its launching immediatelyfollowing hypervelocity impact into a relatively young (lateAmazonian) volcanic source terrain such as is found in theTharsis and Elysium regions

AcknowledgmentsndashThis work has been funded by the NaturalSciences and Engineering Research Council of Canada(NSERC) through research grants awarded to J G Spray andan NSERC PGS-b scholarship awarded to E L Walton Wethank Tim McCoy at the Smithsonian Institution WashingtonDC for loan of the samples Douglas Hall at UNB andMichael Coy at JEOL USA for EM support Cliff Shaw forhelpful discussions and Angel Gomez for draftingConstructive Reviews by Y Ikeda and R F Schmitt aregratefully acknowledged Planetary and Space Science CentreContribution 31

Editorial HandlingmdashDr Edward Scott

REFERENCES

Chen M and El Goresy A 2000 The nature of maskelynite inshocked meteorites Not a diaplectic glass but a glass quenchedfrom shock-induced dense melt at high pressures Earth andPlanetary Science Letters 179489ndash502

El Goresy A Fechtig H and Ottemann J 1968 The opaqueminerals in impact glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMono Book pp 531ndash553

Garrison D H and Bogard D D 2000 Cosmogenic and trappednoble gases in the Los Angeles meteorite (abstract) Meteoriticsamp Planetary Science 35A58

Houmlrz F and Quaide W L 1973 Debye-Scherrer investigations ofexperimentally shocked silicates The Moon 645ndash82

Kenkmann T Hornemann U and Stoumlffler D 2000 Experimentalgeneration of shock-induced pseudotachylytes alonglithological interfaces Meteoritics amp Planetary Science 351275ndash1290

Kieffer S W Schaal R B Gibbons R Houmlrz F Milton D J andDube A 1976 Shocked basalt from Lonar crater India andexperimental analogues Proceedings 7th Lunar and PlanetaryScience Conference pp 1391ndash1412

Kirkpatrick R J 1975 Crystal growth from the melt A reviewAmerican Mineralogist 60798ndash814

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McSween H Y Jr 2002 The rocks of Mars from far and nearMeteoritics amp Planetary Science 377ndash25

Melosh H J 1984 Impact ejection spallation and the origin ofmeteorites Icarus 59234ndash260

Melosh H J 1985 Ejection of rock fragments from planetary bodiesGeology 13144ndash148

Nishiizumi K Caffee M W and Masarik J 2000 Cosmogenicradiocunclides in Los Angeles martian meteorite (abstract)Meteoritics amp Planetary Science 35A120

Nyquist L E Reese Y D Wiesmann H Shih C Y and SchwandtC 2000 Rubidium-strontium age of the Los Angeles shergottiteMeteoritics amp Planetary Science 35A121ndashA122

Nyquist L E Bogard D D Shih C Y Greshake A Stoumlffler D andEugster O 2001 Ages and histories of martian meteoritesChronology and Evolution of Mars 96105ndash164

Rubin A E Warren P H Greenwood J P Verish R S Leshin LA Hervig R L and Clayton R N 2000 Los Angeles The mostdifferentiated basaltic martian meteorites Geology 281011ndash1014

Schaal R B and Houmlrz F 1977 Shock metamorphism of lunar andterrestrial basalts Proceedings 8th Lunar and Planetary ScienceConference pp 1679ndash1729

Schmitt R T 2000 Shock experiments with the H6 chondriteKernouveacute Pressure calibration of microscopic shock effectsMeteoritics amp Planetary Science 3545ndash560

Spray J G 1999 Shocking rocks by cavitation and bubble implosionGeology 27695ndash698

Spray J G and Rae D A 1995 Quantitative electron-microprobeanalysis of alkali silicate glasses A review and user guideCanadian Mineralogist 33323ndash332

Stoumlffler D 1984 Glasses formed by hypervelocity impact Journal ofNon-Crystalline Solids 67465ndash502

Stoumlffler D 2000 Maskelynite confirmed as diaplectic glassIndications for peak shock pressure below 45 GPa in all martianmeteorites (abstract 1170) 31st Lunar and Planetary ScienceConference CD-ROM

Stoumlffler D Ostertag R Jammes C and Pfannschmidt G 1986Shock metamorphism and petrography of the Shergottyachondrite Geochimica et Cosmochimica Acta 50889ndash903

Stoumlffler D Keil K and Scott E R D 1991 Shock metamorphismof ordinary chondrites Geochimica et Cosmochimica Acta 53845ndash3867

Stopler E and McSween H Y Jr 1979 Petrology and origin of theshergottite meteorites Geochimica et Cosmochimica Acta 431475ndash1498

Terribilini D Busemann H and Eugster O 2000 Krypton-81-

Shock-induced melt pockets in Los Angeles 1873

Smaller (lt1 mm) pockets (Types 1 and 2) tend to becrystallite-rich while larger (gt1 mm) pockets (Type 3) tend tobe clast-poor and exhibit flow-structured glasses Opticallyshock effects are manifest as dark brown to black glassy andor microcrystalline enclaves of melt material melt veins anddeposits of sulfides and melt injected into fractures withinsilicates In total these localized regions of melting compriselt1 vol of Los Angeles and they are heterogeneouslydistributed throughout the sample

Melt pockets invariably develop at the grain boundariesof minerals with contrasting shock impedance (shock wavevelocity multiplied by the mineralrsquos density) eg plagioclaseand pyroxene This is most obvious in smaller (pound1 mm) meltpockets (Type 1 and 2) where the original grain positions andboundaries are preserved With increasing volume of melt theoriginal grains and their boundaries become obscured ordestroyed Typically minerals adjacent to the melt pocketshave smooth arcuate margins at the contact with the meltThe composition of the melt pockets corresponds to localizedmelting of bordering host rock minerals Melt veins have beenobserved to form an interlocking network of complexanastomosing branches surrounding unmolten host rockconstituents

The variable compositions of the studied melt pocketscan be attributed to the relatively coarse grain size (2ndash4 mm)of the host rock minerals compared to the diameters of themelt pockets (lt4 mm) The melt pockets are not largeenough to represent a bulk rock melt of Los AngelesInstead they represent localized mineral-scale meltingproducts Melt pocket compositions presented in Table 3differ markedly when comparing those of the smaller meltpockets Types 1 and 2 to the larger Type 3 melt pocketsTypes 1 and 2 are enriched in SiO2 Al2O3 and alkalis ascompared to Type 3 The CIPW norm calculations alsolisted in Table 3 indicate that the precursors of Types 1 and2 melt pockets were mainly quartz + alkali feldspar +plagioclase + pyroxene while those for Type 3 were mainlyplagioclase + pyroxene + whitlockite Therefore theprecursors of Types 1 and 2 melt pockets likely had a largecontribution from late-stage minerals (mesostasis) while thebulk of Type 3 are derived from the main host rock lithologyThis is not surprising considering that shock melting beginsat grain boundary margins where mesostasis is moreabundant Thus smaller (Type 1 and 2) melt pockets willposses a greater contribution of mesostasis to bulkcomposition than larger (Type 3) melt pockets that havemelted approximately the same amount of mesostasis butgreater proportions of host rock phases

Based on compositional and textural lines of evidencewe conclude that the melt pockets formed by the local in situmelting of host rock minerals as opposed to forcefulinjection of extraneous impact melt along fractures in thehost rock Remobilization and injection of opaques and meltfrom the melt region into pre-existing fractures in adjacent

host rock pyroxene could have occurred during compressionand subsequent stress relaxation The contact between themelt pockets and adjacent host minerals is intrusive inplaces The intensity of shock features is observed toincrease with proximity to the melt pockets indicating alocalized increase in shock pressure This pressure gradientfrom melt pocket (60ndash80 GPa) to bulk rock (35ndash40 GPa) iscertainly sufficient to account for forceful injection of Fe-sulfides and melt from the melt pockets into adjacent hostminerals Additionally intrusions could have formed bypressure generated by a volume increase (~10) associatedwith the melting event

The relationship between areas of alkali- and silica-richglass and the melt pockets is puzzling However cominglingand entrapment of liquid blebs from the melt pocket ispossible if the two phases were initially molten at the sametime This could be explained if the alkali- and silica-richglass was formed during the shock event but was quenchedbefore the shock pressure was completely released Melt fromthe melt pocket could then have remained in a liquid stateafter pressure release This is consistent with the presence ofabundant vesicles in the melt pocket glasses indicating lowconfining pressure Stopler and McSween (1979) observedsimilar textural associations in Shergotty and Zagamiconcluding that they may represent shock mixtures of alkali-feldspars and silica

The presence of clasts in one melt pocket with quenchtextures as well as the presence of crosscutting silicate meltsdoes not require multiple impact events That a single impactevent is capable of producing several shock metamorphicfeatures in a target rock is well-documented (eg Stoumlffler1984 Stoumlffler et al 1991) The high concentration ofrelatively cold lithic and crystal fragments as well as theproximity to the melt pocket margin may have facilitatedrapid crystallization of one melt with respect to the otherRapid crystallization could also cause a drastic increase in theviscosity of one melt with respect to the other

The development of abundant crystallites and glass inthe melt pockets attests to rapid cooling of the melt whichimplies that the studied Los Angeles samples were derivedfrom the chilled margins of a larger ejected body (severalmeters to tens of meters in diameter) or were never muchlarger than the recovered meteorite sizes (tens of cm indiameter) The high abundance of relatively cold clasts inthe smaller melt pockets (Type 1) probably contributed tothe rapid crystallization (generating abundant crystallites)and an absence of flow textures due to the presence ofcopious heterogeneous nucleation sites Vesiculationprobably occurred during shock decompression withvesicles persisting in the melt under lower pressureconditions (Kenkmann et al 2000) However this does notpreclude the development of high pressure high temperaturepolymorphs within the melt pockets produced by cavitationand bubble implosion (Spray 1999) Melt veins and melt

1874 E L Walton and J G Spray

dikelets occur in direct contact with the melt pockets and areconsidered morphologically distinct from typical ldquoshockveinsrdquo that transect meteorites and that form by acombination of shock and frictional melting The concentriczoning observed in the margins of clinopyroxenes in contactwith melt pockets coincides with a progressive reduction inbirefringence which suggests that shock excursions werefocused as point sources and that these were generated in thewake of the planar shock front that induced bulk shockeffects We infer that the larger melt pockets (Type 3) wereformed by higher energy excursions relative to the smallerpockets (Type 1 and 2)

Based on experimental data by Kieffer et al (1976)Schaal and Houmlrz (1977) and Schmitt (2000) shock pressureand temperature excursions of 60ndash80 GPa and 1600ndash 2000degCare estimated for Los Angeles 7058 Because pyroxene is notobserved as clasts in any of the melt pockets in Los Angelesthe higher pressure of ~80 GPa is favored coincident with theobserved melting of pyroxene in experimentally shockedbasalts (Schaal and Houmlrz 1977) These discrete excursionsare therefore approximately twice the magnitude of the bulkshock (35ndash40 GPa)

A young formation age 170 plusmn 8 Ma for the Los Angelesmeteorite (Nyquist et al 2001) precludes it recording multipleimpact events that would be more typical of target rocks fromthe older impact-scarred Noachian terrains of the southernhemisphere Instead Los Angeles records evidence of asingle shock event which favors its launching immediatelyfollowing hypervelocity impact into a relatively young (lateAmazonian) volcanic source terrain such as is found in theTharsis and Elysium regions

AcknowledgmentsndashThis work has been funded by the NaturalSciences and Engineering Research Council of Canada(NSERC) through research grants awarded to J G Spray andan NSERC PGS-b scholarship awarded to E L Walton Wethank Tim McCoy at the Smithsonian Institution WashingtonDC for loan of the samples Douglas Hall at UNB andMichael Coy at JEOL USA for EM support Cliff Shaw forhelpful discussions and Angel Gomez for draftingConstructive Reviews by Y Ikeda and R F Schmitt aregratefully acknowledged Planetary and Space Science CentreContribution 31

Editorial HandlingmdashDr Edward Scott

REFERENCES

Chen M and El Goresy A 2000 The nature of maskelynite inshocked meteorites Not a diaplectic glass but a glass quenchedfrom shock-induced dense melt at high pressures Earth andPlanetary Science Letters 179489ndash502

El Goresy A Fechtig H and Ottemann J 1968 The opaqueminerals in impact glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMono Book pp 531ndash553

Garrison D H and Bogard D D 2000 Cosmogenic and trappednoble gases in the Los Angeles meteorite (abstract) Meteoriticsamp Planetary Science 35A58

Houmlrz F and Quaide W L 1973 Debye-Scherrer investigations ofexperimentally shocked silicates The Moon 645ndash82

Kenkmann T Hornemann U and Stoumlffler D 2000 Experimentalgeneration of shock-induced pseudotachylytes alonglithological interfaces Meteoritics amp Planetary Science 351275ndash1290

Kieffer S W Schaal R B Gibbons R Houmlrz F Milton D J andDube A 1976 Shocked basalt from Lonar crater India andexperimental analogues Proceedings 7th Lunar and PlanetaryScience Conference pp 1391ndash1412

Kirkpatrick R J 1975 Crystal growth from the melt A reviewAmerican Mineralogist 60798ndash814

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McSween H Y Jr 2002 The rocks of Mars from far and nearMeteoritics amp Planetary Science 377ndash25

Melosh H J 1984 Impact ejection spallation and the origin ofmeteorites Icarus 59234ndash260

Melosh H J 1985 Ejection of rock fragments from planetary bodiesGeology 13144ndash148

Nishiizumi K Caffee M W and Masarik J 2000 Cosmogenicradiocunclides in Los Angeles martian meteorite (abstract)Meteoritics amp Planetary Science 35A120

Nyquist L E Reese Y D Wiesmann H Shih C Y and SchwandtC 2000 Rubidium-strontium age of the Los Angeles shergottiteMeteoritics amp Planetary Science 35A121ndashA122

Nyquist L E Bogard D D Shih C Y Greshake A Stoumlffler D andEugster O 2001 Ages and histories of martian meteoritesChronology and Evolution of Mars 96105ndash164

Rubin A E Warren P H Greenwood J P Verish R S Leshin LA Hervig R L and Clayton R N 2000 Los Angeles The mostdifferentiated basaltic martian meteorites Geology 281011ndash1014

Schaal R B and Houmlrz F 1977 Shock metamorphism of lunar andterrestrial basalts Proceedings 8th Lunar and Planetary ScienceConference pp 1679ndash1729

Schmitt R T 2000 Shock experiments with the H6 chondriteKernouveacute Pressure calibration of microscopic shock effectsMeteoritics amp Planetary Science 3545ndash560

Spray J G 1999 Shocking rocks by cavitation and bubble implosionGeology 27695ndash698

Spray J G and Rae D A 1995 Quantitative electron-microprobeanalysis of alkali silicate glasses A review and user guideCanadian Mineralogist 33323ndash332

Stoumlffler D 1984 Glasses formed by hypervelocity impact Journal ofNon-Crystalline Solids 67465ndash502

Stoumlffler D 2000 Maskelynite confirmed as diaplectic glassIndications for peak shock pressure below 45 GPa in all martianmeteorites (abstract 1170) 31st Lunar and Planetary ScienceConference CD-ROM

Stoumlffler D Ostertag R Jammes C and Pfannschmidt G 1986Shock metamorphism and petrography of the Shergottyachondrite Geochimica et Cosmochimica Acta 50889ndash903

Stoumlffler D Keil K and Scott E R D 1991 Shock metamorphismof ordinary chondrites Geochimica et Cosmochimica Acta 53845ndash3867

Stopler E and McSween H Y Jr 1979 Petrology and origin of theshergottite meteorites Geochimica et Cosmochimica Acta 431475ndash1498

Terribilini D Busemann H and Eugster O 2000 Krypton-81-

1874 E L Walton and J G Spray

dikelets occur in direct contact with the melt pockets and areconsidered morphologically distinct from typical ldquoshockveinsrdquo that transect meteorites and that form by acombination of shock and frictional melting The concentriczoning observed in the margins of clinopyroxenes in contactwith melt pockets coincides with a progressive reduction inbirefringence which suggests that shock excursions werefocused as point sources and that these were generated in thewake of the planar shock front that induced bulk shockeffects We infer that the larger melt pockets (Type 3) wereformed by higher energy excursions relative to the smallerpockets (Type 1 and 2)

Based on experimental data by Kieffer et al (1976)Schaal and Houmlrz (1977) and Schmitt (2000) shock pressureand temperature excursions of 60ndash80 GPa and 1600ndash 2000degCare estimated for Los Angeles 7058 Because pyroxene is notobserved as clasts in any of the melt pockets in Los Angelesthe higher pressure of ~80 GPa is favored coincident with theobserved melting of pyroxene in experimentally shockedbasalts (Schaal and Houmlrz 1977) These discrete excursionsare therefore approximately twice the magnitude of the bulkshock (35ndash40 GPa)

A young formation age 170 plusmn 8 Ma for the Los Angelesmeteorite (Nyquist et al 2001) precludes it recording multipleimpact events that would be more typical of target rocks fromthe older impact-scarred Noachian terrains of the southernhemisphere Instead Los Angeles records evidence of asingle shock event which favors its launching immediatelyfollowing hypervelocity impact into a relatively young (lateAmazonian) volcanic source terrain such as is found in theTharsis and Elysium regions

AcknowledgmentsndashThis work has been funded by the NaturalSciences and Engineering Research Council of Canada(NSERC) through research grants awarded to J G Spray andan NSERC PGS-b scholarship awarded to E L Walton Wethank Tim McCoy at the Smithsonian Institution WashingtonDC for loan of the samples Douglas Hall at UNB andMichael Coy at JEOL USA for EM support Cliff Shaw forhelpful discussions and Angel Gomez for draftingConstructive Reviews by Y Ikeda and R F Schmitt aregratefully acknowledged Planetary and Space Science CentreContribution 31

Editorial HandlingmdashDr Edward Scott

REFERENCES

Chen M and El Goresy A 2000 The nature of maskelynite inshocked meteorites Not a diaplectic glass but a glass quenchedfrom shock-induced dense melt at high pressures Earth andPlanetary Science Letters 179489ndash502

El Goresy A Fechtig H and Ottemann J 1968 The opaqueminerals in impact glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMono Book pp 531ndash553

Garrison D H and Bogard D D 2000 Cosmogenic and trappednoble gases in the Los Angeles meteorite (abstract) Meteoriticsamp Planetary Science 35A58

Houmlrz F and Quaide W L 1973 Debye-Scherrer investigations ofexperimentally shocked silicates The Moon 645ndash82

Kenkmann T Hornemann U and Stoumlffler D 2000 Experimentalgeneration of shock-induced pseudotachylytes alonglithological interfaces Meteoritics amp Planetary Science 351275ndash1290

Kieffer S W Schaal R B Gibbons R Houmlrz F Milton D J andDube A 1976 Shocked basalt from Lonar crater India andexperimental analogues Proceedings 7th Lunar and PlanetaryScience Conference pp 1391ndash1412

Kirkpatrick R J 1975 Crystal growth from the melt A reviewAmerican Mineralogist 60798ndash814

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McSween H Y Jr 2002 The rocks of Mars from far and nearMeteoritics amp Planetary Science 377ndash25

Melosh H J 1984 Impact ejection spallation and the origin ofmeteorites Icarus 59234ndash260

Melosh H J 1985 Ejection of rock fragments from planetary bodiesGeology 13144ndash148

Nishiizumi K Caffee M W and Masarik J 2000 Cosmogenicradiocunclides in Los Angeles martian meteorite (abstract)Meteoritics amp Planetary Science 35A120

Nyquist L E Reese Y D Wiesmann H Shih C Y and SchwandtC 2000 Rubidium-strontium age of the Los Angeles shergottiteMeteoritics amp Planetary Science 35A121ndashA122

Nyquist L E Bogard D D Shih C Y Greshake A Stoumlffler D andEugster O 2001 Ages and histories of martian meteoritesChronology and Evolution of Mars 96105ndash164

Rubin A E Warren P H Greenwood J P Verish R S Leshin LA Hervig R L and Clayton R N 2000 Los Angeles The mostdifferentiated basaltic martian meteorites Geology 281011ndash1014

Schaal R B and Houmlrz F 1977 Shock metamorphism of lunar andterrestrial basalts Proceedings 8th Lunar and Planetary ScienceConference pp 1679ndash1729

Schmitt R T 2000 Shock experiments with the H6 chondriteKernouveacute Pressure calibration of microscopic shock effectsMeteoritics amp Planetary Science 3545ndash560

Spray J G 1999 Shocking rocks by cavitation and bubble implosionGeology 27695ndash698

Spray J G and Rae D A 1995 Quantitative electron-microprobeanalysis of alkali silicate glasses A review and user guideCanadian Mineralogist 33323ndash332

Stoumlffler D 1984 Glasses formed by hypervelocity impact Journal ofNon-Crystalline Solids 67465ndash502

Stoumlffler D 2000 Maskelynite confirmed as diaplectic glassIndications for peak shock pressure below 45 GPa in all martianmeteorites (abstract 1170) 31st Lunar and Planetary ScienceConference CD-ROM

Stoumlffler D Ostertag R Jammes C and Pfannschmidt G 1986Shock metamorphism and petrography of the Shergottyachondrite Geochimica et Cosmochimica Acta 50889ndash903

Stoumlffler D Keil K and Scott E R D 1991 Shock metamorphismof ordinary chondrites Geochimica et Cosmochimica Acta 53845ndash3867

Stopler E and McSween H Y Jr 1979 Petrology and origin of theshergottite meteorites Geochimica et Cosmochimica Acta 431475ndash1498

Terribilini D Busemann H and Eugster O 2000 Krypton-81-

Shock-induced melt pockets in Los Angeles 1875

krypton cosmic ray exposure ages of martian meteoritesincluding the new shergottite Los Angeles (abstract) Meteoriticsamp Planetary Science 35A155ndashA156

Warren P H Greenwood J P Richardson J W Rubin A E andVerish R S 2000 Geochemistry of Los Angeles A ferroan La-

and Th-rich basalt from Mars (abstract 2001) 31st Lunar andPlanetary Science Conference CD-ROM

Xirouchakis D Draper D S Schwandt C S and Lanzirotti A2002 Crystallization of Los Angeles a basaltic martianmeteorite Geochimica et Cosmochimica Acta 661867ndash1880