3. MINERALOGY AND TEXTURES OF IRON-TITANIUM OXIDE · PDF fileMINERALOGY AND TEXTURES OF...

Von Herzen, R. P., Robinson, P. T., et al., 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 118

3. MINERALOGY AND TEXTURES OF IRON-TITANIUM OXIDE GABBROSAND ASSOCIATED OLIVINE GABBROS FROM HOLE 735B1

Kazuhito Ozawa,2 Peter S. Meyer,3 and Sherman H. Bloomer4

ABSTRACT

Abundant iron-titanium (Fe-Ti) oxide gabbro, olivine gabbro, and troctolite were drilled at Hole 735B adjacentto the Atlantis II Fracture Zone of the Southwest Indian Ridge during Leg 118. The Fe-Ti oxide gabbro occurs asintrusive bodies into olivine gabbro with very sharp intrusive contacts. The size of the intrusive bodies varies froma millimeter to a few tens of meters. Mineralogical parameters, such as anorthite content of plagioclase andMg/(Mg+Fe) ratios of mafic minerals exhibit bimodal distributions corresponding to olivine and Fe-Ti oxidegabbros, respectively. When the two major gabbro types are looked at separately, several downhole mineralogicalcycles are recognized. The Fe-Ti oxide gabbros exhibit two such cycles with plagioclase becoming more sodic andmafic minerals becoming more iron-rich downward in the drill core. The olivine gabbros and troctolites, however,exhibit two cycles showing an upward increase in sodium in plagioclase and iron in mafic minerals. Themineralogical variations of these gabbros and the intrusive contact relationships probably resulted from downwardintrusion of evolved magma into underlying solid or almost solidified olivine gabbros and troctolite. The denseevolved melt at the top of the cumulus pile probably formed from the crystallization of olivine gabbro cumulatesfollowed by extreme fractional crystallization of residual melt in an isolated, ephemeral magma chamber. Theinterlayered occurrence of evolved and primitive gabbros from Hole 735B represents a typical section of lowerocean crust formed at a very slow spreading ridge.

INTRODUCTION

Basalts erupt on the seafloor after undergoing varyingdegrees of crystallization in magma chambers beneath mid-ocean ridges. The crystallization products of mid-ocean ridgebasalts (MORB) form gabbros that make up oceanic layer 3.To date, most oceanic gabbros have been dredged fromfracture zones and rift valley walls (Hodges and Papike, 1976;Miyashiro and Shido, 1980; Tiezzi and Scott, 1980; Hébert etal., 1983; Walker and DeLong, 1984; Batiza and Vanko, 1985;Elthon, 1987; Meyer et al., 1989). The fragmental nature ofdredged samples, however, limits their usefulness for petro-logic modeling of magmatic accretion at mid-ocean ridges.Drilling at Site 735 on Leg 118, however, succeeded inrecovering an almost complete 500-m section of gabbroicrocks that originally formed at the axis of the SouthwestIndian Ridge. The core from Hole 735B provides the first viewof lower oceanic-crust stratigraphy and offers an opportunityto study the remnants of a mid-ocean ridge magmatic system.These gabbroic rocks exhibit remarkable chemical variationsand may represent accumulated crystals at various stages inthe evolution of a mid-ocean ridge magma chamber.

This study reports petrographic and mineralogical charac-teristics of Fe-Ti oxide gabbros that may be related to alocalized occurrence of evolved magma beneath a slow-spreading ridge. The Fe-Ti oxide gabbros are present through-out the whole sequence of the drilled section as intrusivebodies into olivine gabbros and rarely into troctolite. Thismixed occurrence of evolved and primitive gabbros with aclear mineralogical gap may be representative of lower oce-

1 Von Herzen, R. P., Robinson, P. T., et al., 1991. Proc. ODP, Sci.Results, 118: College Station, TX U.S.A. (Ocean Drilling Program).

Geological Institute, Faculty of Science, University of Tokyo, Tokyo,113, Japan.

3 Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.4 Department of Geology, 675 Commonwealth Ave., Boston University,

Boston, MA 02215, U.S.A.

anic crust at slow spreading ridges particularly in the proxim-ity of transform faults.

Olivine-Bearing Gabbro, Olivine Gabbro,and Troctolite

The petrography of olivine-bearing gabbro, olivine gabbro,and troctolite has been summarized by Bloomer et al. (thisvolume). Olivine-bearing and olivine gabbros are the mostabundant rock types recovered from Hole 735B. They are themain rock types in lithologic Units II, V, and VI, and containtrace amounts of interstitial orthopyroxene and opaque min-erals, which are commonly sulfide or ilmenite. Modal propor-tions of olivine, plagioclase, and clinopyroxene vary consid-erably, which reflects modal layering or irregular distributionof crystals in coarse-grained rocks. The troctolites from Hole735B exhibit either medium-grained poikilitic or fine-grainedequigranular textures. They contain trace amounts of spinel(up to 1%) and sulfide, but no primary Fe-Ti oxides have beenobserved. Troctolites or troctolitic gabbros are common inUnit VI and their frequency tends to increase toward thebottom of the hole. Some olivine gabbros contain clinopyrox-ene that is vermicularly intergrown with orthopyroxene +pale brown amphibole. Such olivine gabbros tend to be moreprimitive than those without clinopyroxene free of the vermic-ular intergrowth (Fig. 1).

Iron-Titanium Oxide GabbroIron-titanium (Fe-Ti) oxide gabbros generally contain

greater than 2% (rarely less than 1%) Fe-Ti oxides (ilmeniteand magnetite). In addition to Fe-Ti oxides, these gabbroscontain clinopyroxene with Mg#s (100 × Mg/(Mg+Fe)) <75,plagioclase with An contents <50, ±olivine (Mg#s <65),±orthopyroxene, and ±inverted pigeonite. Accessory pri-mary phases include common brown hornblende and rareapatite and zircon.

Most of these Fe-Ti oxide gabbros have been metamor-phosed and contain green hornblende, actinolite, epidote,albite, chlorite, and sphene as metamorphic minerals. They

41

K. OZAWA, P. S. MEYER, S. H. BLOOMER

A 90

B

20 30 40 50

20

70 80

α>α>

rox

Q.O

ort

h

»•—o

12

g/(

2oo

8 0 .

7 0 .

60 .

50 .

40

O

° °ooC< O

ooooo

•

o•D

•

H

B

+

B B +

ù

Fe-Ti oxide gabbroFe-Ti oxide gabbro near contactOlivine gabbroOlivine gabbro near contactTroctolitβVermicular cpx olivine gabbroSW Indian Ridge 54°S-7°16 E

30 40 50 60 70 80

42

An mol% in plagioclase

Figure 1. Average An content in plagioclase vs. average Mg/(Mg+Fe) ratios of maficminerals (A, Olivine. B, Clinopyroxene. C, Orthopyroxene) in gabbros from Hole 735B.Solid small symbols imply the gabbros in contact with each other in thin sections. Thosedata for olivine gabbro were obtained from cores 1-3 cm away from the contact. Smallsolid squares and small pluses are the data obtained by P. S. Meyer at MIT (Bloomer etal., this volume) and those for deformed and metamorphosed samples in Unit 1,respectively. Data for Southwest Indian Ridge are taken from Meyer et al. (1989) andthose for Skaergaard Intrusion are from Wager and Brown (1967).

FE-TI OXIDE GABBROS AND ASSOCIATED OLIVINE GABBROS

Table 1. Petrographic table of the studied Fe-Ti oxide gabbros from Hole 735B.

Core, section, interval(cm)

118-735B-

I2R-1, 45-47I4R-1, 93-9719R-5, 47-5019R-5, 55-6020R-I, 111-11421R-1-2, 82-89 FETI21R-2, 27-3223R-3, 33-3723R-3, 34-3724R-2, 95-9724R-2, 120-12628R-1, 109-11428R-3, 72-7734R-4, 8-1236R-4, 5-937R-3-2. 71-76 FETI37R-3, 76-7937R-3, 80-8238R-3, 85-8838R-4, 24-2840R-5, 0-440R-5. 4-843R-3, 95-10243R-4, 64-6644R-2, 16-2445R-1, 5-1246R-1,48-58 FETI46R-3. 121-12846R-4. 109-11347R-2. 117-12747R-3. 56-6147R-3-2, 143-149 FETI48R-2, 56-6548R-2, 109-11348R-3, 112-11648R-4, 82-8449R-1,42-4649R-2, 94-10050R-2, 43-475OR-3, 62-6751R-1,94-9951R-I, 99-10352R-1, 91-10052R-3. 121-12352R-4, 88-9453R-1,47-5453R-I, 123-12753R-2, 31-3954R-1, 131-13654R-3, 20-2454R-3, 125-12754R-5, 117-11955R-2, 101-10555R-2, 110-12055R-3. 83-8656R-2, 11-1473R-4-2, 82-87 FETI73R-5-2, 72-78 FETI74R-6, 27-3576R-1-2, 63-70 FETI76R-1, 70-73 FETI76R-1, 99-110 FETI76R-3-2, 35-41 FETI76R-4, 12-1977R-2, 5-877R-2, 101-10779R-7-2, 2-9 FETI80R-3, 67-7180R-6, 130-13580R-7, 10-1880R-7, 23-2582R-3-2, 0-5 FETI86R-4, 123-13086R-6, 143-145

Depth(mbsf)

39.9251.9883.0283.1285.8990.4791.06

102.61102.61107.94108.20127.47129.54163.23174.80180.13180.18180.23184.25184.88195.45195.49209.45210.23212.37216.07221.47224.68225.83228.52229.26230.10232.80233.28234.53235.51236.34237.21239.34240.82244.03244.09248.93251.60252.67253.45254.12254.55259.25260.84261.81264.39267.50267.62268.81271.52370.23371.58382.13394.69394.74395.08397.06398.20404.96405.89422.82427.35431.98432.23432.32445.72486.60489.57

Opaque(%)

<2<2tr.< l< l5-10

520

5-78-10

10-20<55-1020< l

11-32-3

52-33-5

21-10/loczl-2/locz

101-5/locz

14/locz

101015

10-205

3-55-10

51510

10-2010-155-105-15

52

108

5-105-10

65-10107

1310-15

55-105-105-10

55-10

1553

< l15

5-105

<55

105-10

5

Olivine(%)

0?tr./alt.

00

70-8020

0'?0000

0?2.55-101-3

52-3

1-2 ? alt.2-3

510530

1-5

501

1-25

1-24

2 - 32-1

22-32-3

11

1-22

2-31/alt.

21-2

11020

15-202-31-24-7

53000

< l00

3-5< l5-10

10-205-105-10

10-203-5

70'.'5-10

2

Opx + pig(%)

10-5< l

10-155212

3-55-102-5

10-202-33-5

11-2< l1.51-310

5-1010

2-31-5

110

10-20< l< l

1< l

1< l

2< l ?1—21-2< l1-21-21-2

22-3

1< l< l

< < l< l

< < l< l

1111

< l< l

< < l2-31-210

15-202

5-105-1020•)

4 - 8512

< l< l10< l

3

Ca-poor px

Opx + pig?OpxOpxOpxOpx + pigOpx (pig?)Opx + pigOpx + pigOpx + pigOpxPigPig+opxPigOpx + pigOpxOpx + pig ?Opx + pigPigPigPig+opxOpxOpxOpxOpxOpx + pigOpx + pig(locz)OpxOpxOpx (pig?)OpxOpx + pigOpxPig

Opx + pigOpxOpx + pig?Opx + pigOpx + pigOpx + pigOpx + pig?Opx + pigOpxOpxOpxOpx(pig>)Opx + pig?OpxOpxOpxOpxOpxOpx + pigOpxOpxOpxOpxOpxOpxOpxOpxOpxOpxOpx + pig

OpxOpxOpx + pig?Opx + pigOpxOpxOpxOpxOpx

Metamorphism

ExtensiveMinorModWeakWeak-modExtensiveWeak-modWeak-modWeakMod-extModWeak-modWeak-modWeakWeak-modMinorModWeak-modWeak-modWeakMinorMinorMinorNoWeakModMinorWeakWeakWeakWeakWeakWeak-modMinorWeak-modWeakMinorWeakWeakWeakWeak-modWeakWeak-modModWeak-modWeakModModWeakModWeakWeakWeakModWeak-modNoMinorMinorMinorMinorMinorMinorMinorMinorWeakMinorMinorWeakWeakModWeak-modModModWeak-mod

Deformation

PorphyMyloniticPorphyPorphyPorphyWeak porphyWeak local pphWeakWeakWeak-modWeakMod-porphyModWeakMod-porphyWeakWeakWeakModWeakMinor-noMinor-noVery MinorNoWeakVery minorNoNoWeakVery minorWeakMinorWeakNoWeakWeakNoWeakNoWeakWeakWeakMinorWeak-porphyWeak-porphyWeak porphyWeak-modWeak-modPorphyWeak-mod pphPorphyMod-porphyMod-porphyModPorphy-mylMyloniticWeak-mod pphNoNoNoNoNoNoNoWeak-modWeak-mod pphPorphy-mylPorphyPorphyWeakWeakWeak-modPorphyPorphy

Foliation/layer

Def?

? sz-mod lay+

+ siz lay++ siz lay+

+ + mod layWeak+ siz-mod lay+ siz-mod lay+ + mod lay+ ?+ siz-mod lay+ ++ siz-mod lay

Weak+++ siz-mod lay+++t• f

++ mod-siz lay?+ mod lay+ mod-siz lay+-t- ++ +++ siz-mod lay+ siz mod lay

Def ?Def ?Def ?

Weak

Def?

+ + mod lay+ + siz-mod layDef ?Weak

Siz lay def?++ mod lay

Grainsize

5.07.04.06.04.07.0

18.09.0

15.04.07.04.0

30.015.02.00.51.02.5

20.04.07.01.04.03.0

10.03.03.50.8

20.015.05.05.04.00.34.02.52.55.04.04.05.04.54.02.04.01.04.03.04.08.03.07.57.09.01.02.02.03.00.4

10.08.08.00.81.00.87.53.06.09.00.80.53.06.53.0

Rock type

GabbronoriteInst ol gabbro?GabbronoriteGabbronoritePI wehrliteInst ol gabbroPig gabbroPig gabbroPig gabbroGabbronoritePig gabbroPig gabbroPig gabbroInst ol gb +rDiss ox ol gbDiss ox ol gbOl pig gabbroOl pig gabbroOl pig gabbroOl pig gabbroOl gabbronortDiss ox ol gbOl gabbronortDiss ox ol gbPig gabbroOl pig gabbroDiss ox ol gb?Diss ox ol gbPig gabbroInst ol gabbroInst ol gb +rInst ol gabbroInst ol gabbroOl feti microgbInst ol gabbroInst ol gabbroInst ol gabbroInst ol gb +rInst ol gabbroInst ol gabbroInst ol gabbroInst ol gabbroInst ol gb +rInst ol gabbroInst ol gabbroInst ol gabbroInst ol gabbroInst ol gabbroOl feti gabbroOl feti gabbroOl feti gabbroInst ol gabbroInst ol gabbroOl feti gabbroOl feti gabbroOl feti gabbroGabbronoriteGabbronoritePoik gabbnortGabbronoriteGabbronoriteGabbronoriteOl gabbronortGabbronoriteDiss ox ol gbOl gabbronortOl gabbronortOl feti gabbroOl feti gabbroOl feti microgbOl feti microgbGabbronoriteOl feti gabbroInst ol gabbro

Note: Abbreviations are: loclz, localized; alt., altered; tr., trace; cpx, clinopyroxene; opx, orthopyroxene; pig, inverted pigeonite; mod, moderate; ext, extensive; porphyor pph, porphyroclastic; myl, mylonitic; def, deformation; siz, size; mod, modal; lay, layering; ol, olivine; opq, opaque mineral; +, strong magmatic lamination; + + ,very strong magmatic lamination; inst ol gabbro, interstitial olivine-bearing oxide gabbro; pi, plagioclase; inst ol gb +r, interstitial olvine-bearing oxide gabbro withcorona structure; diss ox ol gb, disseminated Fe-Ti oxide olivine gabbro; ol feti microgb, olivine Fe-Ti oxide microgabbro; ol feti gabbro, olivine Fe-Ti oxide gabbro;poik gabbnort, poikilitic gabbronorite. Fe-Ti oxide gabbro in contact with olivine gabbro is indicated by "FETI" in the interval column. Grain size is representativeone for clinopyroxene.

43


commonly exhibit porphyroclastic or mylonitic textureswith remarkable foliation and lineation, indicating that thesegabbros were subjected to strong subsolidus deformation.Some specimens also exhibit a magmatic lamination whichlikely resulted from hypersolidus deformation of the cumu-late pile (Dick et al., this volume). Magmatic lamination isalways cut by deformation foliation, which can be clearlyseen by the redistribution patterns of Fe-Ti oxides (Robin-son, Von Herzen, et al., 1989). Magmatic lamination can bediscriminated from deformation foliation by the absence ofhighly deformed plagioclase and olivine exhibiting strongwavy extinction, kink bands, or recrystallization. Magmaticlamination at levels shallower than 170 mbsf is weak orabsent, or possibly overprinted by deformation (Table 1;Fig. 7 in Robinson, Von Herzen, et al., 1989).

In spite of these secondary processes, primary magmaticstructures, textures, and mineralogical characteristics are wellpreserved. Because this study focuses on igneous processes,only magmatic features are presented in the following petro-graphic descriptions. Some gabbros exhibit magmatic modaland/or size layering. They also commonly exhibit magmaticlamination defined by an elongate aggregate of Fe-Ti oxides orpreferred orientations of elongate plagioclase and clinopyrox-ene crystals. Fe-Ti oxide gabbros commonly occur as centi-meter to meter scale veins and layers within the olivinegabbro. The boundaries between the Fe-Ti oxide gabbros andolivine gabbro are very sharp and are generally marked by theabrupt appearance of Fe-Ti oxides. This mixed occurrence ofevolved and primitive gabbros over a 500-m depth range is themost distinctive feature of the gabbros from Hole 735B. Somezones having thicknesses of more than a few meters alsooccur: these are located in the gabbronorite in the top 28 m ofUnit I and in the 50-m-thick massive Fe-Ti oxide gabbro zoneUnit IV. In the upper two thirds of Unit V, Fe-Ti oxide gabbrois almost absent (Fig. 2). In Unit IV, two zones of olivinegabbros having thicknesses of 50 cm and 1 m are present nearthe top of the massive Fe-Ti oxide gabbro. The proportion ofFe-Ti oxide gabbro increases downward from Units I to IVand from Units V to VI (Robinson, Von Herzen, et al., 1989).These two zones in the core are also marked by a downwardincrease in the abundance of Fe-Ti oxides and by downwardtrends to more evolved mineral compositions, e.g., lower Ancontents in plagioclase and lower Mg#s in olivines andclinopyroxenes (Fig. 2 and 3).

Fe-Ti oxide gabbros can be divided into eight types accord-ing to modal proportions of primary phases and texturalrelationships (Table 1). These are gabbronorite, olivine gab-bronorite, disseminated Fe-Ti oxide olivine gabbro, olivinepigeonite gabbro, pigeonite gabbro, interstitial olivine-bearingFe-Ti oxide gabbro, olivine Fe-Ti oxide gabbro, olivine Fe-Tioxide microgabbro.

GEOLOGICAL SETTING OF HOLE 735BHole 735B is located on a shallow platform in about 700 m

of water on the east rim of the Atlantis II Fracture Zone,which is a 210-km left lateral offset formed during ridgeextension of the Southwest Indian Ridge at about 58 Ma(Robinson, Von Herzen, et al., 1989). The platform is 9 kmlong in a north-south direction and 4 km wide, and is one of aseries of uplifted blocks aligned to form a linear ridge parallelto the Atlantis II Fracture Zone. This platform has a flat,probably wave-cut, surface and locally is covered with a thinlayer of sediments. The crustal age of Site 735 is inferred to beabout 12 Ma on the basis of magnetic anomaly patterns on theeastern transform wall. The surface feature is characterizedby development of a steeply dipping foliation and a regularpattern of faults and joints, (Robinson, Von Herzen, et al.,1989). The section drilled in Hole 735B was originally divided

into six lithologic units on the basis of primary igneous modalmineralogy, mineral and bulk chemical compositions, anddegree and style of deformation (Robinson, Von Herzen, etal., 1989). Dick et al. (this volume) redescribed the entire coreat the Ocean Drilling Repository, dividing it into six units,three of which were further subdivided into two to foursubunits. In this paper, this lithostratigraphic division isemployed in the description and discussion.

PETROGRAPHYGabbros from Hole 735B can be divided into primitive

gabbros (olivine-bearing gabbro, olivine gabbro, and trocto-lite) and evolved gabbros (gabbronorite, pigeonite gabbro,olivine iron-titanium (Fe-Ti) oxide gabbro). Olivine gabbrosmostly without Fe-Ti oxide are the main constituent of thehole. These two groups can be discriminated by the presenceor absence of Fe-Ti oxides and by the compositions ofplagioclase, clinopyroxene, and olivine which exhibit bimodalpopulations in An mol% (100 × Ca/(Ca+Na+K)) of plagio-clase and Mg# (100 × Mg/(Mg+Fe)) of mafic minerals (Fig.4). As described later, the contacts between olivine gabbroand Fe-Ti oxide gabbro generally are sharp (mineralogicalgradients are confined to zones less than 1 cm) resulting in fewgabbros with intermediate compositions.

Cinopyroxenes in Fe-Ti oxide gabbros are generally sub-hedral to euhedral and have (001) exsolution lamellae andirregular orthopyroxene patches (Fig. 22 of Robinson, VonHerzen, et al., 1989), whereas those in olivine gabbro aregenerally anhedral and have no (001) exsolution lamellae.

GabbronoriteGabbronorite consists mainly of orthopyroxene, clinopy-

roxene, plagioclase, and Fe-Ti oxides (PI. 1, Fig. 1). Modalabundances of Fe-Ti oxide are generally less than a fewpercent. The oxides exhibit scattered local concentration thatdefines a thin layer, which is parallel to the lamination. Traceamounts of olivine are rarely present. Reddish-brown amphib-ole is present as a minor phase at the rim of or partiallyreplacing clinopyroxene. Zircon is present as an isolatedeuhedral grain in some gabbronorites. Orthopyroxene is sub-hedral to anhedral and exhibits various types of intergrowthwith clinopyroxene. The most common type of intergrowth(0.1-1 mm in size) is characterized by a host orthopyroxeneand optically continuous clinopyroxene having a poikiliticappearance (e.g., Sample 118-735B-76R-4, 12-19 cm). Themargins of euhedral orthopyroxene rarely exhibit fine inter-growths (10-60 µm in size) with optically continuous clinopy-roxene (Sample 118-735B-19R-5, 47-50 cm).

A rare poikilitic gabbronorite occurs as an intrusion intoolivine gabbro (118-735B-74R-6 [Piece 1 through Piece 4C]). Itconsists of euhedral clinopyroxene, euhedral plagioclase, and5- to 10-mm oikocrysts of orthopyroxene containing abundantslightly corroded plagioclase and opaque minerals aschadacrysts and very rare clinopyroxene chadacrysts. Euhe-dral apatite is also present. Euhedral to subhedral magnetiteand ilmenite are homogeneously distributed, being indepen-dent of orthopyroxene oikocrysts. Such euhedral Fe-Ti oxidesare rare among the rest of the Fe-Ti oxide gabbros from Hole735B, whose Fe-Ti oxides are generally anhedral and fill theinterstitial spaces between the silicate minerals.

A gabbronorite sample (Sample 118-735B-76R-1, 63-70cm), which is in contact with olivine gabbro, contains ananhedral grain of olivine surrounded by an intergrowth oforthopyroxene and Fe-Ti oxide (PI. 1, Fig. 4). This coronastructure is located 1 cm from the contact. The occurrence ofthe corona structure at the contact suggest a reaction ofolivine in the olivine gabbro with the evolved melt thatcrystallized gabbronorite. A similar orthopyroxene-magnetite

44

30

200 300 400

500

^>Fe-Ti oxide gabbro ^>Olivine gabbro

C 9 0

500

100 200 300 400 500

D 90Φc

s2 80.Q.Oto

7 0 .

ΦLJL 6 0 .+en

σ> 50.

oo

40

Ia ibj

Unit II m; IV

δ J

I VIbici d

100 200 300 400 500

Depth (mbsf)

Figure 2. Downhole mineralogical variations (average An content in plagioclase (A) and average Mg/(Mg+Fe) ratios for the main mafic minerals (B-D)) forgabbros from Hole 735B. Solid symbols indicate data near the contact between olivine gabbro and iron-titanium oxide gabbro. The data for olivine gabbros andtroctolites in contact with iron-titanium oxide gabbro are analyses of cores 1-3 cm away from the contact. Bars indicate standard deviations. The unit boundaries(Dick et al., this volume) are shown in (D). Small solid squares in (A), (B), and (C) are the data obtained by P. S. Meyer at MIT (Bloomer et al , this volume).Pluses in Unit I are for fairly deformed and metamorphosed samples, original rock types of which are difficult to identify.

o


• Gabbronoπtθ

D Olivmβ gabbronoπtθ

Q Disseminated Fe-Ti oxide olivine gabbro

O Olivine pigeonile gabbro

• Pigβonite gabbro

Δ Interstitial ol-beaπng Fe-Ti oxide gabbro I

+ Interstitial ol-beaπng Fe-Ti oxide gabbro II

A Olivme Fβ-Ti oxide gabbro

× Olivine Fe-Ti oxide microgabbro

500

500

500

100 200 300

Deoth (mbsf)

400 500

46

Figure 3. Downhole mineralogical variations (average An content in plagioclase (A) andaverage Mg/(Mg+Fe) ratios of the main mafic minerals (B-D)) for Fe-Ti oxide gabbros fromHole 735B. Interstitial olivine-bearing Fe-Ti oxide gabbro II contains, but I does not have acorona structure. The corona structure consists of calcium-poor pyroxenes (invertedpigeonite or orthopyroxene) surrounded by clinopyroxene, which is further surrounded byolivine. Intergrowth of olivine and clinopyroxene rarely surrounds calcium-poor pyroxenes.Bars indicate standard deviations. The unit boundaries (Dick et al., this volume) are shownin (D). Three main cycles are depicted with arrows in (A) and (C).


intergrowth around primary cumulus olivine has been noted inseveral layered intrusions and has been interpreted as areaction between cumulus olivine and pore liquid (e.g., Am-bler and Ashley, 1977).

Olivine GabbronoriteOlivine gabbronorite consists of olivine, orthopyroxene,

clinopyroxene, plagioclase, and Fe-Ti oxides (PI. 1, Fig. 2).Minor amounts of brown hornblende are included in clinopy-roxene, which is commonly associated with the opaque min-erals. Hornblende also occurs at the contact between clinopy-roxene and interstitial Fe-Ti oxides. The modal abundance ofFe-Ti oxides is generally less than a few percent. Texturally,Fe-Ti oxides are anhedral, except for those that are includedin pyroxenes and are locally concentrated, defining a remark-able modal layering. Olivine is commonly present as isolatedgrains or aggregates. Orthopyroxene is anhedral to subhedral,and the subhedral grains are elongate and show preferredorientation. Anhedral orthopyroxene is poikilitic and enclosesplagioclase, olivine, and opaque minerals. Olivine gab-bronorite commonly exhibits lamination and/or size andmodal layering. In some layered specimens (e.g., Sample118-735B-43R-3, 95-102 cm), olivine is present as an intersti-tial phase rimming opaque minerals in the olivine-poor andopaque-rich upper portion. In another layered specimen (Sam-ple 118-735B-45R-1, 5-12 cm), a lower olivine gabbronoritelayer is in contact with an opaque-rich pigeonite gabbro layerwithout olivine. The layer boundary is nearly horizontal and ismarked by a sudden increase in opaque phases. Weak lami-nation, defined by elongate clinopyroxene, plagioclase, andFe-Ti oxides, is nearly vertical and thus oblique to the layerboundary. The modal abundance of orthopyroxene decreasesdownward suddenly in one layered specimen (Sample 118-735B-40R-5, 0-8 cm) and creates the appearance of dissemi-nated Fe-Ti oxide olivine gabbro, which will be describedlater. In this specimen, the Fe-Ti oxide is rich in the olivinegabbronorite layer, but is poor in the disseminated oxideolivine gabbro layer.

Disseminated Fe-Ti Oxide GabbroThis gabbro consists of olivine, clinopyroxene, Fe-Ti

oxides, ±orthopyroxene, and brown hornblende as an ac-cessory primary phase (PI. 1, Fig. 3). Disseminated Fe-Tioxide olivine gabbro has a modal abundance similar toolivine gabbro, but shows more evolved mineralogical char-acteristics and always contains minor amounts of Fe-Tioxides. Its modal abundance also is similar to an olivineFe-Ti oxide gabbro, except for the difference in the opaqueabundance and mineral chemistries. The modal abundanceof Fe-Ti oxides is mostly less than 1%. Orthopyroxene ispresent (less than 1%) as anhedral grains, often in contactwith olivine or clinopyroxene. Olivine is present as isolatedgrains or in aggregates. Disseminated Fe-Ti oxide olivinegabbro commonly shows layering and a marked lamination.Olivine shows local concentrations in layers parallel to othertypes of modal or size layering. Magmatic lamination is oftenbowed and commonly oblique to the layer boundaries. Onedisseminated Fe-Ti oxide olivine gabbro (Sample 118-735B-40R-5, 0-8 cm) is in contact with upper olivine gab-bronorite. The disseminated Fe-Ti oxide olivine gabbrocontains slightly more sodic plagioclase and Fe-rich maficminerals than the olivine gabbronorite.

Olivine Pigeonite GabbroOlivine pigeonite gabbro consists mainly of olivine, in-

verted pigeonite, clinopyroxene, plagioclase, Fe-Ti oxides,and ± orthopyroxene. Modal abundance of the Fe-Ti oxides is

variable and ranges from 1% to 5%, which is distinctly higherthan that of gabbronorite, olivine gabbronorite, and dissemi-nated Fe-Ti oxide olivine gabbro. The modal abundance ofolivine is commonly less than 5%, but some gabbros exhibitmarked concentration of olivine, up to 70%, giving rise towehrlitic lithology. Olivine and Fe-Ti oxides show localconcentrations forming thin, olivine and/or opaque-rich lay-ers. Olivine is present either as isolated aggregates or asinterstitial grains generally associated with opaque mineral-s.In rare cases, olivine occurs in intergrowths with clinopy-roxene in close proximity to inverted pigeonite. Invertedpigeonite is present as subhedral isolated grains (PI. 1, Fig. 5)or in cores of clinopyroxenes sharing its crystallographic caxis; thus (001) clinopyroxene exsolution lamellae in invertedpigeonite have the same extinction position as its host (PI. 1,Fig. 6). The contacts between host clinopyroxene and in-verted pigeonite are fairly irregular. This mode of occurrenceof inverted pigeonite, which is more common in pigeonitegabbro and olivine-bearing Fe-Ti oxide gabbro, suggests thatthe pigeonite became unstable at some stage of crystallizationand started to corrode, and then clinopyroxene grew over it.Such corroded inverted pigeonite in clinopyroxene is alsoobserved in olivine Fe-Ti oxide gabbro, but is quite rare.Modal abundance of inverted pigeonite ranges from 1% to 7%.In one specimen of olivine pigeonite gabbro (Sample 118-735B-45R-1, 5-12 cm), olivine and pigeonite appear to beincompatible and are present in different layers. The pigeo-nite-bearing layer is rich in opaque minerals in contrast to theolivine-rich layer. In a remarkably laminated specimen (Sam-ple 118-735B-38R-4, 24-28 cm), containing a 5-mm-thickolivine and an opaque-rich layer (steeply inclining at 60°),inverted pigeonite occurs within 1 cm above the olivine-richbands. Below this, orthopyroxene appears instead of invertedpigeonite, both of which are further sandwiched by dissemi-nated Fe-Ti oxide olivine gabbro.

Pigeonite GabbroPigeonite gabbro consists mainly of inverted pigeonite,

clinopyroxene, ±orthopyroxene, plagioclase, and Fe-Ti ox-ides (Figs. 19A 19B in Robinson, Von Herzen, et al., 1989).Olivine is absent and trace amounts of brown amphibole occurat the rim of, or included in, clinopyroxene. Euhedral apatiteis rarely present. Modal abundance of Fe-Ti oxides rangesfrom 3% to 20%, which is distinctly greater than that of thegabbros described above. They are anhedral filling interstitialspaces between pyroxenes and plagioclase. The contactsbetween the Fe-Ti oxides and pyroxenes are bowed and arecharacterized by the common occurrence of cusps of Fe-Tioxides projecting into clinopyroxene. Inverted pigeonite oc-curs either as anhedral single grains enclosed by clinopyrox-ene or as isolated euhedral-to-subhedral grains commonlyassociated with the Fe-Ti oxides. The modal abundance ofinverted pigeonite is commonly less than a few percent, butrarely reaches 15% (e.g., Sample 118-735B-24R-2, 120-126cm). Pigeonite gabbros are generally coarser (with grains up to30 mm) than the other types of gabbros (Table 1). Intergrowthof Fe-Ti oxides and calcium-poor pyroxene is rarely present.The intergrowth may be a product of a reaction betweenolivine and liquid (see discussion above under gabbronorite).

Interstitial Olivine-Bearing Fe-Ti Oxide GabbroThis type of gabbro is characterized by the occurrence of

small amounts of intercumulus olivine, commonly associatedwith Fe-Ti oxides (PI. 2, Figs. 1 to 4). It consists of clinopy-roxene, plagioclase, Fe-Ti oxides, olivine, ±inverted pigeo-nite, and ±orthopyroxene. The modal abundance of Fe-Tioxides varies from a few percent to 15%. They are fairly

47


anhedral, and fill the interstitial spaces between plagioclaseand clinopyroxene. The modal abundance of olivine is lessthan a few percent. Olivine occurs as an interstitial phasebetween Fe-Ti oxides and clinopyroxene (PI. 2, Fig. 1) orsometimes as a filmy mantle around oxides adjacent to pla-gioclase (50-500µm; PI. 2, Fig. 2). A similar occurrence ofolivine has been reported from the Skaergaard Intrusion(Wager and Brown, 1967). Olivine is, furthermore, oftenpresent as a fine intergrowth with clinopyroxene, whichsometimes surrounds inverted pigeonite (PI. 2, Fig. 4). Olivineis rarely present as a monomineralic aggregate. Clinopyroxeneis subhedral and commonly rounded with cusps of Fe-Tioxides projecting into clinopyroxene. Inverted pigeonite andorthopyroxene are present in the cores of clinopyroxenes.They have fairly irregular corroded shapes (PI. 2, Fig. 3).Some interstitial olivine-bearing Fe-Ti oxide gabbros have acorona structure exhibiting a reaction relation between calci-um-poor pyroxene and silicate melt. Calcium-poor pyroxene(inverted pigeonite or orthopyroxene) is surrounded by cli-nopyroxene-olivine intergrowth, which is further surroundedby olivine (PI. 2, Fig. 4; Fig. 23 in Robinson, Von Herzen, etal., 1989). This type of corona structure is wholly or partlyenclosed by Fe-Ti oxides. The intergrowth (without a core ofcalcium-poor pyroxene) is common for more than one-half ofthe interstitial olivine-bearing gabbros. Interstitial olivine-bearing Fe-Ti oxide gabbros often show modal and sizelayering or banding; plagioclase-poor, clinopyroxene- andopaque-rich bands are common. These gabbros are alsocharacterized by well-developed igneous lamination which inmost cases is parallel to the layering.

Olivine Fe-Ti Oxide GabbroOlivine Fe-Ti oxide gabbro is characterized by the presence

of abundant, large, euhedral to subhedral olivine (PI. 2, Fig. 5),suggesting that this olivine was a cumulus phase. Olivine Fe-Tioxide gabbro consists of olivine clinopyroxene Fe-Ti oxidesplagioclase, ±orthopyroxene, and + inverted pigeonite and canbe easily discriminated from olivine gabbro and disseminatedFe-Ti oxide olivine gabbro by its evolved chemical nature and bythe occurrence of much more abundant Fe-Ti oxides. Brownhornblende and apatite are common minor phases and zircon ispresent in some olivine Fe-Ti oxide gabbros. In rare cases, themodal abundance of apatite reaches 2%-4%, and it is present asan aggregate with Fe-Ti oxides. The modal abundance of olivineranges from a few percent to 10%. Olivine also occurs as aninterstitial phase that is commonly associated with Fe-Ti oxides.The modal abundance of Fe-Ti oxides ranges from a few percentto 20%. These oxides are anhedral, filling interstitial spacesbetween the silicate minerals. Inverted pigeonite is rarely pre-sent, but if present, is always in the core of clinopyroxene as ananhedral corroded grain. Orthopyroxene also is present, rim-ming euhedral olivine crystals. Layering is not developed inolivine Fe-Ti oxide gabbro, and the magmatic lamination isweak.

Olivine Fe-Ti Oxide MicrogabbroOlivine Fe-Ti oxide microgabbro consists mainly of oliv-

ine, clinopyroxene, plagioclase, and Fe-Ti oxides (PI. 2, Fig.6). Minor amounts of brown hornblende are also present.Calcium-poor pyroxenes are absent with the exception ofirregular orthopyroxene patches in the clinopyroxene. Olivineis anhedral and commonly associated with Fe-Ti oxides. Themicrogabbro often exhibits layering and weak lamination. Inmedium-grained layers in contact with fine-grained layers, theolivine is clearly interstitial to clinopyroxene and plagioclase,indicating that the olivine Fe-Ti oxide microgabbro is afine-grained interstitial olivine-bearing Fe-Ti oxide gabbro.

Downhole Petrographic Variation

Even though these Fe-Ti oxide gabbros occur mixed witholivine gabbros at various scales, they exhibit a regularpattern of downhole petrographic and mineral chemical vari-ations. As the depth increases from Unit I to IV, the Fe-Tioxide gabbros tend to become more evolved. Three majorcycles can be identified (Fig. 3; Table 1). From Units I to IV,the types of gabbro change as follows: gabbronorite, olivine-bearing Fe-Ti oxide gabbro, olivine pigeonite gabbro, pigeo-nite gabbro (the first cycle), disseminated Fe-Ti oxide olivinegabbro, olivine gabbronorite, olivine pigeonite gabbro, pigeo-nite gabbro, olivine-bearing Fe-Ti oxide gabbro, and olivineFe-Ti oxide gabbro (the second cycle) (Fig. 3, Table 1).Olivine Fe-Ti oxide gabbro, the most evolved Fe-Ti oxidegabbro, is in particular restricted to a 50-m thick zone ofmassive Fe-Ti oxide gabbro at the base of Unit IV. BelowUnit V, the variation is more erratic but still shows the samedownward tendency (the third cycle); gabbronorite, olivinegabbronorite, and disseminated Fe-Ti oxide olivine gabbro arepresent in the shallower levels (<400 mbsf), and olivine-bearing Fe-Ti oxide gabbro and olivine Fe-Ti oxide gabbro arepresent in the deeper levels (>400 mbsf, Fig. 3; Table 1).

Modal abundances of Fe-Ti oxides also show systematicdownhole variations that follow the petrographic cycles men-tioned above. Abundances increase from Unit I to the bottomof Unit II where pigeonite gabbro occurs. In Unit IIIB, wheregabbronorite and disseminated Fe-Ti oxide olivine gabbrooccur, the abundances abruptly decrease and then increaseagain until reaching a maximum in the middle of Unit IV,where pigeonite gabbro and olivine-bearing oxide gabbrooccur. Near the bottom of Unit IV, the abundances of Fe-Tioxides tend to decrease. Below Unit V, the abundances showa steady increase downward. Modal abundances of olivineincrease from Unit I to Unit IV, where the proportion ofolivine reaches 20%, with the exception of local concentra-tions (e.g., Sample 118-735B-20R-1, 111-114 cm). For Units Vand VI, olivine tends to increase downward.

Fe-Ti oxide gabbros commonly exhibit magmatic lamina-tion. This magmatic lamination is developed in Units III andIV (below 180 mbsf, down to 250 mbsf). In the Unit IV, thelamination is well-developed, but is very weak or absent nearthe bottom, where olivine Fe-Ti oxide gabbro is present. Thelamination dips at 45° in Unit III. The dip of this laminationgradually decreases from 45° at the top of Unit IV to almosthorizontal near the bottom of the unit (Robinson, VonHerzen, et al., 1989). The gradual decrease in the dips ofmagmatic lamination, deformational foliation, and stable mag-netic inclination (all by —40°) towards the bottom of Unit IV(Robinson, Von Herzen, et al., 1989) suggest a progressiverotation possibly due to faulting along the mylonite zone at thebottom of Unit IV.

The primary grain size of Fe-Ti oxide gabbros exhibitsdownhole variation (Table 1). The representative grain size(the most common grain size) of clinopyroxene ranges from0.2 to 30 mm. Grain size ranges 2 to 10 mm in Unit I andupper half of Unit II, 2 to 30 mm in the bottom half of UnitII, and 0.2 to 20 mm in Units III and IV. Thus, the maximumgrain size first increases downward and then reaches amaximum at —130 mbsf, followed by a decrease to thebottom of Unit IV. The minimum grain size tends todecrease from Unit I to Unit IV. In Units V and VI, the grainsize ranges from 0.3 to 10mm.

Fe-Ti oxide gabbro commonly shows size and/or modalbanding on a scale of a few millimeters (Robinson, Von Herzen,et al., 1989). This type of banding is best developed in Units IIIand IV, where lamination is well developed (Table 1).

48


MINERAL CHEMISTRY

Minerals were analyzed by an electron microprobe ana-lyzer (JEOL JCMA 733 Mk-II) at the Geological Institute, theUniversity of Tokyo, under the following conditions: 2-3 µmbeam size, 15 kV accelerating voltage, 1.2 × 10~8 A specimencurrent, and ZAF correction procedure. Other conditions arethe same as those applied by Nakamura and Kushiro (1970).In the area analyses, the specimen stage was automaticallydriven at 20-150 µm/s. The pixel size ranged from 10 to 100µm. The beam size ranged from 5 to 50 µm.

Major and minor element abundances in cumulus miner-als in gabbros can be used as indicators of magma evolution,but first the effects of postcumulus crystallization and sub-solidus reequilibration must be considered. In Hole 735Bgabbros, subsolidus effects are probably negligible for allelements in plagioclase and most elements (except for Ca,Mg, and Fe) in clinopyroxene, because plagioclase andclinopyroxene exhibit magmatic zoning (rarely oscillatory)for these elements. Magnesium and iron in mafic mineralsare most affected by subsolidus exchange reactions asindicated by the Mg-Fe distribution coefficients. However,the good correlations of Mg#s of all the mafic mineralsindicate that the Mg#s can be used as a parameter of degreeof the magma evolution.

Clinopyroxene grains are typically strongly zoned towardtheir rims. Core compositions, therefore, are more repre-sentative of cumulus compositions and better indicators ofmagma evolution. So most of the clinopyroxene analyses areof cores. Olivine is always fairly homogeneous so rims andcores have not been discriminated in the following discussion.The analyses of orthopyroxenes are of subhedral cumulusgrains (in gabbronorite or olivine gabbronorite), hosts ofinverted pigeonites (in olivine pigeonite gabbro or pigeonitegabbro), patchy grains (in Fe-Ti oxide gabbro), and interstitialgrains (in olivine gabbro and troctolite). Plagioclase in theFe-Ti oxide gabbros generally exhibits normal zoning, so mostof the analyses are of cores. Average chemical compositionsof olivine, plagioclase, clinopyroxene, and orthopyroxene arelisted in Tables 3 to 6. In Table 2 average An mol% ofplagioclase and Mg#s of mafic minerals are listed with thenumber of analyses for each mineral.

Relationships Between Rock Typeand Mineral Chemistry

The mineral chemistry of Fe-Ti oxides gabbros from Hole735B shows distinct correlation with rock types (modalvariation) and textural characteristics (e.g., grain size andreaction texture). The An content in plagioclase and Mg# ofmafic minerals decrease with overlap in the following order:gabbronorite, olivine gabbronorite, disseminated Fe-Ti ox-ide olivine gabbro, olivine pigeonite gabbro, pigeonite gab-bro, interstitial olivine-bearing Fe-Ti oxide gabbro, andolivine Fe-Ti oxide gabbro (Fig. 5). Olivine Fe-Ti oxidemicrogabbros have similar mineral chemistries to olivine-bearing Fe-Ti oxide gabbros, but their plagioclase is slightlymore An-rich. Olivine-bearing Fe-Ti oxide gabbros withrelatively unevolved mineralogical characteristics often ex-hibit corona structures (PI. 2, Fig. 4; Fig. 5). Gabbronoritesand olivine pigeonite gabbros that exhibit intergrowths ofcalcium-poor pyroxene and Fe-Ti oxides (rarely surroundingolivine) have mineralogical characteristics similar to the lessevolved olivine-bearing Fe-Ti oxide gabbros. The co-varia-tion trend of An content in plagioclase and Mg# of maficminerals for the gabbros from Hole 735B is nearly parallel tothat of the layered series of the Skaergaard Intrusion (Wagerand Brown, 1967; Naslund, 1976; Hoover, 1978), and is

characterized by a rapid decrease in Mg# of mafic mineralsfor relatively evolved gabbros with smaller variations in theAn content of plagioclase, compared with olivine gabbro andtroctolite (Fig. 4). These facts suggest that the Fe-Ti oxidegabbros were derived from the same magma which formedthe host troctolites and olivine gabbros.

The variation of mineral chemistry, reaction texture, andappearance and disappearance of minerals all indicate thatas crystallization proceeds, the liquidus mafic phases vary asfollows: orthopyroxene + clinopyroxene ± olivine, pigeo-nite + clinopyroxene ± orthopyroxene ± olivine, pigeonite+ clinopyroxene, olivine + clinopyroxene. These changes incumulus phase are similar to those observed in the Skaer-gaard Intrusion, although in Hole 735B, orthopyroxene wasan important liquidus phase before pigeonite started tocrystallize. Temporary cessation of olivine crystallization issuggested by either the complete absence of olivine (e.g., inpigeonite gabbros) or by the presence of interstitial olivine inplace of isolated cumulus grains (e.g., in interstitial olivine-bearing Fe-Ti oxide gabbros). This reaction relationship andthe disappearance of olivine has been documented experi-mentally in the system MgO-FeO-SiO2 (Bowen and Schairer,1935). The Fo content of olivine marking the disappearanceand reappearance of olivine for Hole 735B is nearly the sameas for the Skaergaard Intrusion; Fo = 50-52 and Fo =40-45, respectively. The An content in plagioclase is, how-ever, much more sodic than that of the Skaergaard Intru-sion. The common occurrence of inverted pigeonite in thecores of clinopyroxenes in interstitial olivine-bearing Fe-Tioxide gabbro suggests restricted adcumulus growth andextensive reaction between cumulus pigeonite and intersti-tial evolved melt occurred. The Mg# of mafic mineralsmarking the first appearance of pigeonite as a liquidus phaseranges from 60 to 70, which indicates that the temperature ofa magma crystallizing pigeonite or olivine pigeonite gabbroswas approximately 1050°C (Davidson and Lindsley, 1989).

Downhole Mineralogical Variationsin the Fe-Ti Oxide Gabbros

The primary minerals in the Fe-Ti oxide gabbros showsystematic downhole chemical variations consistent with thedownhole distribution of rock types (Fig. 3). The An contentin plagioclase and Mg# (100 × Mg/(Mg+Fe)) of maficminerals show positive correlations and tend to decreasefrom Unit I to the bottom of Unit IV, with two major cycles,which are characterized by progressive downward evolution(Fig. 3). These cycles are further subdivided into a fewminor cycles. The start of each cycle is marked by an abruptincrease in An content and Mg# of mafic minerals. Within acycle, those values decrease gradually until they abruptlyincrease again, and the next cycle begins (Fig. 3). The cyclesidentified in Units I to IV are:. (1) from 75 to 125 mbsf, (2)from 175to 184 mbsf, (3) from 195to 234 mbsf, (4) from 237to260 mbsf, and (5) from 262 to 271 mbsf. In spite of thesedownward repetitions of mineral chemistry, An content andMg# of mafic minerals show a steady decrease as a wholeuntil the bottom of Unit IV is reached and olivine Fe-Tioxide gabbro occurs, which contains the most sodic plagio-clase (An30) and the most iron-rich olivine (Fo30) in Hole735B (Fig. 3). Below Unit V, An content and Mg# of maficminerals tend to decrease downward as in the case of UnitsI to IV. The downhole mineralogical variations of the Fe-Tioxide gabbros are in marked contrast to those observed forthe olivine gabbros and troctolites, which are characterizedby two cycles showing a gradual downward increase in theAn content of plagioclase and the Mg# of mafic minerals(Fig. 2).

49


Table 2. Average An mol% of plagioclase and magnesium of mafic minerals in gabbros from Hole 735B.

Core, section,interval (cm)

118-735B-8D-1,45-48

12R-1, 45-4714R-1,93-9719R-5, 47-5019R-5, 55-6020R-1, 111-11421R-1-1, 82-89 OLGB21R-1-2, 82-89 FETI21R-2, 27-3221R-2, 49-5122R-3, 118-12023R-3, 33-3724R-2, 95-9724R-2, 120-12627R-1-1, 26-33 OLGB28R-1, 109-11428R-3, 72-7731R-2-1, 116-119 OLGB31R-2, 120-12234R-4, 8-1236R-4, 5-937R-3-1, 71-76 OLGB37R-3-2, 71-76 FETI37R-3, 76-7937R-3, 80-8238R-3, 85-8838R-4, 24-2840R-5, 0-440R-5, 4-843R-3, 95-10243R-4, 64-6644R-1-1, 113-120 CLGB44R-2, 16-2445R-1, 5-1246R-1-1, 48-58 OLGB46R-1-2, 48-58 FETI46R-3, 121-12846R-4, 109-11347R-2, 117-12747R-3, 50-52 OLGB47R-3, 56-6147R-3-1, 143-149 OLGB47R-3-2, 143-149 FETI48R-2, 56-6548R-2, 109-11348R-3, 112-11648R-4, 82-8449R-1,42-4649R-2, 94-10050R-2, 43-4750R-3, 62-6751R-1, 94-9951R-1, 99-10352R-1, 91-10052R-3, 121-12352R-4, 88-9453R-1,47-5453R-1, 123-12753R-2, 31-3954R-1, 131-13654R-3, 20-2454R-3, 125-12754R-5, 117-11955R-2, 101-10555R-2, 110-12055R-3, 83-8656R-2, 11-1459R-3, 70-7259R-4, 29-3560R-1, 18-2062R-4, 21-2471R-2, 82-8472R-5, 123-12573R-3, 73-7573R-4-1, 82-87 OLGB

Depth(mbsf)

31.2939.9251.9883.0283.1285.8990.4790.4791.0691.3899.00

102.61107.94108.20121.80127.47129.54145.94145.%163.23174.80180.13180.13180.18180.23184.25184.88195.45195.49209.45210.23212.13212.37216.07221.47221.47224.68225.83228.52229.20229.26230.10230.10232.80233.28234.53235.51236.34237.21239.34240.82244.03244.09248.93251.60252.67253.45254.12254.55259.25260.84261.81264.39267.50267.62268.81271.52288.73289.65290.68304.30353.01362.15369.02370.23

Rock type

Ol gabbroGabbronoriteInst of gabbro?GabbronoriteGabbronoritePI wehrliteOl gabbroInst of gabbroPig gabbroOl gabbroOl gabbroPig gabbroGabbronoritePig gabbroOl gabbroPig gabbroPig gabbroOl gabbroOl gabbroInst ol gb +rDiss ox ol gbOl gabbroDiss ox ol gbOl pig gabbroOl pig gabbroOl pig gabbroOl pig gabbroOl gabbronortDiss ox ol gbOl gabbronortDiss ox ol gb.01 gabbroPig gabbroOl pig gabbroOl gabbroDiss ox ol gb?Diss ox ol gbPig gabbroInst of gabbroOl gabbroInst ob gb +rOl gabbroInst of gabbroInst of gabbroOl feti microgbInst of gabbroInst of gabbroInst of gabbroInst ol gb +rInst of gabbroInst of gabbroInst of glabbroInst of glabbroInst of gb +rInst of gabbroInst of gabbroInst of gabbroInst of gabbroInst of gabbroOl feti gabbroOl feti gabbroOl feti gabbroInst of gabbroInst of gabbroOl feti gabbroOl feti gabbroOl feti gabbroOl gabbroOl gabbroOl gabbroOl gabbroOl gabbroOl gabbroOl gabbroOl gabbro

Fo(mol%)

53.73

75.4373.81

75.50

54.6760.17

53.7559.35

54.1160.3059.0059.1658.2680.40

53.53

58.%52.79

56.6579.8349.9076.5751.67

46.9851.4648.5247.8957.5752.1154.81

53.8547.6546.01

39.4043.63

30.7931.4133.0541.3939.8939.0437.4133.9469.1970.8872.1471.4375.0178.0477.9477.88

FoSD

0.95

1.121.61

0.40

1.362.87

0.313.07

0.950.881.851.820.500.52

1.17

0.561.52

1.430.440.540.520.28

2.301.131.760.741.021.682.28

1.322.141.11

1.850.52

0.610.821.102.293.120.921.781.770.731.251.230.511.920.180.520.41

No.ol

0000040004500000040670240066

10101030408

110475420844366406

10120630

136

127756898557486

An(mol%)

61.2342.5138.3642.7643.7638.0456.9538.3336.2359.2059.1736.7433.8736.2459.1233.8035.4959.2557.7542.6341.3461.8641.3739.2938.9038.1838.5143.7442.9941.4543.4464.2836.1539.3158.9741.4041.1737.9439.7063.9738.8261.8936.0436.2537.4437.0437.5637.8537.9838.5936.6437.7737.1936.5536.5435 .%34.1533.6534.0231.1530.1232.8037.8435.1133.4235.2730.2256.0558.7957.8962.3858.8462.1766.5560.67

AnSD

;;

;

.92

.09(.88>.Ol.48.47.57.23.68.78

J.60.89.03

0.401.182.282.421.072.581.641.681.980.973.152.350.511.963.672.311.371.890.291.101.380.481.071.930.861.791.301.300.651.000.861.710.291.361.212.082.630.790.810.242.100.631.192.431.561.261.942.672.071.662.231.180.560.851.644.723.684.622.730.496.482.06

No.Pi

8635654554656465646

138327856689

123472

111266883

10875

1287

11647

10198876

114

107968

12111010464

156

XMgcpx

84.7068.7369.1271.6073.5170.7880.2269.9370.1083.1281.3169.4065.4269.6681.4468.2569.7584.2382.7869.2774.3284.%70.7870.3069.2268.4770.9073.8472.2071.4671.8485.9267.5268.0183.5372.3870.2670.3670.7685.1769.4685.4769.1468.0966.5569.5867.8268.3170.0068.8168.5067.5067.7567.8367.2063.8462.8063.1863.7757.0355.4958.2565.7961.9463.2060.7759.5977.9780.6278.8680.9980.8183.8184.5582.95

XMgcSD

0.601.101.612.342.571.351.111.050.860.791.881.232.920.830.881.171.270.552.471.112.060.642.081.931.311.551.501.781.361.691.670.450.611.71

0.911.492.081.780.461.050.531.471.741.901.770.981.101.611.070.880.501.031.150.801.241.370.860.781.032.691.532.132.082.891.451.901.082.360.311.441.830.531.000.72

No.cp

31468344339797464426

156345

1356449

1534419

11494

1037587

15967677

12218

115556

127686

117

124574

275

XMgopx

60.7959.4164.9768.0463.46

61.8060.52

78.1564.5456.2860.69

59.5158.52

73.7560.8365.24

63.2364.4360.6359.3062.7866.1862.6566.6066.51

58.3262.13

63.25

61.1363.57

61.14

57.8160.5355.0759.9759.0159.0561.9260.0059.5458.0359.8657.9557.2153.83

51.9353.4347.5342.6148.4456.9051.0657.9551.9845.8172.0374.1373.6474.8976.84

79.86

XMgoSD

1.920.910.561.380.82

0.440.52

1.162.901.02

1.190.76

0.620.12

0.171.900.830.700.170.35

2.420.82

1.941.14

0.94

1.24

1.84

1.68

1.250.930.692.360.581.122.030.731.060.811.10

0.830.410.450.231.441.562.42

0.131.881.451.270.161.85

0.61

No.op

074854035017340360092032434515304504031060151243345934220224334311284322070

50


Table 2 (continued).


73R-4-2, 82-87 FETI73R-5-1, 72-78 OLGB73R-5-2, 72-78 FETI73R-7-1, 0-5 OLGB74R-6, 27-3576R-1-1, 63-70 OLGB76R-1-2, 63-70 FETI76R-1,70-73 FETI76R-1, 99-110 FETI76R-3-1,35-41 OLGB76R-3-2, 35-41 FETI76R-4, 12-1977R-2, 5-877R-2, 101-10779R-2-1, 55-61 OLGB79R-7-1, 2-9 OLGB79R-7-2, 2-9 FETI79R-7, 99-10280R-3, 67-7180R-6, 130-13580R-7, 10-1880R-7, 23-2581R-5, 1-781R-7, 64-6682R-3-1, 0-5 OLGB82R-3-2, 0-5 FETI83R-2, 3-983R-7, 77-8186R-4, 123-13086R-6, 143-145

Depth(mbsf)

370.23371.58371.58373.56382.13394.69394.69394.74395.08397.06397.06398.20404.96405.89416.49422.82422.82423.74427.35431.98432.23432.32433.54442.33445.72445.72453.86460.84486.60489.57

Rock type

GabbronoriteOl gabbroGabbronoriteOl gabbroPoik gabbnortOl gabbroGabbronoriteGabbronoriteGabbronoriteOl gabbroOl gabbronortGabbronoriteDiss ox ol gbOl gabbronortOl gabbroOl gabbroOl gabbronortTroct. microgbOl feti gabbroOl feti gabbroOl feti microgbOl feti microgbOl gabbroOl gabbroOl gabbroGabbronoriteTroctoliteTroctoliteOl feti gabbroInst of gabbro

Fo(mol%)

74.13

77.15

76.1063.64

75.8658.5559.9757.8956.8765.3582.0334.0178.0735.5836.1334.5539.1777.8676.8380.6143.2282.5884.2042.9041.40

FoSD

0.57

0.48

1.080.54

0.591.810.040.770.720.150.620.770.961.401.111.410.640.450.730.491.150.140.740.930.04

No.ol

0306033005727948596765

107438

1162

An(mol%)

28.0959.0631.8066.5441.2461.7742.5347.4137.3560.3141.7442.4343.1540.6452.4774.0129.4968.5632.1429.5334.1136.6964.4060.4964.8230.0266.8572.4436.0338.17

AnSD

2.002.202.042.432.842.593.164.152.191.930.982.260.811.501.335.071.373.441.050.701.271.534.533.002.010.792.604.071.347.42

No.Pi

5656

10479

1366

13374

126

285986

166939

1096

XMgcpx

62.4784.6867.73

69.4483.8071.8472.9773.1684.0770.6371.3873.0171.4280.9488.0557.3485.3761.1258.3761.3963.6784.7082.9987.4556.9386.5888.9164.7161.51

XMgcSD

1.090.722.41

0.961.600.581.702.05

(

.74

.32

.19

.21

.02).74.12.64

2.782.552.021.341.240.852.610.742.660.461.211.333.18

No.cp

35809457

1525

114

1545855674

265796843

XMgopx

51.89

60.43

63.4381.2663.3466.1763.49

64.3465.55

64.68

53.1348.4150.8753.4479.80

47.15

86.5755.4350.74

XMgoSD

2.89

0.34

0.54

1.561.342.39

1.12

1.11

1.820.84

0.05

2.12

0.161.582.15

No.op

40205139

1601

100500002712100

100244

Note: The standard deviations and number of analyses for each specimen are also shown.Abbreviations are: SD, standard deviation; Mg#, Mg/(Mg+Fe); cpx or cp, clinopyroxene; XMgc, Mg# of clinopyroxene; XMgo, Mg# of orthopyroxene; opx or op,

orthopyroxene; No., number of analyses. Other abbreviations are the same as in Table 1. Contacting olivine gabbro and Fe-Ti oxide gabbro are indicated by"OLGB" and "FETI" in the interval column. Depth in meter below sea floor.

Minor Element Abundance in Constituent Minerals

Clinopyroxene

The TiO2 content of clinopyroxene in olivine gabbro in-creases as the An content in plagioclase and Mg# of maficminerals decrease (Fig. 6A). Clinopyroxene in troctoliticmicrogabbros exhibit a wide range in TiO2 from 0.5 to 1.4wt%, with averages for individual samples as high as 1.1 wt%.Clinopyroxenes in coarse troctolites exhibit a similar range inTiO2 as those in olivine gabbros but on average individualsamples have lower TiO? contents (<0.6 wt%). The TiO2content of clinopyroxene in the Fe-Ti oxide gabbros tends todecrease as An content in plagioclase and Mg# of maficminerals decrease (Fig. 6A). TiO2 contents in clinopyroxenecores reach a maximum in pigeonite gabbros and interstitialolivine-bearing Fe-Ti oxide gabbros, which are rich in Fe-Tioxides. TiO2 decreases remarkably as the Mg# decreases ininterstitial olivine-bearing Fe-Ti oxide gabbros and olivineFe-Ti oxide gabbros. Gabbronorite, olivine gabbronorite, anddisseminated Fe-Ti oxide olivine gabbro, all of which arerelatively poor in Fe-Ti oxide, show a slight increase in TiO2content of clinopyroxene as An content in plagioclase de-creases.

For olivine gabbro and troctolite, the TiO2 content ofclinopyroxene markedly increases from the core to the rim,whereas in Fe-Ti oxide gabbro, the TiO2 content is almostconstant or decreases slightly. This change in the TiO2 zoningpattern and the variation in TiO2 content with Mg# can beexplained by the crystallization of Fe-Ti oxides, which beganduring the crystallization of the gabbronorite and reached a

maximum during the crystallization of pigeonite gabbro orinterstitial olivine-bearing Fe-Ti oxide gabbro.

The Na2O content of clinopyroxenes increases as the Mg#decreases from olivine gabbro to pigeonite gabbro (Fig. 6B). Afurther decrease in Mg# of clinopyroxene leads to a slightdecrease in Na2O content. The Na2O content of clinopyrox-ene, consequently, reaches a maximum in the pigeonite gab-bro, whose grain size is the largest among the Fe-Ti oxidegabbros. There is a consistent relationship between Na2Ocontent and the grain size of clinopyroxene; the Na2O contentincreases with an increase in grain size. This relationshipsuggests that the Mg#-Na2O relationship of clinopyroxene iscontrolled not only by magma composition but by sub-liquidusprocesses, such as adcumulus growth or reaction with trappedmelt. The Na2O content tends to be high in troctolite ascompared to that for olivine gabbros.

The A12O3 content of clinopyroxene decreases steadilyfrom troctolite to olivine Fe-Ti oxide gabbro as the Mg# ofclinopyroxene decreases; the clinopyroxene of the troctolitecontains 3 to 4 wt% A12O3, whereas that of the olivine Fe-Tioxide gabbro contains only 1.0 to 1.6 wt% A12O3 (Fig. 7A).This decrease in A12O3 content is probably due to the fraction-ation of plagioclase in each batch of magma, which is consis-tent with the decrease in A12O3 content in MORB glass withthe decrease in MgO content (BVSP, 1981). Clinopyroxene inolivine gabbros showing vermicular intergrowth with ortho-pyroxene tends to be poor in A12O3.

The MnO content in clinopyroxene shows a steady in-crease from troctolite to olivine Fe-Ti oxide gabbro, as Mg#sdecrease; from 0.1 wt% for clinopyroxenes in troctolites to 0.5wt% for those in olivine Fe-Ti oxide gabbros (Fig. 7B). The

51


Table 3. Average chemical composition of olivine in gabbros from Hole 735B.


118-735B-

76R-4, 12-19

40R-5, 0-4

43R-3, 95-102

76R-3, 34-41 CT

77R-2, 101-107

36R-4, 5-9

37R-3, 71-76 CT

40R-5, 4-8

43R-4, 64-66

46R-3, 121-128

77R-2, 5-8

20R-1, 111-114

37R-3, 76-79

38R-4, 24-28

45R-1.5-12

47R-2, 117-127

47R-3, 143-149 CT

48R-3, 112-116

48R-4, 82-84

49R-1, 42-46

50R-2, 43-47

50R-3, 62-67

51R-1, 94-99

52R-3, 121-123

53R-1, 47-54

53R-1, 123-127

54R-5, 117-119

55R-2, 101-105

34R-4, 8-12

47R-3, 56-61

49R-2, 94-100

52R-1, 91-100

54R-1, 131-136

54R-3, 20-24

54R-3, 125-127

55R-2, 110-120

55R-3, 83-86

Rock type

Gabbronorite

Olivine gabbronorite




Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Interst ol ox gabbro I













Interst ol ox gabbro II




Olivine oxide gabbro





SiO2

36.160.19

36.740.12

36.390.42

36.400.26

36.260.16

36.760.17

36.150.23

36.520.33

36.510.23

35.990.25

36.500.22

36.180.18

36.080.56

36.130.16

35.950.18

36.220.18

36.400.57

35.800.13

35.100.17

35.410.38

36.120.33

35.990.25

36.080.24

35.060.20

34.540.43

34.750.20

34.290.53

34.100.29

36.040.19

35.440.24

36.570.23

35.110.36

32.950.14

33.390.33

33.000.31

34.070.21

34.170.29

FeO

35.870.07

35.120.44

35.371.43

35.411.47

37.090.76

35.311.46

39.690.52

35.521.46

36.030.39

39.231.10

36.130.64

39.290.54

36.061.17

39.080.68

39.001.27

37.011.79

36.882.87

40.981.64

42.211.06

41.190.45

39.961.06

38.671.85

39.301.12

44.150.75

48.500.89

45.821.08

46.651.33

46.871.94

38.170.86

41.770.66

37.020.89

43.561.32

52.160.40

54.030.88

51.230.49

48.510.81

49.770.93

Olivine

MnO

0.5480.1050.5850.0610.6170.0490.5980.0490.6510.032

0.6910.0380.6450.0500.6260.0670.5910.0380.6730.0610.6190.031

0.6800.0160.6920.0350.6460.0480.7240.093

0.6990.0530.5790.0580.6880.0600.6280.0460.6660.0820.6700.0380.6340.0730.7050.0800.7130.0260.8790.0830.8770.1090.8270.0770.8830.048

0.6810.0690.7%0.0510.6350.0400.6960.055

1.1590.0441.1250.1051.0580.0390.9430.0470.8570.036

MgO

30.160.00

29.520.28

28.571.16

28.070.95

27.440.33

28.851.06

25.880.01

28.901.06

28.210.37

24.590.99

27.670.36

25.610.65

26.580.22

25.860.64

25.200.50

27.120.30

27.682.15

24.360.32

22.331.02

23.010.43

24.641.07

26.321.24

25.730.71

21.110.65

17.711.08

19.900.22

18.501.23

17.481.57

25.840.86

23.340.20

28.110.65

21.911.01

13.190.20

12.931.22

14.150.49

17.430.39

16.700.95

CaO

0.0310.0170.0380.0220.0430.0090.0380.0110.0290.016

0.0410.0150.0550.0220.0370.0230.0460.0150.0460.0230.0740.019

0.0650.0420.0510.0190.0450.0070.0560.023

0.0540.0170.0370.0200.0590.0100.0460.0230.0540.0060.0480.0110.0470.0320.0510.0150.0310.0270.0370.0260.0300.0260.0690.0190.0780.014

0.0510.0100.0550.0220.0450.0140.0660.026

0.0610.0310.0930.0310.0680.0210.0640.0060.0710.010

NiO

0.0130.0260.0210.0360.0060.012

0.0110.0110.0330.0230.0160.027

0.0500.0420.0250.029

0.0100.0130.0240.0270.0300.0300.0080.0090.0130.023

0.0090.0110.0010.0010.0160.020

0.0080.0090.0020.0040.0070.016

0.0190.027

0.0330.0400.0340.028

0.0060.0110.0070.0090.0230.0210.0030.0060.0120.013

52




56R-2, 49-52

56R-2, 49-52

80R-3, 67-71

80R-6, 130-135

86R-4, 123-130

48R-2, 109-113

80R-7, 10-18

80R-7, 23-25

83R-7, 77-81

83R-2, 3-9

79R-7, 99-102

80R-1, 41-45

73R-3, 73-75

81R-5, 1-7

71R-2, 82-84

81R-7, 64-66

62R-4, 21-24

72R-5, 123-125

59R-4, 29-35

21R-2, 49-51

60R-1, 18-20

59R-3, 70-72

Rock type






01 oxide microgabbro



Troctolite

Troctolite

Troctolite

01 gabbro-Troctolite

Vermic cpx ol gabbro




Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

SiO2

33.870.29

33.570.11

33.590.14

34.000.21

34.360.30

35.320.37

33.660.22

33.740.37

40.610.14

40.180.20

39.260.27

39.790.23

39.470.19

39.370.16

38.820.11

38.950.24

38.740.10

39.780.19

38.560.26

38.440.41

38.280.26

38.250.06

FeO

51.291.03

51.940.44

50.691.47

50.681.00

45.791.37

43.521.47

51.801.03

47.710.46

15.190.68

16.750.15

20.300.87

17.490.50

20.540.52

20.580.40

22.050.23

21.240.61

26.240.54

20.810.39

26.450.96

22.040.60

25.650.58

27.900.61

Olivine

MnO

0.9980.0341.0410.0341.1470.1191.1810.0410.8590.061

0.7050.0660.9840.0700.7210.057

0.2380.0280.2760.0510.3570.0400.3010.029

0.3020.0280.2940.0310.3360.0280.2950.031

0.4150.0470.3170.0240.4140.0420.3250.0470.3890.0420.4110.049

MgO

15.120.89

14.740.20

15.710.55

16.090.55

19.290.29

21.661.26

15.100.57

17.400.28

45.410.50

44.560.22

40.560.99

43.130.40

40.700.30

40.610.43

39.060.41

39.870.43

36.500.34

41.490.35

36.130.96

37.901.21

35.781.04

35.140.51

CaO

0.0550.0120.0470.0100.0690.0230.0670.0170.0550.031

0.0510.0130.0510.0180.0520.013

0.0370.0110.0540.0210.0570.0270.0420.011

0.0390.0150.0390.0110.0390.0100.0330.019

0.0620.0080.0580.0230.0470.0160.0340.0110.0390.0070.0430.014

NiO

0.0160.0170.0100.021

0.0360.021

0.0260.035

0.2060.032

0.1520.042

0.0890.0330.0930.0320.0%0.0390.0600.015

0.0820.0290.0940.0450.0620.0200.0700.028

Note: Standard deviations are also shown below the average of oxide weight percent.Abbreviations are: dissem ox ol gabbro, disseminated Fe-Ti oxide olivine gabbro; ol pigeonite gabbro,

olivine pigeonite gabbro; interst ol ox gabbro I, interstitial olivine-bearing Fe-Ti oxide gabbro;olivine oxide gabbro, olivine Fe-Ti oxide gabbro; ol oxide microgabbro, olivine Fe-Ti oxidemicrogabbro; ol gabbro, olivine gabbro; vermic cpx ol gabbro, vermicular clinopyroxene olivinegabbro; CT, near contacts between Fe-Ti oxide gabbro and olivine gabbro.

MnO content in both olivine and orthopyroxene also increasesas Mg#s decrease.

The Cr2O3 content markedly decreases as the Mg# ofclinopyroxene decreases as rock types change from troctoliteto olivine gabbro (Fig 7C). The content of Cr2O3 is less than0.1 wt% for all of the Fe-Ti oxide gabbros. This remarkabledecrease can be explained by early-stage fractionation ofchromian spinel and clinopyroxene, as observed in troctoliteand magnesian olivine gabbros (Robinson, Von Herzen, et al.,1989).

Orthopyroxene

As in the case of clinopyroxene, the A12O3 content oforthopyroxene decreases as its Mg# decreases (Fig. 8B). Inolivine gabbro, orthopyroxenes contain up to 2.0 wt% A12O3,whereas they contain less than 0.5 wt% A12O3 in the mostevolved olivine Fe-Ti oxide gabbro. The MnO content oforthopyroxene increases steadily from 0.2 wt% in troctolites

to 1.0 wt% in olivine Fe-Ti oxide gabbros as the Mg#decreases (Fig. 8C). The TiO2 content of orthopyroxeneshows a similar variation to that in clinopyroxene; as theMg# decreases, it reaches a maximum in pigeonite gabbrosand olivine pigeonite gabbros (up to 0.4 wt%) and thendecreases in interstitial olivine-bearing Fe-Ti oxide gabbrosand olivine Fe-Ti oxide gabbros (Fig. 8A). In the olivineFe-Ti oxide gabbros the TiO2 content is less than 0.2 wt%.The Cr2O3 contents of orthopyroxenes show the samevariation as those in clinopyroxenes; decreasing from 0.25wt% in troctolites to less than 0.1 wt% in Fe-Ti oxidegabbros.

Plagioclase

Plagioclase in Fe-Ti oxide gabbros commonly appearsdusty in thin section in plane light (PI. 1, Fig. 2). The dustyappearance is due to the abundance of tiny exsolution lamellaeof opaque iron oxides, suggesting that the plagioclase in Fe-Ti

53


Table 4. Average chemical composition of plagioclase in gabbros from Hole 735B.


118-735B-

12R-1,45-47

19R-5, 47-50

19R-5, 55-60

74R-6, 27-35

76R-4, 12-19

40R-5, 0-4

43R-3, 95-102

76R-3, 34-41 CT

77R-2, 101-107

36R-4, 5-9

37R-3, 71-76 CT

40R-5, 4-8

43R-4, 64-66

46R-3, 121-128

77R-2, 5-8

20R-1, 111-114

37R-3, 76-79

37R-3, 80-82

38R-3, 85-88

38R-4, 24-28

45R-1, 5-12

21R-2, 27-32

23R-3, 34-37

24R-2, 120-126

28R-1, 109-114

28R-3, 72-77

44R-2, 16-24

46R-4, 109-113

14R-1, 93-97

21R-1, 82-89 CT

47R-2, 117-127

47R-3, 143-149 CT

48R-2, 56-65

48R-3, 112-116

48R-4, 82-84

49R-1, 42-46

50R-2, 43-47

Rock type

Gabbronorite

Gabbronorite

Gabbronorite

Gabbronorite

Gabbronorite





Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro










SiO2

58.570.29

58.400.81

58.320.35

58.620.60

56.740.50

57.410.80

57.800.57

58.150.48

58.290.27

58.100.32

58.950.28

57.590.57

57.620.57

58.740.60

58.460.54

59.300.67

58.490.75

58.230.76

57.450.24

58.710.47

58.890.40

60.130.38

58.710.48

59.550.26

59.950.41

59.530.61

59.240.35

59.760.49

59.171.11

60.010.14

58.560.24

59.230.32

60.060.34

59.640.16

59.550.59

59.400.48

60.170.97

A12O3

26.290.27

26.160.59

27.140.16

26.050.32

26.360.41

26.490.61

26.040.17

26.020.27

26.080.26

26.600.21

26.520.04

26.310.29

26.490.43

26.200.37

26.480.22

25.590.43

26.120.40

25.670.35

25.240.11

25.800.30

25.660.26

25.710.08

25.540.27

25.450.15

25.120.35

25.520.58

24.880.28

25.360.24

25.790.69

25.580.16

26.070.14

25.690.19

25.580.23

25.670.06

25.370.32

25.410.44

25.770.43

Plagioclase (wt%)

Fe2O3

0.1900.0520.1890.0450.1590.0560.3010.1200.3220.134

0.2230.0660.1710.0320.1790.0530.1950.043

1.1830.1220.1170.0970.2490.2140.1790.0450.2260.1050.1730.072

0.1050.0510.1980.1440.1360.0420.2310.0590.1260.0500.2070.064

0.2080.0790.2060.0490.1490.0690.1760.0530.1660.0380.1380.0490.2390.203

0.1490.0870.1320.0960.1490.0770.2050.0890.2110.0470.1410.0420.1670.0790.2310.0920.2100.088

CaO

8.400.168.190.728.960.298.170.528.580.38

8.720.678.260.258.310.168.190.31

8.410.368.290.108.520.408.840.408.290.408.590.58

7.470.298.010.597.760.487.690.027.700.437.780.30

7.410.477.770.277.420.166.930.487.270.467.370.287.520.07

7.620.807.610.147.950.327.540.217.300.227.500.147.470.277.520.267.650.52

Na2O

6.200.236.670.426.290.266.350.406.490.21

6.130.466.380.206.340.186.550.24

6.560.236.350.186.180.306.310.216.480.206.200.13

6.740.216.790.436.690.266.830.246.720.206.550.21

7.100.236.890.217.150.147.420.297.240.277.050.156.710.22

6.670.386.730.366.570.357.220.247.010.116.940.066.800.266.740.156.690.30

K2O

0.1160.0210.0600.0390.1120.0240.1380.0190.0880.023

0.1070.0120.0980.0140.1110.0150.0990.018

0.0550.0320.0990.0420.1080.0180.0830.0140.1040.0360.0930.020

0.0720.0350.0760.0340.0780.0500.0970.0280.1050.0400.1440.013

0.1550.0310.1000.0440.0900.0270.1370.0540.1720.0190.2140.0760.1340.020

0.1290.0500.0730.0530.1350.0160.0990.0410.1350.0230.1550.0210.0940.0410.1220.0360.0970.044

54




5OR-3, 62-67

51R-1, 94-99

51R-1, 99-103

52R-3, 121-123

52R-4, 88-94

53R-1, 47-54

53R-1, 123-127

53R-2, 31-39

54R-5, 117-119

55R-2, 101-105

82R-6, 73-78

34R-4, 8-12

47R-3, 56-61

49R-2, 94-100

52R-1,91-100

54R-1, 131-136

54R-3, 20-24

54R-3, 125-127

55R-2, 110-120

55R-3, 83-86

56R-2, 11-14

56R-2, 49-52

80R-3, 67-71

80R-6, 130-135

86R-4, 123-130

48R-2, 109-113

80R-7, 10-18

80R-7, 23-25

83R-7, 77-81

83R-2, 3-9

79R-7, 99-102

80R-1, 41-45

73R-3, 73-75

81R-5, 1-7

71R-2, 82-84

81R-7, 64-66

62R-4, 21-24

Rock type


























Ol oxide microgabbro



Troctolite

Troctolite

Troctolite

Troctolite-01 gabbro





Olivine gabbro

SiO2

59.230.65

59.320.68

60.020.32

59.710.41

60.250.43

61.160.38

60.670.44

59.560.69

59.350.45

59.910.60

59.820.59

58.410.67

59.130.35

59.150.66

58.980.46

60.920.55

61.670.87

59.800.64

60.270.22

59.940.27

62.250.44

60.710.31

61.070.35

61.080.36

59.630.21

59.860.37

59.830.25

59.330.51

50.771.06

52.020.86

51.050.87

51.631.20

52.141.41

52.391.15

53.330.73

52.770.63

53.640.98

A12O3

25.160.26

25.280.06

25.500.16

25.600.17

25.160.36

24.980.31

24.960.27

24.890.35

25.370.53

25.120.40

25.310.38

26.160.34

25.740.26

25.700.44

25.790.49

24.150.29

24.380.44

24.650.31

24.980.07

25.270.20

24.480.19

24.560.28

24.330.32

24.310.16

25.250.23

25.610.36

25.150.27

25.140.15

31.700.70

30.740.51

30.770.46

30.580.92

30.091.02

30.190.61

29.030.59

29.530.60

21.900.77

Plagioclase (wt%)

Fe2O3

0.4080.6670.1900.0750.2290.1080.1340.0430.2620.1660.1660.0550.1660.0930.2860.2690.1800.0470.1730.0720.1540.052

0.2720.1010.2430.2070.2910.1850.2440.139

0.2810.1760.1810.0940.2420.1110.1620.0390.1870.0640.2270.1260.2250.0500.1690.0410.2290.0510.2610.217

0.1820.0540.1850.0910.1400.054

0.1060.0440.0730.0480.1310.1250.1260.073

0.1910.0520.1730.1200.2360.550.0900.035

0.2850.042

CaO

7.200.127.560.147.380.177.510.107.010.396.710.396.630.346.710.257.590.397.080.437.250.35

8.410.337.750.247.580.457.540.39

6.080.235.940.536.550.386.710.317.070.136.120.166.220.436.360.195.923.277.310.28

7.580.307.030.257.360.25

14.660.93

13.290.54

13.750.60

13.600.91

13.361.11

12.910.73

11.990.58

12.220.58

12.560.87

Na2O

6.780.226.630.426.800.117.150.137.030.207.060.377.130.197.290.236.820.167.140.306.890.26

6.190.366.660.226.740.267.170.33

7.470.157.460.277.360.287.280.257.080.147.680.207.360.277.300.257.690.247.060.23

6.670.727.430.256.970.29

3.060.443.680.373.470.423.500.46

3.710.603.930.564.590.294.400.35

4.150.54

K2O

0.1440.0190.1610.0250.1380.0160.0940.0340.1010.0530.1430.0300.1330.0340.1150.0410.1130.0370.1430.0410.1240.026

0.1090.0400.1340.0180.1530.0270.1100.075

0.1910.0810.2400.0430.0980.0440.1690.0190.1370.0450.1150.0650.1410.0550.1850.0380.1810.0190.1710.037

0.1030.0370.1150.0360.0830.025

0.0270.0090.0250.0170.0270.0110.0420.021

0.0380.0160.0420.0240.0720.0190.0260.007

0.0570.011

55




72R-5, 123-125

59R-4, 29-35

8D-1,45-48

22R-3, 118-120

21R-2, 49-51

60R-1, 18-20

31R-2, 120-122

59R-3, 70-72

Rock type

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

SiO2

52.420.23

54.291.32

53.270.55

53.540.62

53.700.13

53.640.92

54.320.39

55.000.38

A12O3

30.060.09

29.270.80

29.730.33

29.020.36

28.740.18

28.790.70

29.020.35

28.700.32

Plagioclase (wt%)

Fe2O3

0.2400.0510.3060.0770.1380.0810.1370.0490.1230.0500.3060.0870.1260.0350.2810.090

CaO

12.490.22

11.980.98

12.380.39

11.780.47

11.560.11

11.720.72

11.510.50

11.150.29

Na2O

4.160.074.590.524.320.234.470.314.460.324.670.424.630.314.790.21

K2O

0.0560.0090.0740.0290.0230.0240.0300.0170.0290.0250.0590.0200.0430.0230.0640.014

Note: Standard deviations are also shown below the average of oxide weight percent. Abbreviations arethe same as in Table 3.

oxide gabbros contains abundant iron, which exsolved as ironoxide during cooling. This is reflected in the plagioclaseanalyses; in spite of erratic FeO content (because the EPMAanalyses were made with a focused beam), it increases as theAn content decreases. The K2O content in plagioclase in-creases from 0.02 wt% to 0.2 wt% as An content decreases(Fig. 9).

Olivine

The MnO content in olivine steadily increases as a linearfunction of Fo mol% (Fig. 10B), from 0.2 wt% in troctolites to1.4 wt% in olivine Fe-Ti oxide gabbros. The MnO variation isquite similar to that in pyroxenes. The NiO content decreasesexponentially as Fo content decreases (Fig. 10A), from 0.2wt% in troctolites to less than 0.04 wt% in all the Fe-Ti oxidegabbros. The variation is due to the fractionation of olivineinto which Ni is strongly partitioned compared to silicate melt.

Amphibole

Reddish brown to pale brown amphibole is always pre-sent as a minor phase occurring in interstitial areas withFe-Ti oxides or as inclusions in clinopyroxene. It shows thechemical variations consistent with other silicate phases.This fact and the difference in its mode of occurrence fromobviously metamorphic amphiboles suggest that amphiboleis magmatic.

The TiO2 contents of amphiboles show the same variationpattern as in pyroxenes; as Mg# of amphibole decreases, itfirst increases from troctolite (1 wt%) to gabbronorite (2-4wt%) and reaches a maximum (up to 5 wt%) for pigeonitegabbro or interstitial olivine-bearing Fe-Ti oxide gabbro,followed by a rapid decrease in olivine Fe-Ti oxide gabbro(4-1 wt%). The K2O content of amphibole tends to increasevery slightly as the Mg# decreases. It reaches a maximum inpigeonite gabbros and interstitial olivine bearing gabbros(0.5-1 wt%) and decreases slightly in olivine Fe-Ti oxidegabbros. The Na2O contents in amphiboles increase as Mg#sdecrease, and is highest in pigeonite gabbros. The Cr2O3contents of amphiboles decrease exponentially with decreas-ing Mg#, from 2 wt% in troctolites to less than 0.1 wt% inFe-Ti oxide gabbros. MnO contents in amphiboles increasesteadily as Mg#s decrease down to 0.4 wt% in olivine Fe-Tioxide gabbros. A12O3 contents in amphiboles decrease withdecreasing Mg#.

IlmeniteThe MgO content of ilmenite in Fe-Ti oxide gabbros tends

to decrease from 2-3 wt% in disseminated Fe-Ti oxide olivinegabbros and olivine gabbronorites to less than 1 wt% in olivineFe-Ti oxide gabbros as the Mg# of mafic minerals and Ancontent of plagioclase decrease.

RELATIONSHIPS BETWEEN FE-TI OXIDEAND OLIVINE GABBROS

In Hole 735B, Fe-Ti oxide gabbros are commonly foundadjacent to and cross-cutting olivine gabbros with sharpcontacts between the two (PI. 3, Figs. 1 and 2). These contactsare marked by abrupt changes in modal mineralogy includingthe appearance of Fe-Ti oxides in the Fe-Ti oxide gabbro,changes in clinopyroxene morphology, and changes in mineralchemistry (PI. 3).

Fe-Ti oxide gabbros tend to be more deformed more thanthe olivine gabbros, possibly because of the presence ofabundant Fe-Ti oxides which are more easily deformed thanthe silicates. For example, centimeter-thick bands of Fe-Tioxide gabbro are porphyroclastic or mylonitic, whereas thehost olivine gabbro shows only minor deformation marked bythe presence of wavy extinction in plagioclase and kink bandsin olivine. In one such sample, element concentration mapsindicate a sharp contact that has not been affected by plasticdeformation at all, clearly indicating that deformation tookplace after the two gabbros were juxtaposed by a magmaticprocess.

The contacts are generally gently inclined or horizontal,but vertical or steeply dipping contacts are also present, (e.g.,Sample 118-735B-82R-3, 0-9 cm). About 1-cm to 1-mm thickveins of Fe-Ti oxide gabbro are observed locally to branch outfrom thicker Fe-Ti oxide gabbros (e.g., Samples 118-735B-82R-3, 0-9 cm and 47R-3, 143-149 cm). Map analyses of theseboundaries indicate that Mg#s of mafic minerals and Ancontent in plagioclase show marked variations within the hostolivine gabbro, whereas they are fairly homogeneous in theFe-Ti oxide gabbro. Across the contacts, there are very steepgradients in chemical plagioclase composition which occurwithin a few mm of the contacts, (Fig. 11) but more gradualgradients in the Mg# of mafic phases which occur over adistance of several centimeters. Similar gradients in mineralcompositions have been observed across contacts in othergabbros from the Southwest Indian Ridge by Meyer et al.

56


Table 5. Average chemical composition of clinopyroxene in gabbros from Hole 735B.


118-735B-

37R-3, 80-82

38R-3, 85-88

38R-4, 24-28

45R-1, 5-12

21R-2, 27-32

23R-3, 34-37

24R-2, 120-126

28R-1, 109-114

28R-3, 72-77

44R-2, 16-24

46R-4, 109-113

14R-1, 93-97

21R-1,82-89CT

47R-2, 117-127

47R-3, 143-149 CT

48R-2, 56-65

48R-3, 112-116

48R-4, 82-84

12R-1, 45-47

19R-5, 47-50

19R-5, 47-50

19R-5, 55-60

74R-6, 27-35

76R-4, 12-19

40R-5, 0-4

43R-3, 95-102

76R-3, 34-41 CT

77R-2, 101-107

36R-4, 5-9

37R-3, 71-76 CT

40R-5, 4 -8

43R-4, 64-66

46R-3, 121-128

77R-2, 5-8

20R-1, 111-114

37R-3, 76-79

Rock type

Ol pigeonite gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro








Gabbronorite

Gabbronorite

Gabbronorite

Gabbronorite

Gabbronorite

Gabbronorite





Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbro

Ol pigeonite gabbro

Ol pigeonite gabbro

SiO2

51.200.41

50.440.37

52.270.32

51.690.45

52.750.06

51.800.27

51.750.21

51.870.13

52.350.00

51.190.18

51.940.27

51.750.38

51.900.11

51.400.23

51.850.13

52.080.02

51.780.06

51.250.25

52.290.24

52.430.43

51.950.38

52.850.54

52.040.36

51.020.25

51.650.12

51.540.39

51.900.38

51.840.39

51.700.14

52.640.35

51.670.16

51.670.44

51.800.18

52.030.46

52.270.31

52.070.56

A12O3

1.9220.3081.9000.3891.7170.1212.1270.417

2.2940.1412.1850.1512.2360.2541.8380.0631.7170.0002.0680.1122.3350.023

2.2720.1592.1720.0462.4000.1112.1850.2282.1120.0092.0110.2631.9550.184

1.6820.2741.6990.1891.8200.2492.1240.6031.9650.2072.2810.147

2.3040.2402.1190.2081.9690.2141.9750.338

2.3720.2381.9570.2%2.2120.0982.3000.2092.3470.1152.2380.325

1.8970.1902.0710.301

TiO2

0.6040.1210.6660.2270.5140.0830.7730.151

0.7790.1210.7210.0510.8860.0570.7520.1150.6590.0000.8460.1280.7940.141

0.6770.0820.7050.0450.9020.0570.7080.1090.7460.0110.6690.1170.6330.069

0.4570.0840.5000.1180.4920.1190.6670.1730.7260.0880.6930.068

0.7580.1010.7260.0650.5810.0680.6110.128

0.7910.0600.6470.1580.7520.0520.7140.0670.8360.0460.7070.200

0.6250.1180.6720.208

FeO

10.880.74

11.290.88

10.510.99

12.031.15

10.080.34

10.240.39

10.310.54

11.140.33

10.560.00

10.900.33

10.320.97

10.360.59

10.320.569.800.86

10.920.53

11.730.63

11.180.54

11.000.74

10.700.548.970.34

11.140.929.021.05

10.480.26

10.060.61

9.180.85

10.250.71

10.861.209.930.48

9.150.43

10.481.44

10.170.949.720.85

10.690.859.490.67

10.%2.02

10.040.88

MnO

0.3520.0250.3170.0710.3040.0450.4260.022

0.3530.040.3410.0580.330.0050.4180.0120.33900.3610.0110.350.031

0.3170.0310.3340.0440.3180.0230.3050.0290.3540.0110.3370.050.330.056

0.3140.0390.2580.030.3580.0580.2450.1080.370.0520.3260.029

0.2810.0140.3080.0390.3580.0320.3280.05

0.320.0790.3710.0410.3070.0570.3230.0430.3410.0250.2860.02

0.3720.0230.3340.032

Clinopyroxene

MgO

13.290.15

13.620.11

14.090.36

13.860.91

13.260.31

13.430.25

13.160.16

12.910.15

13.160.00

12.780.23

13.230.07

13.240.22

13.450.06

13.510.37

13.450.31

13.300.18

13.160.06

12.770.71

13.320.21

14.050.20

13.230.59

14.020.21

13.490.33

14.120.33

14.340.28

14.250.42

13.980.43

13.920.44

14.010.26

14.230.23

14.490.44

14.120.23

13.460.55

14.380.25

13.700.47

13.810.57

CaO

20.200.66

20.000.70

20.111.37

18.511.48

20.720.52

20.420.78

20.550.24

20.420.07

20.330.00

20.270.43

19.940.13

21.010.51

20.390.096

19.950.40

19.980.57

20.070.88

20.060.36

20.010.75

21.040.29

21.570.42

20.071.38

21.050.78

20.740.21

20.210.66

20.270.43

19.481.16

18.831.07

20.780.82

20.520.30

19.981.07

19.281.38

20.500.66

19.661.31

20.101.00

19.751.84

20.150.40

Na2O

0.4280.0390.4430.0880.3930.0210.3840.096

0.6200.0260.5410.0540.5510.0600.5200.0580.5310.0000.5560.0520.5710.016

0.4230.0520.5340.0530.5590.0510.5090.0770.4440.0170.4570.0650.4210.068

0.3500.0610.3820.0390.4120.0390.4170.0670.4350.0590.4760.036

0.4010.0870.3910.0440.4020.0430.4130.054

0.4970.0380.3%0.0280.3760.1050.4400.0540.5210.0460.3970.020

0.4240.0220.4580.019

Cr2O3

0.0060.080.0000.0010.0050.0120.0090.016

0.0070.0120.0070.0070.0330.0390.0300.0080.0300.0000.0010.0020.0150.021

0.0170.0200.0110.0090.0040.0090.0040.0060.0130.0180.0300.0430.0140.015

0.0100.0120.0310.0350.0180.0160.0320.0400.0110.0160.0200.030

0.0260.0260.0230.0250.0000.0000.0130.020

0.0280.0380.0000.0000.0030.0030.0160.0180.0060.0060.0320.049

0.0310.0370.0050.005

Wo

0.4280.0140.4190.0130.4200.0270.3930.037

0.4410.0110.4340.0140.4380.0090.4340.0030.4340.0000.4350.0050.4300.008

0.4420.0070.4320.0140.4300.0120.4230.0130.4200.0170.4260.0080.4320.023

0.4390.0070.4480.0050.4250.0270.4420.0140.4350.0040.4240.015

0.4280.0090.4120.0230.4030.0260.4340.017

0.4350.0070.4170.0230.4070.0280.4300.0150.4210.0280.4230.019

0.4170.0400.4270.010

En

0.3920.0030.3970.0030.4090.0130.4090.021

0.3920.0100.3970.0090.3900.0020.3820.0000.3910.0000.3820.0010.3970.007

0.3880.0040.3970.0070.4050.0110.3960.0080.3880.0060.3890.0030.3830.016

0.3870.0050.4060.0020.3900.0160.4100.0050.3940.0060.4120.008

0.4210.0090.4190.0120.4160.0110.4040.013

0.4130.0070.4130.0100.4260.0140.4120.0070.4010.0160.4210.009

0.4020.0130.4070.011

Fs

0.1800.0120.1840.0140.1710.0170.1990.016

0.1670.0050.1700.0070.1720.0080.1850.0030.1760.0000.1830.0060.1740.014

0.1700.0100.1710.0100.1650.0130.1810.0090.1920.0110.1850.0080.1850.009

0.1740.0080.1460.0070.1840.0170.1480.0180.1720.0050.1650.009

0.1510.0140.1690.0120.1810.0180.1620.008

0.1510.0070.1700.0210.1680.0150.1590.0120.1790.0140.1560.012

0.1810.0330.1660.015

57




49R-1, 42-46

50R-2, 43-47

50R-3, 62-67

51R-1, 94-99

51R-1, 99-103

52R-3, 121-123

52R-4, 88-94

53R-1,47-54

53R-1, 123-127

53R-2, 31-39

54R-5, 117-119

55R-2, 101-105

82R-6, 73-78

34R-4, 8-12

47R-3, 56-61

49R-2, 94-100

52R-1, 91-100

54R-1, 131-136

54R-3, 20-24

54R-3, 125-127

55R-2, 110-120

55R-3, 83-86

56R-2, 11-14

56R-2, 49-52

8OR-3, 67-71

80R-6, 130-135

86R-4, 123-130

48R-2, 109-113

80R-7, 10-18

80R-7, 23-25

83R-7, 77-81

83R-2, 3-9

79R-7, 99-102

73R-3, 73-75

81R-5, 1-7

71R-2, 82-84

81R-7, 64-66

Rock type































Troctolite

Troctolite

Troctolite





SiO2

51.460.30

52.170.52

51.440.16

51.590.33

51.690.16

51.730.35

51.390.24

51.980.27

51.580.55

51.150.33

52.040.17

51.470.77

52.000.46

51.560.27

51.720.29

52.350.29

51.340.35

50.800.19

51.490.11

50.800.19

51.690.14

51.770.29

51.660.18

51.530.25

51.350.24

51.460.27

52.090.20

51.910.20

51.410.44

50.990.53

52.330.09

52.600.33

51.930.47

52.890.43

52.740.45

52.510.41

52.461.20

A12O3

2.0240.1942.0730.1202.1870.2232.2090.1892.2650.1002.0610.3131.8260.1731.8310.2061.7660.1341.7%0.2261.8010.2381.4240.2662.0400.206

2.2450.2012.3170.1881.7720.3362.1690.280

1.4640.0431.1670.1891.3050.0481.7690.4661.5850.0911.4470.1801.4450.0291.4690.1801.3300.0941.7430.093

1.8980.1991.5460.2001.8430.139

3.6080.1943.3550.2613.1910.531

2.7340.2542.6730.2792.5710.2552.7680.837

TiO2

0.7160.0870.6720.1270.7550.0190.7550.1110.8380.0740.7230.1640.6670.1110.6860.0840.6970.1190.7040.1550.6170.1450.4540.1090.6830.082

0.7780.0910.8350.0790.5400.1080.7810.138

0.6400.0490.4130.0930.4780.0690.5180.1210.5700.0770.5040.1370.4930.0690.4900.0760.5190.1080.4130.129

0.6740.1000.5020.1130.5690.100

0.5760.0540.5120.0331.0860.223

0.5050.1020.5200.1210.5930.2550.5580.129

FeO

10.960.59

10.830.57

10.770.70

11.310.57

11.170.56

11.200.44

12.600.57

12.700.56

12.630.46

12.410.34

12.101.16

13.930.81

10.621.00

10.600.67

10.430.42

10.681.13

11.220.74

14.090.45

15.881.50

14.810.30

12.341.58

14.101.08

14.490.15

14.490.59

13.520.94

14.601.10

12.400.54

12.541.17

13.460.63

12.980.47

3.900.544.690.414.390.09

5.500.635.240.506.801.146.570.52

MnO

0.3690.0080.3630.0280.3580.0540.360.0360.340.0450.3370.0310.4120.0270.420.0680.4240.0440.4330.0380.3710.0590.440.0380.4040.032

0.3270.0460.3550.0290.3520.0450.3580.046

0.4460.0110.4980.0510.4930.0330.4040.0670.410.0420.4550.0110.5030.0580.5280.0490.5730.0450.3970.065

0.3440.0440.4410.0640.3850.012

0.1250.0380.1430.0420.1750.04

0.1530.030.1640.0270.1430.0370.2110.015

Clinopyroxene

MgO

13.030.19

13.120.15

12.930.37

13.010.17

13.070.13

12.950.31

12.250.32

12.280.19

12.160.08

12.260.18

12.830.25

11.840.21

13.280.056

13.110.25

13.190.09

13.840.40

12.940.28

10.780.20

10.880.52

10.860.08

12.330.75

12.160.43

11.460.75

11.470.29

11.920.46

11.470.21

12.490.08

13.560.64

12.000.17

12.200.43

16.320.46

16.971.02

15.870.20

16.460.93

16.220.53

16.501.95

17.570.62

CaO

20.320.32

20.440.46

20.090.69

20.121.00

20.280.47

20.630.40

19.950.56

20.510.61

20.000.80

19.800.53

19.550.53

19.240.29

20.710.42

20.190.59

20.650.51

19.921.19

20.230.74

19.450.99

19.220.72

19.560.42

19.700.39

19.061.07

19.021.26

19.240.64

20.130.29

19.520.98

20.200.30

18.720.87

19.870.63

19.130.98

21.830.71

20.701.19

22.360.51

21.021.49

21.391.09

19.522.66

18.530.39

Na2O

0.4770.0520.4690.0420.5130.0340.5410.0390.5020.0550.4360.0560.4370.0520.4520.0470.4150.0320.4700.0390.4210.0290.4400.0610.4410.052

0.5000.0560.5310.0330.4200.0840.5350.062

0.5360.0470.3980.0270.3890.0270.4630.0300.4360.0530.4840.0890.4890.0180.4230.0490.4700.0350.4850.061

0.4020.0620.4580.0500.4400.071

0.4900.0490.3760.0680.4420.075

0.3320.0420.3510.0520.3300.0550.4650.162

Cr2O3

0.0140.0210.0050.0100.0060.0110.0070.0100.0090.0160.0070.0150.0030.0050.0180.0210.0170.0330.0260.0240.0060.0080.0120.0140.0170.035

0.0220.0180.0060.0080.0100.0100.0250.032

0.0080.0090.0020.0050.0080.0090.0090.0130.0070.0130.0110.0160.0030.0060.0130.0140.0060.0130.0000.000

0.0160.0150.0240.0210.0110.006

1.2370.0220.8330.3740.5390.090

0.1800.0350.2150.0370.1860.0500.0970.041

Wo

0.4320.0080.4340.0100.4320.0180.4280.0180.4300.0100.4350.0090.4260.0140.4320.0110.4280.0120.4250.0090.4170.0120.4130.0090.4360.012

0.4320.0140.4380.0100.4190.0250.4310.17

0.4280.0180.4110.0160.4230.0080.4240.0050.4060.0240.4110.0260.4140.0140.4260.0050.4160.0190.4270.006

0.3950.0220.4220.0110.4140.021

0.4590.0170.4320.0290.4670.005

0.4360.0330.4450.0220.4090.0640.3850.015

En

0.3860.0040.3870.0040.3870.0080.3850.0080.3850.0030.3800.0070.3640.0090.3600.0070.3620.0030.3660.0050.3810.0070.3540.0070.3890.012

0.3900.0070.3890.0040.4050.0100.3830.006

0.3300.0090.3240.0150.3270.0030.3690.0220.3600.0130.3440.0230.3430.0070.3510.0130.3400.0060.3680.002

0.3980.0170.3550.0070.3670.013

0.4770.0100.4920.0240.4610.005

0.4750.0240.4700.0150.4800.0470.5080.009

Fs

0.1820.0090.1790.0090.1810.0100.1880.0110.1850.0090.1850.0070.2100.0080.2090.0080.2110.0100.2080.0070.2020.0190.2330.0110.1750.016

0.1770.0100.1730.0070.1760.0190.1860.012

0.2420.0100.2650.0250.2500.0050.2070.0260.2340.0160.2440.0030.2430.0100.2230.0160.2430.0190.2050.009

0.2060.0160.2230.0110.2190.008

0.0640.0090.0760.0060.0720.001

0.0890.0100.0850.0080.1110.0160.1060.006

58




62R-4, 21-24

72R-5, 123-125

59R-4, 29-35

8D-1, 45-48

22R-3, 118-120

21R-2, 49-51

60R-1, 18-20

31R-2, 120-122

59R-3, 70-72

Rock type

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

SiO2

52.600.34

53.030.33

52.500.77

52.450.49

52.210.35

51.710.37

50.720.41

52.720.44

51.790.16

A12O3

2.8670.3492.7160.1463.3000.4762.9640.2742.4250.5282.7270.0883.0540.2032.6650.1452.9620.248

TiO2

0.8920.1510.5580.0580.6360.1940.5420.1140.6450.0410.6490.1020.9310.3120.6050.1690.8300.064

FeO

6.710.605.750.366.420.805.170.045.770.515.730.486.600.376.021.007.460.56

MnO

0.230.0730.1740.0250.180.0260.1740.0360.1910.0390.1840.0170.2130.0080.1620.0230.2250.044

Clinopyroxene

MgO

16.041.04

16.660.44

16.311.52

15.630.27

14.860.22

15.480.82

14.781.00

16.190.51

15.000.32

CaO

20.361.37

20.801.05

20.331.73

22.360.27

21.201.23

21.200.49

21.340.26

20.941.04

21.040.66

Na2O

0.4210.0560.3550.0370.4000.0640.3590.0080.2920.0020.3190.0390.4160.0010.3600.0380.4510.070

Cr2O3

0.1350.0800.1560.0370.2310.0810.2530.1770.1640.0190.0700.0220.1200.0350.1210.0380.1080.055

Wo

0.4250.0300.4290.0200.4240.0410.4650.0010.4570.0210.4490.0190.4540.0150.4350.0210.4410.014

En

0.4660.0250.4780.0140.4720.0390.4520.0010.4460.0120.4560.0140.4370.0210.4680.0160.4370.010

Fs

0.1090.0110.0930.0060.1040.0130.0840.0010.0970.0100.0950.0060.1100.0080.0980.0160.1220.009

Note: Standard deviations are also shown below the average of oxide weight percent. Abbreviations are the same as in Table 3.

(1989) who attributed the steeper plagioclase gradients to veryslow diffusion rates in plagioclase. In gabbros from Hole735B, different diffusion rates in plagioclase and mafic phaseshave produced disequilibrium phase assemblages near con-tacts between gabbro types, e.g., co-existing calcic plagio-clase and iron-rich olivine (Fig. 12). Because of this, onlyanalyses of minerals at least 2 cm from contacts have beenused in the downhole mineral plots (Figs. 2, 3) and in mineralco-variation plots (Figs. 4, 5).

Seventeen contacts between Fe-Ti oxide gabbro and olivinegabbro have been examined (PI. 3, Figs. 4-6). At the sharpestcontact (Sample 118-735B-79R-7, 2-9 cm), an oscillatory zonedcalcic plagioclase in an olivine gabbro is sharply cut by arelatively homogeneous sodic plagioclase within an Fe-Ti oxidegabbro having an irregularly serrated boundary (PI. 3, Figs. 3 and4; Figs. 11A and 11B). Under the optical microscope, twinboundaries in the plagioclase can be traced continuously overthe boundary, with a slight kinking at the contact because of thecompositional difference (PI. 3, Figs. 1-3). These observationssuggest that sodic plagioclase grew on the disrupted calcicplagioclase of the olivine gabbro. An overgrowth of iron-richclinopyroxene on olivine in olivine gabbro was also observed inthis sample (PI. 3, Fig. 4). Consequently, we infer that the meltfrom which the Fe-Ti oxide gabbro crystallized intruded intoeither solid or almost solidified olivine gabbro and that theboundaries are intrusive contacts.

In general, minerals in Fe-Ti oxide gabbro veins exhibit awide range in compositions compared to their host olivinegabbros (Fig. 13). Furthermore, there is an inverse relation-ship between mineral compositions in Fe-Ti oxide gabbroveins and mineral compositions in host olivine gabbros (Fig.13). Thus, the most evolved mineral compositions are found inFe-Ti oxide gabbro veins that intrude the most primitiveolivine gabbros. In addition to this complementary relation-ship, variable chemical exchange between Fe-Ti oxide gabbroveins and host olivine gabbro is indicated by an inverserelationships between delta An content (difference in plagio-clase composition in the vein and in the unreacted gabbrohost) and the thickness of the reaction zone. The thickness of thereaction zone is determined either by measuring the distancewithin a single crystal that is zoned away from the contact(diffusion distance in Fig. 14) or by measuring the distance overwhich multiple plagioclase grains exhibit chemical zoning awayfrom the contact (penetration distance in Fig. 14).

The variation in the diffusion and penetration distance mayhave been caused by difference in thermal history (tempera-ture or heating duration), chemical contrast between intrudedmelt and its host, and amounts of trapped melt in the hostolivine gabbro. The negative correlation of the diffusion andpenetration distance with the difference in the mineral com-positions, however, can only be explained by decreases intemperature (or heating duration) and in the abundance oftrapped melt as the host olivine gabbro becomes more primi-tive. There may be some trapped melt in the host olivinegabbro showing large penetration distance at the contact withFe-Ti oxide gabbro, because in such samples plagioclasecomposition changes along grain boundaries deeply into theolivine gabbro (PI. 3, Fig. 5; Fig. 11C), which cannot beexplained by grain boundary diffusion.

The evolved magma might have intruded into the cumuluspile of olivine gabbro whose lower, more primitive portionwas almost solidified, while the relatively evolved upperportion still contained trapped melt. The common occurrenceof dikes or sills of evolved Fe-Ti oxide gabbro and fairlyprimitive troctolitic microgabbro from Hole 735B (Robinson,Von Herzen, et al., 1989) suggest that the disruption ofsolidified or partially solidified lower oceanic crust commonlytook place even near mid-oceanic ridge, at least in the regionnear the fracture zones. The steady downcore increase of thediffusion distance in plagioclase at the contact between olivineand Fe-Ti oxide gabbros suggests that cooling from the base orwall of a magma chamber, which was modeled by Morton andSleep (1985) assuming a deep hydrothermal cell. The deephydrothermal activity is substantiated by abundant hydrother-mal alteration even at the base of Hole 735B. The close associ-ation of the hydrothermal alteration and Fe-Ti oxide gabbrossuggest that the fracture system filled by evolved melts wasresumed during the subsequent hydrothermal circulation.

ORIGIN OF THE DOWNHOLE MINERALOGICALVARIATIONS IN THE FE-TI OXIDE GABBROSAs described above, the Fe-Ti oxide gabbros are charac-

terized by several downhole mineralogical cycles that aremarked by decreases in the An content of plagioclase and theMg# of mafic minerals (Figs. 2 and 3). These trends areopposite from those observed among the olivine gabbros andtroctolites. Reverse fractionation trends have been reportedfrom the marginal series of the Jimberlana Intrusion (Camp-

59


Table 6. Average chemical composition of orthopyroxene in gabbros from Hole 735B.


118-735B-12R-1, 45-47

19R-5, 47-50

19R-5, 55-60

74R-6, 27-35

76R-4, 12-19

40R-5, 0-4

43R-3, 95-102

76R-3, 34-41 CT

77R-2, 101-107

36R-4, 5-9

37R-3, 71-76 CT

40R-5, 4-843R-4, 64-66

20R-1, 111-114

37R-3, 76-79

37R-3, 80-82

38R-3, 85-88

38R-4, 24-28

45R-1, 5-1221R-2, 27-32

23R-3, 34-37

24R-2, 120-126

28R-1, 109-114

28R-3, 72-77

44R-2, 16-24

46R-4, 109-113

14R-1, 93-97

21R-1, 82-89 CT

47R-2, 117-12747R-3, 143-149 CT48R-2, 56-65

48R-3, 112-11648R-4, 82-84

49R-1, 42-46

50R-2, 43-47

5OR-3, 62-67

51R-1, 94-99

51R-1, 99-103

52R-3, 121-123

Rock type

Gabbronorite

Gabbronorite

Gabbronorite

Gabbronorite

Gabbronorite





Dissem ox ol gabbro

Dissem ox ol gabbro

Dissem ox ol gabbroDissem ox ol gabbro

Ol pigenoite gabbro

Ol pigenoite gabbro

Ol pigenoite gabbro

Ol pigenoite gabbro

Ol pigenoite gabbro

Ol pigenoite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro

Pigeonite gabbro



Interst ol ox gabbro IInterst ol ox gabbro IInterst ol ox gabbro I

Interst ol ox gabbro IInterst ol ox gabbro I







SiO2

52.%0.18

52.690.60

54.260.25

53.130.15

52.510.29

53.250.21

52.940.49

52.610.17

52.830.24

53.130.34

53.540.13

52.6553.470.16

52.900.07

53.380.36

51.980.21

51.420.17

53.000.23

52.65

53.570.15

53.280.35

52.860.32

52.610.26

52.730.31

52.210.22

52.450.37

52.500.44

53.190.14

52.7852.9453.390.11

53.0852.360.99

52.020.18

53.120.52

52.380.46

52.280.11

52.980.01

52.620.27

A12O3

0.8030.0800.9870.1981.1260.1100.8020.0720.9960.121

1.0200.0611.0390.2281.1460.0831.0170.064

1.0100.2830.8510.0821.0881.0170.031

0.7950.0510.7430.0590.7760.1040.7970.0970.7840.0770.855

0.7730.2260.8570.0410.6550.1130.8180.0600.7120.0630.6950.1230.9740.121

0.8240.2260.8920.2101.1210.6970.7620.0770.7700.7190.0891.0540.0800.7500.0810.8070.1300.7110.1790.8460.1000.8510.511

TiO2

0.2470.0970.2780.0680.3360.0500.4540.0340.3460.064

0.3620.0300.3460.0500.2810.0330.3060.028

0.3790.1100.2380.0750.3140.3230.018

0.3420.0460.1750.0910.2440.0630.2870.0730.3550.1400.314

0.2260.0550.2830.0450.2890.0790.2740.0210.2470.0260.3020.0480.3210.015

0.3180.0850.3130.0830.3650.2460.2630.0530.3120.2900.0400.2700.0330.2820.0390.2850.0300.2730.0380.2950.0330.2100.031

FeO

24.331.02

22.662.13

20.370.09

22.150.46

21.641.05

20.860.45

20.181.73

21.560.23

21.840.59

21.350.07

23.090.13

22.8220.750.56

22.270.77

21.500.64

23.880.44

25.010.52

22.300.77

23.30

24.060.44

21.480.60

23.841.03

25.030.04

25.040.78

25.191.09

23.220.64

24.350.36

23.420.07

22.1325.4123.960.95

25.2424.760.35

24.530.54

23.930.32

24.260.66

26.071.13

24.370.15

25.870.83

MnO

0.6110.0700.5620.1170.4900.0230.6550.0460.5880.041

0.5560.0310.5320.0450.5580.0090.5900.045

0.6170.0070.6230.0290.6030.5240.015

0.5930.0470.5930.0490.6600.0560.6880.0720.5840.0410.657

0.7420.0920.5900.0430.6820.0540.7510.0020.7400.0310.7390.0990.6350.061

0.6420.0530.6780.0400.5310.6450.6220.0390.7490.6590.0250.6810.0670.6690.0360.6710.0600.6570.0780.6700.0000.6820.027

Orthopyroxene

MgO

20.500.55

21.511.39

23.320.03

21.560.24

22.980.32

23.090.19

22.550.66

21.650.02

22.150.55

22.480.18

22.250.09

21.4823.120.24

21.460.50 1

21.871.16

20.630.40

20.440.19

21.710.45 (

21.07

20.700.14 (

22.510.50 (

20.640.40 (

20.060.06 (

19.750.36 (

19.470.89 (

20.490.51 (

20.000.56 (

21.26

CaO

1.180D.2671.374D.3901.2973.1621.4143.1401.5043.439

1.4373.4021.1613.1441.0773.2251.1253.188

1.1453.2871.0373.0371.0881.4243.161

1.5623.4233.8683.0703.9973.1061.1303.0291.4143.4271.615

1.2673.102.359

).2521.359).3661.130). 1581.2833.2751.279).3121.167).130

1.182).1O81.072

0.33 0.25021.6619.5320.620.71 (

20.4420.03

.201

.889

.112). 118.269.122

0.66 0.02819.84 0.9560.15 0.122

20.34 .4250.20 0.136

20.030.51 (

19.79

.105).197.003

0.78 0.24520.75 .4360.10 0.363

19.40 0.9930.02 0.060

Cr2O3

0.0230.0280.0150.0150.0100.0180.0050.0050.0130.017

0.0070.0090.0110.0150.0200.0110.0100.012

0.0000.0000.0030.0040.0540.0060.005

0.0050.0050.0210.0300.0160.0320.0120.0210.0020.0030.024

0.0140.0170.0100.0130.0110.0180.0300.0380.0180.0410.0100.0170.0130.011

0.0130.0250.0300.0250.0120.0330.0040.0090.0000.0010.0020.0330.0130.0100.0170.0030.0070.0270.0330.0260.0180.0000.000

Wo

0.02420.00550.02800.00780.02610.00320.02900.00280.02990.0090

0.02880.00790.02400.00270.02240.00470.02300.0039

0.02330.00590.02070.00070.02230.02860.0032

0.03210.00900.01810.00180.02060.00210.02300.00050.02880.00860.0329

0.02590.00210.02750.00520.02790.00770.02330.00320.02660.00570.02660.00650.02440.0027

0.02460.00210.02190.00520.02470.03860.02290.00250.02570.02320.00090.02000.00250.02940.00280.02300.00380.02050.00490.02910.00730.02060.0015

En

0.58570.01350.61050.03400.65360.00260.61590.00590.63480.0077

0.64450.00480.65000.02340.62710.00040.62900.0092

0.63710.00500.61890.00270.61250.64610.0068

0.61180.00950.63270.01990.59380.00810.57930.00710.61600.00640.5969

0.58950.00520.63330.00950.58990.01170.57450.00300.56880.00670.56400.02490.59630.0118

0.57950.00%0.60450.00710.62000.55570.59140.01560.57560.57660.01150.57870.00790.58470.00540.58160.00950.56300.01750.58520.00410.56030.0071

Fs

0.39000.01740.36150.03760.32030.00130.35500.00510.33530.0134

0.32670.00680.32600.02380.35050.00430.34800.0107

0.33950.00090.36040.00200.36520.32530.0088

0.35610.00830.34930.01800.38560.00840.39760.00660.35510.01330.3702

0.38460.00520.33920.01050.38220.01000.40230.00020.40460.01110.40940.01850.37930.0124

0.39590.00840.37360.00300.35530.40560.38570.01700.39870.40010.01070.40130.00600.38590.00300.39540.01220.41650.02080.38570.00320.41910.0086

60




52R-4, 88-94

53R-1, 123-127

53R-2, 31-3954R-5, 117-119

55R-2, 101-105

82R-6, 73-78

34R-4, 8-12

47R-3, 56-61

49R-2, 94-100

52R-1, 91-100

54R-1, 131-136

54R-3, 20-24

54R-3, 125-127

55R-2, 110-12055R-3, 83-8656R-2, 11-14

56R-2, 49-52

80R-3, 67-71

80R-6, 130-135

86R-4, 123-130

80R-7, 10-1880R-7, 23-25

83R-7, 77-81

73R-3, 73-75

81R-5, 1-771R-2, 82-84

62R-4, 21-24

59R-4, 29-35

60R-1, 18-20

31R-2, 120-12259R-3, 70-72

Rock type



Interst ol ox gabbro IInterst ol ox gabbro I










Olivine oxide gabbroOlivine oxide gabbroOlivine oxide gabbro





Olivine oxide gabbroOlivine oxide gabbro

Troctolite


Vermic cpx ol gabbroVermic cpx ol gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbro

Olivine gabbroOlivine gabbro

SiO2

51.700.35

51.970.30

51.4252.380.18

51.410.35

52.040.35

52.610.18

52.820.25

53.170.30

52.200.37

51.160.14

51.260.05

50.730.06

52.5752.6151.510.02

51.150.20

51.570.28

51.650.24

52.150.20

51.1951.58

57.500.12

55.400.35

55.2554.620.31

54.990.09

54.980.36

53.710.35

53.5254.310.43

A12O3

0.6940.0190.4200.0160.6960.8710.1450.4480.0990.7460.170

0.9860.1510.7850.1660.7900.0730.9890.291

0.4250.0620.4100.0460.3650.0060.8040.4350.4670.0210.5270.0630.5070.1180.5380.0820.6380.0670.7600.792

0.7390.208

1.7530.5261.9321.2280.138

1.4480.0701.2520.1131.4120.0092.0551.3660.242

TiO2

0.2650.0470.1740.0130.2900.2540.0500.1790.0460.2430.031

0.3030.0520.2680.0420.2220.0420.2250.026

0.0790.0410.1690.0210.1150.0190.2%0.2030.1980.0350.2390.0080.1960.0350.1800.0510.1980.0510.2290.209

0.0920.037

0.2370.0560.1650.3780.161

0.3480.0770.2340.0430.3510.0810.3610.2970.100

FeO

26.890.67

28.560.54

26.9125.890.77

28.310.96

26.750.67

23.650.42

23.671.12

23.341.35

25.450.52

30.620.12

33.070.13

29.290.01

25.5628.5631.400.14

29.820.17

28.111.19

30.590.78

26.511.06

29.1227.32

9.010.06

12.940.32

12.9014.630.98

16.110.09

16.510.83

16.210.84

16.5217.810.99

MnO

0.8000.0010.8380.0680.6580.6730.0470.8510.0600.7630.120

0.6170.0590.6180.0340.6100.0580.6370.048

0.9740.0210.9600.0850.8940.0420.7140.7620.9590.0040.8920.0860.8880.0810.9840.0370.7800.0450.8570.716

0.2320.036

0.2620.0440.2770.3660.013

0.3840.0400.3930.0220.3590.0010.2900.4060.051

Orthopyroxene

MgO

17.590.34

17.310.26

17.1219.180.65

16.591.03

18.620.029

20.350.25

20.890.63

21.311.03

19.600.51

15.560.26

13.780.18

14.930.08

19.7717.3514.890.01

15.680.27

17.870.55

16.130.32

18.500.50

16.9217.61

32.580.64

28.810.53

28.5927.24

1.00

26.940.38

26.560.67

26.090.30

26.0525.770.99

CaO

1.2230.0101.2380.4482.1981.1360.0891.4460.54631.2180.208

1.2770.4381.0910.1751.0920.1991.0870.130

0.8330.0291.4650.2852.9840.0711.2102.1481.1060.0571.1930.0831.2230.0551.3110.2261.3250.2661.2391.095

0.5560.148

0.8800.1340.7530.9640.184

0.9890.2640.7410.0970.9300.1291.1590.8720.171

Cr2O3

0.0050.0070.0190.0270.0060.0010.0030.0060.0100.0300.033

0.0110.0160.0200.0290.0100.0080.0100.011

0.0020.0030.0110.0180.0220.0300.0100.0000.0030.0040.0250.0190.0040.0050.0090.0130.0020.0050.0000.000

0.0780.108

0.0930.0390.1230.0950.030

0.0200.0140.0440.0160.0390.0390.0170.0260.018

Wo

0.02620.00020.02600.00920.04680.02370.00200.03100.01220.02540.0043

0.02660.00910.02240.00350.02230.00430.02260.0028

0.01800.00070.03150.00570.06400.00130.02490.04420.02390.00120.02580.00200.02550.00100.02760.00490.02770.00560.02610.0233

0.01050.0030

0.01720.00250.01490.01920.0038

0.01940.00530.01460.00190.01860.00250.02300.01720.0035

En

0.52420.01060.50580.00340.50650.55560.01640.49490.02930.53960.0077

0.58920.00730.59760.01670.60550.02520.56540.0107

0.46670.00450.41270.00100.44560.00070.56510.49690.44720.00180.47130.00370.51780.01830.47120.00790.53890.01320.49550.5222

0.85660.0041

0.78490.00540.78610.75360.0210

0.73430.00560.73050.01490.72780.01380.72050.70790.0207

Fs

0.44960.01080.46830.01260.44670.42080.01440.47400.01780.43500.0115

0.38420.00610.38000.01890.37220.02170.41200.0102

0.51530.00430.55580.00540.49040.00200.41000.45890.52900.00060.50290.00170.45680.01730.50130.01040.43330.01730.47840.4545

0.13290.0011

0.19790.00620.19900.22720.0172

0.24630.00030.25490.01410.25360.01130.25650.27480.0177

Note: Standard deviations are also shown below the average of oxide weight percent. Abbreviations are the same as in Table 3.

bell, 1977) and from the marginal zone in the Muskox Intrusion(Bahattacharji and Smith, 1964). These marginal cumulates oc-cupy the lower portions of the intrusions filling connecting pipes.In the upper border group of the Skaergaard Intrusion, a down-ward trend to more evolved compositions has been interpretedas crystallization from the roof downward in the intrusion(Wager and Brown, 1967; Naslund, 1984).

The unimodal distribution of the mineral chemistryamong the Fe-Ti oxide gabbros (Fig. 1) indicates that theymight have been derived from a common, moderately frac-tionated magma, which may have existed either at the top oron the side of the cumulus pile of olivine gabbro. In the

former case, the evolved magma may have intruded down-ward into the pile with progressive fractional crystallization.In the latter case, the chamber may have been reverselystratified and lateral intrusion occurred. The lateral intrusionmodel seems unrealistic because it is difficult to maintain areversely stratified magma chamber (Sparks and Huppert,1984); the melt in equilibrium with the most evolved gabbros,the olivine Fe-Ti oxide gabbros, deep in the section likely washigher in silica and lower in iron so had a lower density than themelt in equilibrium with the less evolved gabbronorites andpigeonite gabbros higher in the section so would have risenthrough the cumulus pile.

61


12

Total number of samples: 119

35 40 45 50 55 60 65 70 75

Fo mol% in olivine


60 65 70 75 80 85

100Mg/(Mg+Fe) of cliπopyroxene



Figure 4. A. Frequency histograms of average Fo mol% in olivine. B.Average 100 Mg/(Mg+Fe) of clinopyroxene. C. Average An mol% inplagioclase in gabbros from Hole 735B. Note that histograms showbimodal distribution pattern, specifically for An mol% and Mg/(Mg+Fe) ratio of clinopyroxene. Each peak corresponds to olivinegabbro-troctolite and iron-titanium oxide gabbros.

The downward intrusion of fractionated magma into asolidifying cumulus pile is plausible if magma chambersbeneath mid-ocean ridges are chemically zoned, as proposedby Pallister and Hopson (1981) and Casey and Karson(1981). In their models, magma situated at the margin of thechamber, and away from the spreading center, is moreevolved than that at the center of the chamber. A similarzonation may be attained in a small ephemeral magmachamber with limited supply of primitive magmas. In such a

case, evolved melt may overly a completely- or almost-solidified cumulus pile of primitive gabbros. Disrupting thecumulus pile may have caused the overlying evolved magmato fill the fractures and crystallize as a dike- and sill-complex. Progressive downward intrusion may have causedcontinued evolution; thus, the deeper intrusives show moreevolved characteristics. The contact relationship betweenolivine gabbro and Fe-Ti oxide gabbro suggests that themagma body should have been cooled from the base. In asteady state magma chamber model, this situation may onlybe achieved if the intrusion took place away from the ridgecenter, assuming a deep hydrothermal cell (Morton andSleep, 1985). This indicates that the evolved melt may haveexisted away from the accretion center, which is consistentwith the zoned magma chamber model mentioned above.The downward evolution of Fe-Ti oxide gabbros in Unit IVcan be explained by downward crystallization from the roofof a thick dike. Accordingly, the base of Unit IV maycorrespond to the proximity of the center of the dike, thelower half of which was displaced by faulting and is missingfrom the section of Hole 735B.

SUMMARY

The major petrological characteristics of gabbros fromHole 735B are (1) the intimate association of evolved (Fe-Tioxide gabbro) and primitive (olivine gabbro and troctolite)gabbros, with rare gabbros having intermediate mineralchemistry, (2) the likely co-genetic relationship betweenevolved and primary gabbros suggested by systematic min-eralogical variations among all the gabbros from Hole 735B,and (3) the sharp contacts between evolved and primitiveand evolved gabbros, suggesting that these contacts wereformed by the intrusion of evolved magma into solid oralmost solidified olivine gabbro host.

We suggest that a body of fairly homogeneous olivinegabbro cumulates formed on the floor and walls of a magmachamber. This isotropic (unlayered) unit of gabbro cumu-lates was overlain by a body of isolated melt which fraction-ated to produce gabbronorites and dense iron-rich melts thatmigrated back into the cumulate pile along cooling anddeformation fractures. Downward penetration of melt led tofurther fractionation and the production of evolved gabbrosrich in Fe-Ti oxides. This model requires an unreplenishedephemeral magma chamber (Meyer et al., 1989) which isconsistent with low magma supply rates along the SouthwestIndian Ridge (Dick et al., 1984). High cooling rates andextensive fractional crystallization may have been enhancedby the proximity to the Atlantis II Fracture Zone.

ACKNOWLEDGMENTS

We wish to thank the scientists, technicians, officers, andcrew aboard the JOIDES Resolution, and the shore-basedODP staff for their support and cooperation during and afterLeg 118. We are grateful to Toshitsugu Fujii, Hiroko Naga-hara, and Natsuko Takahashi for fruitful discussions andencouragements. This paper benefitted considerably fromthe many comments by Peter Thy.

REFERENCES

Ambler, E. P., and Ashley, P. M., 1977. Vermicular orthopyroxene-magnetite symplectites from the Wateranga layered mafic intru-sion, Queensland, Australia. Lithos, 10:163-172.

Batiza, R., and Vanko, D. A., 1985. Petrologic evolution of largefailed rifts in the eastern Pacific: petrology of volcanic and plutonicrocks from the Mathematician Ridge area and the GuadalupeTrough. J. Petrol, 26:564-602.

62


Bhattacharji, S., and Smith, C. H., 1964: Flowage differentiation.Science, 145:150-153.

Bowen, N. L., and Schairer, J. F., 1935. The system MgO-FeO-SiO2.Am. J. Sci., 26:151-217.

BVSP, 1981. Basaltic Volcanism on the Terrestrial Planets: (Lunarand Planetary Institution), New York (Pergamon Press).

Casey, J. F., and Karson J. A., 1981. Magma chamber profiles fromthe Bay of Islands ophiolite complex. Nature, 292:295-301.

Campbell, I. H., 1977. A study of macro-rhythmic layering andcumulus processes in the Jimberiana Intrusion, Western Austra-lia. Part I: The upper layered series. J. Petrol., 189:183-215.

Davidson, P. M., and Lindsley, D. H., 1989. Thermodynamic analy-sis of pyroxene-olivine-quartz equilibria in the system CaO-MgO-FeO-SiO2. Am. Mineral., 74:18-30.

Elthon D., 1987. Petrology of Gabbroic rocks from the Mid-CaymanRise spreading center. J. Geophys. Res., 92:658-682.

Hébert, R., Bideau, D., and Hekinian, R., 1983. Ultramafic and maficrocks from the Garret Transform Fault near 13°N30'S on the EastPacific Rise: igneous petrology. Earth Planet. Sci. Lett., 65:107-125.

Hodges, F. N., and Papike, J. J., 1976. DSDP site 334: Magmaticcumulates from oceanic layer 3. J. Geophys. Res., 81:4135-4151.

Hoover, J. D., 1978. Petrologic features of the Skaergaard marginalborder group. Year Book, Carnegie Instn. Wash., 77:732-739.

Meyer, P. S., Dick, J. B., and Thompson, G., 1989. Cumulate gab-bros from the Southwest Indian Ridge, 54°S-7°16'E: implicationsfor magmatic processes at a slow spreading ridge. Contrib. Min-eral. Petrol., 103:44-63.

Miyashiro, A., and Shido, F., 1980. Differentiation of gabbros in theMid-Atlantic Ridge near 24°N. Geochem. J. 14:145-154.

Morton, J. L., and Sleep, N. H., 1985. A mid-ocean ridge thermalmodel: constraints on the volume of axial hydrothermal heat flux.J. Geophys. Res., 90:11345-11353.

Nakamura, Y., and Kushiro, I., 1970. Compositional relationshipsof coexisting orthopyroxene, pigeonite and augite in a tholeiiticandesite from Hakone volcano. Contrib. Mineral. Petrol.,26:265-275.

Naslund, H. R., 1984. Petrology of the Upper Border Series of theSkaergaard intrusion. J. Petrol., 25:185-212.

, 1976. Mineralogical variations in the upper part of theSkaergaard intrusion, East Greenland. Year Book, Carnegie Instn.Wash., 75:640-644.

Pallister, J. S., and Hopson, C. A., 1981. Samail ophiolite plutonicsuite: field relations, phase variation, cryptic variation and layer-ing, and a model of a spreading ridge magma chamber. / . Geophys.Res., 86:2593-2644.

Robinson, P. T., Von Herzen, R., et al., 1989: Proc. ODP, Init.Repts., 118: College Station, TX (Ocean Drilling Program).

Sinton, J. M., Wilson, D. S., Christie, D. M., Hey, R. N., andDelaney, J. R., 1983. Petrologic consequences of rift propagationon oceanic spreading rides. Earth Planet. Sci. Lett., 62:193-207.

Sparks, R. S., and Huppert, H. E., 1984. Density changes during thefractional crystallization of basaltic magmas: fluid dynamic impli-cations. Contrib. Mingral. Petrol., 85:300-309.

Tiezzi. L. J., and Scott, R. B., 1980. Crystal fractionation in acumulate gabbro, Mid-Atlantic Ridge, 26°N. J. Geophys. Res.,85:5438-5454.

Wager, L. R., and Brown, G. M., 1967. Layered Igneous Rocks: SanFrancisco (Freeman).

Walker, D., and Delong, S. E., 1984. A small Soret effect in spreadingcenter gabbros. Contrib. Mineral. Petrol., 85:203-208.

Date of initial receipt: 19 October 1989Date of acceptance: 26 June 1990Ms 118B-125

63


A 70

60-Φ

">"δc— 50-o

4 0 -

30

Gabbronorite


Disseminated Fe-Ti oxide olivine gabbro

Olivine pigeonite gabbro

Pigeonite gabbro

Interstitial ol-bearing Fe-Ti oxide gabbro I

Interstitial ol-bearing Fe-Ti oxide gabbro II

Olivine Fe-Ti oxide gabbro

Olivine Fe-Ti oxide microgabbro

A

AA A Δ

A A

'Δ oθΔ

25 30 35 40 45

45


Figure 5. Average An content in plagioclase vs. average Mg/(Mg+Fe) ratios of maficminerals (A, Olivine. B, Clinopyroxene (augite-diopside), C, Orthopyroxene) in Fe-Tioxide gabbros from Hole 735B. Symbols are the same as in Figure 3.

64


1.2

1.0.

<DXO> .Q .Oc"üc

wt%

CM

o

0.8.

0.6.

0.4.

0.2

50

B

50

oo

o

«bo o

o'oo o o o o cP•o 0 ° °

o o•o

+ + + +D +

mx

60 70 80 90 100

α>cα>X

oQ.

mo

o

OCMCO

Z

0.6.

0.5.

0.4.

0.3.

0.2

oo _

° o

ooo

+o

o oQoO ocp° €SQO

oCD

++

+ •

D V*+m • • *

D

10060 70 80 90

100Mg/(Mg+Fe) of clinopyro×ene

Figure 6. TiO2 content (A) and Na2O content (B) vs. Mg/(Mg+Fe) ratio ofclinopyroxene (augite-diopside). Data are based on average of clinopyroxene cores.Symbols are the same as in Figure 4.

65


B

o

3 .

CO

oçvj< 2J

0.5

0.0

0.5

1.5

1.0.

coO

ü 0.5 J

0.0

Clinopyroxene

L

X

Δ

èJPtfrf>o"• • Λ.

<P •

0.6 0.7 0.8 0.9

GabbronoriteOlivine gabbronoritβDisseminated olivine Fe-Ti oxide gabbroOlivine pigeonite gabbroPigeon ite gabbroInterstitial ol-bearing Fe-Ti oxide gabbro IInterstitial ol-bearing Fβ-Ti oxide gabbro IIOlivine Fe-Ti oxide gabbroOlivine Fe-Ti oxide microgabbroTroctoliteVermicular cpx olivine gabbroOlivine gabbro

0.5 0.6 0.7

Mg/(Mg+Fe)

0.8 0.9

Figure 7. Mg/(Mg+Fe) ratio vs. A12O3 (A), MnO (B), and Cr2O3 (C) contents ofclinopyroxene. Symbols are the same as in Figure 3, except for troctolite (soliddiamond), vermicular clinopyroxene olivine gabbro (open diamond), and olivinegabbro (small solid square).

66


B

0.7 0.8 0.9

1.0

O

0.8.

0.6.

0.4.

0.2

Gabbronorite " % 'A , "

Olivine gabbronorite <ò'

Disseminated ol Fe-Ti oxide gabbro " t

Olivine pigeonite gabbro

Pigeonite gabbro

Interstitial ol-bearing Fe-Ti oxide gabbro I

Interstitial ol-bearing Fe-Ti oxide gabbro II

Olivine Fe-Ti oxide gabbro

Troctolite & Vermicular cpx olivine gabbro

Olivine gabbro

0.4 0.5 0.6 0.7 0.8 0.9

Mg/(Mg+Fe)Figure 8. Mg/(Mg+Fe) ratio vs. TiO2 (A), A12O3 (B), and MnO (C) contents oforthopyroxene. Symbols are the same as in Figure 7.

67


OC\J

J.ó

0.2-

0.1 -

0.0

AA

A A

"c *•

Plagioclase

-

" ° o *

0.2 0.3 0.4 0.5

An mol%

0.6 0.7 0.8

B

Figure 9. Anorthite mol% vs. Orthoclase mol% in plagioclase. Sym-bols are the same as in Figure 7.

o

α Olivine gabbronorite & gabbronoriteα Disseminated Fe-Ti oxide gabbro° Olivine pigeonite gabbroΔ Interstitial ol-bearing Fe-Ti oxide gabbro I+ Interstitial ol-bearing Fe-Ti oxide gabbro IIA Olivine Fe-Ti oxide gabbrox Olivine Fe-Ti oxide microgabbro* Troctolite" Vermicular cpx olivine gabbro" Olivine gabbro

x * Δ -*°

AAV \ Λ Λ Λ \ ^ D.ΔΔΔ>

Olivine

*

0.2 0.3 0.4 0.5 0.6 0.7 0.8

B

/Vt%

)M

nO

O

1.0-

0.8 .

0 . 6 .

0 .4 .

0.2

A

* AA

4 A

*

A

A ^ AA

Δ A ^Δ O'D

• #

* •

oo"en_cσQ.

c

oEc<

Q.c

oE

ò

_CT5O

qQ.C

80-

70 J

60 -

50 -

40-

30-

?0 -

Fe-Ti ox/de

4

gabbro ,

.

Olivine gabbro

82R-3 0-5 cm

1000 2000 3000

1.5-

1.0-

0.5-

n n-

s ' ., >.f N % "

• :

""*.. *

80R-3 0-5 cm

2000

Distance (µim)

3000

20

Olivine gabbro

Fe-Ti oxide gabbro47R-3 143-149 cm

2000 4000 6000 8000

Distance (jjm)

10000 12000

0.2 0.3 0.4 0.5 0.6 0.7

Fo mol%0.8 0.9

Figure 10. Fo mol% vs. NiO (A) and MnO (B) contents of olivine.Symbols are the same as in Figure 7.

Figure 11. Step scanning profiles of plagioclase composition at thecontact between troctolitic olivine gabbro and extremely evolvedFe-Ti oxide gabbro (A)-(B) (Sample 118-735B-82R-3, 0-5 cm) andolivine gabbro and Fe-Ti oxide gabbro (C) (Sample 118-735B-47R-3,143_149). Note the difference of scales in (A)-(B) and (C). Acalcium-rich peak at the contact of Sample 118-735B-47R-3, 143-149cm (C) corresponds to a zoned calcic plagioclase of the olivine gabbro.

68


10060 40

An mol% in plagioclase20

Figure 12. Anorthite content of plagioclase vs. Forsterite mol% ofolivine at contacts between olivine gabbro and Fe-Ti oxide gabbro.Data for each contact are connected by a line. Olivine and plagioclaseoccurring nearby are paired. Bars indicate compositional ranges.Samples are; solid circle: 118-735B-79R-7, 2-9 cm, open circle:118-735B-82R-3, 0-5 cm, solid square: 118-735B-47R-3, 143-149 cm,solid triangle: 118-735B-76R-3, 35-41 cm, open square: 118-735B-10D-2, 20-24 cm, open triangle: 118-735B-76R-1, 63-70 cm. Arrowson the Skaergaard trend indicate the temporal cessation of olivinecrystallization. Bars indicate ranges of the chemical compositions. Ateach contact, upon crossing the boundary from olivine gabbro toFe-Ti oxide gabbro, first Fo mol% changes markedly keeping plagio-clase composition constant, but at the contact An mol% changesdramatically.

-Q

03

α>L L

cα>

o

-Q

cn

C55

CD1 3

CL

ü

30

70 80 90Fo mol% (Olivine in host olivine gabbro)

50 60 70 80

An mol% (Plag in host olivine gabbro)

Mg# (Cpx in host olivine gabbro)

Figure 13. Relationships between chemical composition of olivine (A),plagioclase (B), and clinopyroxene (C) in olivine gabbro and Fe-Tioxide gabbro in contact with each other. ΔFo, ΔAn, and ΔMg# implyFo of olivine, An of plagioclase, and Mg# of clinopyroxene in hostolivine gabbro minus Fo of olivine, An of plagioclase, and Mg# ofclinopyroxene in Fe-Ti oxide gabbro vein, respectively. Bars indicatestandard deviations. The data for olivine gabbro and troctolite wereobtained 1-3 cm away from the contact and exhibit pristine compo-sition without chemical effect of intruded evolved melts.

69


100

10-

1 :

0.1

o

• Diffusion distance

O Penetration distance

11

10 20 30 40 50

Difference in An mol% of plagioclase

Figure 14. Relationships between compositional difference of plagioclase incontacting olivine and Fe-Ti oxide gabbros, and diffusion or penetrationdistance observed in plagioclase. Diffusion distance implies the zoned width insingle plagioclase at the contact. Penetration distance implies the width withinwhich plagioclase in olivine gabbro shows chemical heterogeneity commonlyfrom the grain boundaries toward inside the grains. Vertical lines connect datafor the same sample.

70


i ,: \ - . ;

Plate 1. Thin section photomicrograph of Fe-Ti oxide gabbros. Scale bar is 1 mm long for all the photos. 1. Sample 118-735B-76R-4, 12-19 cm.Gabbronorite containing plagioclase, orthopyroxene and clinopyroxene with small amount of Fe-Ti oxides. 2. Sample 118-735B-40R-5, 0-4 cm.Olivine gabbronorite containing plagioclase, olivine, orthopyroxene, and clinopyroxene with small amount of Fe-Ti oxides. 3. Sample118-735B-46R-3, 121-128 cm. Disseminated Fe-Ti oxide olivine gabbro containing plagioclase, large olivine, clinopyroxene, and Fe-Ti oxides.4. Sample 118-735B-76R-1, 63-70 cm. Gabbronorite in contact with olivine gabbro. The photomicrograph shows orthopyroxene and Fe-Tioxides symplectite surrounding olivine. The corona structure is observed only very close to the contact. 5. Sample 118-735B-38R-3, 80-85 cm.Inverted pigeonite surrounded by Fe-Ti oxides in olivine pigeonite gabbro. 6. Sample 118-735B-38R-3, 85-88 cm. Inverted pigeonite in the coreof clinopyroxene in pigeonite gabbro. Inverted pigeonite is also present as an isolated grain. Abbreviations in the photos are; OL: olivine, PL:plagioclase, OP: orthopyroxene, CP: clinopyroxene, PG: inverted pigeonite, OX: Fe-Ti oxides.

71


Plate 2. Thin section photomicrographs of Fe-Ti oxide gabbros. Scale bar is 1 mm long except for 4, for which it is 0.5 mm long. 1. Sample118-735B-73R-5, 72-78 cm. Interstitial olivine-bearing Fe-Ti oxide gabbro characterized by presence of minor interstitial olivine generallyassociated with Fe-Ti oxide. 2. Sample 118-735B-52R-1, 91-100 cm. Thin olivine mantle present between Fe-Ti oxides and plagioclase ininterstitial olivine-bearing Fe-Ti oxide gabbro. 3. Sample 118-735B-50R-3, 62-67 cm. Corroded and inverted pigeonite in the core ofclinopyroxene in interstitial olivine-bearing Fe-Ti oxide gabbro. 4. Sample U8-735B-49R-2, 94-100 cm. Inverted pigeonite surrounded by anintergrowth of clinopyroxene and olivine, which are further surrounded by Fe-Ti oxide minerals. 5. Sample 118-735B-54R-3, 125-127 cm. OlivineFe-Ti oxide gabbro containing abundant cumulus olivine. 6. Sample 118-735B-48R-2, 109-113 cm. Olivine Fe-Ti oxide microgabbro containingplagioclase, olivine, clinopyroxene, and Fe-Ti oxides. Abbreviations are the same as in Plate 1.

72


opt

\ 1

5,58

4.51

3.52

2.58

1.51

8,68 HINCD! •TßD-4 C5—orL•iöK I oJ

67.5 3B 116-HM-89

Λ 4os osUITV 488 θθunr

Tisrθ 158i?SEle Nft<i)HftX 787(X) 9.22HIN 8(ü) β.88ft 4.221

1H w S O 4 S

Plate 3. Thin section photomicrographs and element distribution map of contact zones between olivine and Fe-Ti oxide gabbros. Scalebar is 1 mm for Figs. 1. and 2., and 0.2 mm for Fig. 3. 1. Sample 118-735B-73R-5, 72-78 cm. Slightly diffuse contact between olivinegabbro (above) and Fe-Ti oxide gabbro (below). Note that clinopyroxene in Fe-Ti oxide gabbro contains many exsolution lamellae ofopaque minerals, whereas no lamellae is present in clinopyroxene of olivine gabbro. 2. Sample, 118-735B-79R-7, 2-9 cm. Sharp contactbetween olivine gabbro (above) and Fe-Ti oxide gabbro (below). Polysynthetic twins are continuous in plagioclase of both gabbros,though extinction position is different because of the difference in An content. This photomicrograph indicates that sodic plagioclase ofFe-Ti oxide gabbro overgrew the fractured calcic plagioclase of olivine gabbro. 3. Enlarged view of the left side of Fig. 2., showing thatchemical composition of plagioclase changes within hundred microns from the olivine gabbro to the Fe-Ti oxide gabbro. Note thecontinuous twin boundaries. 4. Calcium distribution map at the contact of olivine gabbro and Fe-Ti oxide gabbro (the same sample asin Fig. 2). Black area is mostly plagioclase with an An content of 30. Note that the zoning pattern is sharply cut by plagioclase in Fe-Tioxide gabbro. 5. Sample 118-735B-70R-1, 63-68 cm. Sodium distribution map of the boundary between olivine gabbro and Fe-Ti oxidegabbro. The zoning in plagioclase indicates that the boundary is more irregular and diffused than those in Figs. 1 and 2. 6. Sample118-735B-73R-4, 82-87 cm. Fe-Ti oxide dike (1.5 cm thick) in olivine gabbro. The boundary is more diffused than the case in Figs. 1 and2, but is less diffused than that in Fig. 5. White veins are late stage albite. OC: clinopyroxene in olivine gabbro, FC: clinopyroxene inFe-Ti oxide gabbro, OPL: plagioclase in olivine gabbro, FPL: plagioclase in Fe-Ti oxide gabbro, B: boundary of olivine and Fe-Ti oxidegabbros. Other abbreviations are the same as in Plate 1. Numbers besides the brightness bar in the element distribution maps are CaOwt% for Fig. 4 or Na2O wt% for Figs. 5 and 6.

73

3. MINERALOGY AND TEXTURES OF IRON-TITANIUM OXIDE · PDF fileMINERALOGY AND TEXTURES OF...

Documents

Transcript of 3. MINERALOGY AND TEXTURES OF IRON-TITANIUM OXIDE · PDF fileMINERALOGY AND TEXTURES OF...