Osmium isotope constraints on lower crustal recycling andpluton preservation at Lassen Volcanic Center, CA

Garret L. Hart a;�, Clark M. Johnson a, Steven B. Shirey b,Michael A. Clynne c

a Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, USAb Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W.,

Washington, DC 20015, USAc U.S. Geological Survey, 345 Middle¢eld Road MS910, Menlo Park, CA 94025, USA

Received 12 July 2001; received in revised form 22 February 2002; accepted 28 February 2002

Abstract

Osmium isotope compositions of intermediate- to silicic-composition calc-alkaline volcanic rocks from the Lassenvolcanic region of the Cascade arc are significantly more radiogenic (QOs =+23 to +224) than typical mantle. Theseevolved arc rocks in the Lassen region have unradiogenic Sr, Nd, and Pb isotope compositions which overlap withthose of contemporaneous mafic lavas. Crystal fractionation of mafic- to intermediate-composition magmas producesRe/Os ratios that are high enough to evolve to very radiogenic Os isotope compositions in only a few million years,providing a potential fingerprint for detecting the involvement of such young, relatively mafic crust in magmaticsystems. However, the Sr, Nd, and Pb isotope compositions will remain constant over such short time intervals due torelatively low parent/daughter enrichment during magmatic evolution. The radiogenic Os isotope compositions intypically evolved Lassen rocks are interpreted to reflect significant interaction with lower crustal material that hasradiogenic Os isotope compositions. Beneath this section of the Cascade arc, large amounts of such high-QOs lowercrust may have formed and been isolated from MASH zone mixing and homogenization processes during the Plioceneor Late Miocene. The results from this study indicate that Os isotopes may provide a unique glimpse into lowercrustal processes, such as recycling, in primitive orogenic arcs. 9 2002 Elsevier Science B.V. All rights reserved.

Keywords: osmium; rhenium; felsic composition; subduction zones; crust

1. Introduction

Magmatic and tectonic accretion of juvenile

orogenic arcs is generally thought to be one ofthe primary means by which the continentalmass has grown [1^3]. The fact that juvenile arcsare more ma¢c than estimates for bulk continen-tal crust [4,5] suggests that such arcs represent thestarting point for magmatic addition and intra-crustal di¡erentiation which eventually producethe compositionally evolved cratons that accretedto continental cores and may even have producedthe cores themselves. Oxygen, Sr, Nd, and Pb

0012-821X / 02 / $ ^ see front matter 9 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 5 6 4 - 2

* Corresponding author. Tel. : +1-608-262-8960;Fax: +1-608-262-0693.E-mail addresses: glhart@geology.wisc.edu (G.L. Hart),

clarkj@geology.wisc.edu (C.M. Johnson), shirey@dtm.ciw.edu(S.B. Shirey), mclynne@usgs.gov (M.A. Clynne).

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isotopes have been used to identify the manysources involved in the production of orogenicrocks where the isotopic contrast between mantleand crust is large [6^11]. However, in young oro-genic arcs where the O, Sr, Nd, and Pb isotopecontrast between mantle and crust is small, it isdi⁄cult to study processes of assimilation, melt-ing, mixing, and compositional strati¢cation inthe arc crustal column.Alternatively, the Os isotope system, when

combined with other isotopic and elemental evi-dence, has great potential for tracing intra-crustalprocesses in orogenic crustal sections because ofthe stark Os isotope contrast that can exist be-tween mantle and crustal components. Osmiumisotope contrasts between components developbecause of the extreme parent/daughter fractiona-tions (Re/Os) produced during crystallization ofma¢c magmas [12^15]. These extreme parent/daughter ratios may allow signi¢cant radiogenicOs to be generated in primitive ma¢c crust in onlya few million years. The goal of this work is toassess intra-crustal processes by evaluating poten-tial sources of radiogenic Os in intermediate- tosilicic-composition lavas from the Lassen regionof the southern Cascade arc and by focusing onthe role of the lower crust. This study focuses onsilicic rocks because of their potential for crustalinteraction and because of a general lack of silicicOs isotope data. Osmium isotope analyses ofprimitive basalts [16] provide a useful baselinewith which to discuss the evolved rocks in thisstudy.

2. Lassen Volcanic Center (LVC)

2.1. Geologic summary

LVC lies at the southern end of the active Cas-cade arc (Fig. 1) and is built on older volcaniccenters and regional lavas that erupted from mono-genetic volcanoes [19]. The calc-alkaline volcan-ism at LVC includes three stages, where stage I(600^470 ka) and stage II (470^400 ka) representthe cone-building sequences of Brokeo¡ Volcano,and stage III represents the most evolved volcanicepisode, and includes the Loomis (400 ka), Bum-

pass (250^200 ka), Eagle Peak (75^0 ka), andTwin Lakes (300^0 ka) sequences. Estimated vol-umes of erupted magma at LVC include V80km3 of largely stage I and II andesites of Brokeo¡Volcano and V50 km3 of stage III andesites anddacites. LVC is surrounded and/or underlain by¢ve clearly identi¢ed Pleistocene and Pliocene vol-canic centers including the Maidu Volcanic Cen-ter (V2^0.8 Ma) (unpublished data, M.A.Clynne).

2.2. Previous geochemical studies

Strontium and Nd isotope compositions of theLVC silicic lavas match trends observed in otherCascade volcanoes (Fig. 2). The isotopic compo-sitions of Cascade rocks in general and Lassenrocks in particular are among the most mantle-like of continental arcs, overlapping those of

Fig. 1. Generalized map (modi¢ed from [17,18]) of the Cas-cade Range showing the distribution of the major Cascadevolcanoes (triangles) including Lassen Peak, located in Las-sen Volcanic National Park in northern California. Shadingdenotes major areas of Cenozoic igneous rock. LVC rests onPliocene^Quaternary volcanic units which overlie Sierran^Klamath plutonic^metamorphic basement units.

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primitive oceanic arcs such as the Marianas andTonga arcs (references cited in Fig. 2). Based ondetailed study of the most primitive Lassen lavasavailable, Borg et al. [27] call for a heterogeneousmantle source that has both mid-ocean ridge ba-salt (MORB) and ocean island basalt (OIB) a⁄n-ities, coupled with material derived from the slab,as major components in the regional ma¢c lavasin the Lassen region. Borg et al. [27] and Borgand Clynne [28] interpret lavas with high-[Sr/P]Nratios (N refers to normalization to values ofprimitive mantle de¢ned by Sun and McDonough[29]) to have more ‘arc-like’ trace element compo-sitions that re£ect a slab-derived component, andlavas with low-[Sr/P]N ratios to contain a smallerslab component and have more ‘OIB-like’ traceelement and isotopic compositions. In the Lassenregion, [Sr/P]N ratios decrease from west to east,with the lowest ratios occurring at the arc axis.

This pattern is interpreted to re£ect the decreasingin£uence of slab-derived material toward the arcaxis, such that little slab in£uence is thought to bepresent in the predominantly low-[Sr/P]N basalticmantle input of the current arc axis [27]. In addi-tion, the mantle wedge is interpreted to becomeprogressively depleted (i.e. more ‘MORB-like’with lower K2O contents) from east to west as aresult of arc melt extraction [30]. The range inchemical, and Sr, Nd, and Pb isotope composi-tions of primitive ma¢c lavas in the Lassen regionare therefore thought to re£ect mixing betweenmantle source and slab components rather thancrustal contamination [27,31].Osmium isotope compositions of primitive ba-

salts (MgOv 8 wt%) from the Lassen region areinterpreted by Borg et al. [16] to re£ect mixingbetween a slab-dominated source (high-QOs andhigh-[Sr/P]N) and a mantle-dominated source(low-QOs and low-[Sr/P]N) (QOs = ((187Os/188Osmeasured)/ (187Os/188Osmantle)31)U100), consis-tent with earlier interpretations based on Sr iso-tope compositions. The QOs values of the high-QOs(and high-[Sr/P]N) basalts are much higher thanthose previously estimated for sub-arc mantlebased on analyses of xenoliths from a backarcsetting [32], but are within the range of those mea-sured for other arc lavas [33]. The high-QOs basaltsalso contain the lowest whole-rock Os contents,suggesting that if these basalts re£ect a slab-dom-inated source, such a source may also have lowOs contents. As noted by Borg et al. [16], it seemsunlikely that the high-QOs basalts, which have lowOs contents, re£ect extensive crystallization be-cause all Lassen region basalts have similar Recontents. An alternative to the slab-material inter-pretation is that the high-QOs basaltic lavas in theLassen region have been contaminated by radio-genic (high-QOs) crust [16,34], although the uni-formity of the Re contents suggests that such con-tamination could not have been accompanied byextensive fractional crystallization, which seemsunlikely.The silicic magmas at LVC were erupted in the

main axis of the volcanic arc, where the slab con-tribution of radiogenic Os is thought to be mini-mal [16,27]. The origin of the evolved rocks mayre£ect signi¢cant partial melting of the crust, and

Fig. 2. Sr^Nd isotope variations for Quaternary Cascade vol-canic rocks, Juan de Fuca^Gorda MORB, and Marianasand Tonga arcs. The isotopic compositions of the Cascaderocks are among the most sub-arc mantle-like of any conti-nental arc and overlap those of the Marianas and Tongaarcs (dashed line; data from [20^22]), and nearly overlap theSr isotope compositions of Juan de Fuca and Gorda Ridgemid-ocean ridge basalts (JDF-G MORB; [23,24]). Data forCascade volcanoes from [25,26]. Volcanic rocks from theLassen region span the average range of analyzed Cascadevolcanic rocks, and extend to slightly higher 87Sr/86Sr ratios.The range in Sr^Nd isotope compositions for the Cascaderocks is primarily interpreted to re£ect mixing between slab-dominated (slab box) and mantle-dominated (non-slab box)components rather than crustal interaction [27].

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Borg and Clynne [28] have interpreted trace ele-ment contents to be consistent with V10^20%melting of ma¢c lower crust. However, becausethe Sr, Nd, and Pb isotope compositions of thesilicic Lassen rocks overlap those of the ma¢clavas [25,28], these isotopic systems cannot pro-vide a robust test of the trace element models ofcrustal melting and assimilation.

3. Sample preparation and analytical methods

3.1. Sample preparation

Whole-rock Re and Os isotope analyses of sili-cic volcanic rocks are very di⁄cult because Reand Os concentrations are much less than analyt-ical blank levels (unpublished data; G.L. Hart,C.M. Johnson, S.B. Shirey, W. Hildreth). How-ever, because Fe^Ti oxides are thought to host themajority of Re and Os in evolved rocks [35,36],Re^Os analyses of silicic rocks may be obtainedthrough analysis of the magnetite (Fe^Ti oxide)concentrates, which e¡ectively concentrates andraises the Re and Os abundances to measurablelevels. If we assume that virtually all the Os in therocks of this study lies in magnetite, and thatmagnetite comprises 1% of the rock, then 50 pptOs contents in the magnetite imply whole-rock Osabundances of V0.5 ppt. Analyses of magnetiteseparates thus allows Os isotope data to be ob-tained on low abundance samples that would beimpossible otherwise by achieving a V100-foldOs pre-concentration. In the Ferrar province,Brauns et al. [37] show that 187Re/188Os ratiosfrom magnetite-rich separates from basalts aresimilar to whole-rock ratios, suggesting that Osisotope analyses on magnetite separates are repre-sentative of whole-rocks. This assumption may beapplied to the evolved rocks in this study, espe-cially since they are young and relatively littleradiogenic decay has occurred, and whole-rocksilicic rocks have very low Os contents.Fe^Ti oxide concentrates were obtained from

ten silicic volcanic rocks from the Lassen region(see appendix, Background Data Set1). These sili-cic rocks have V1% Fe^Ti oxides [28], a portionof which may come from disaggregated ma¢c in-

clusions which might impart heterogeneous Oscontents and isotope compositions to the samplesdue, in part, to Cr^Ti zoning in the magnetites[30]. For this reason, ma¢c inclusions were pur-posely avoided during the sampling and crushingprocess to minimize the amount of xenocrysticmaterial, which has been shown to produce lessscatter in the major and trace element data sets[18,39]. However, disaggregation and incorpora-tion of xenocrystic material into magma bodiesare part of the intra-crustal processes that Os iso-topes are ideally suited to identify, particularly ifthe xenocrystic material has radiogenic Os isotopecompositions (as discussed below).Approximately 15^20 kg of rock sample was

crushed using a tungsten carbide (W-C) hydraulicrock breaker to avoid iron contamination.Although tungsten is a potential contaminantfor Re^Os isotope studies, any W-C fragmentswould not be present in the magnetic fractionsused in this study. Potential iron contaminationfrom hammer marks was removed by cleavingfresh surfaces using the W-C rock breaker. Thesesteps are essential because metal rock-processingequipment has V1000 times the Os content ofmagnetite concentrates from crustal rocks (Table1). Minerals were initially separated using densityvariations (shaker style ‘gold-table’), followed byseveral magnetic separations in a water slurry.Sample purity is visually estimated at 70^90%;the impurities, which are mainly silicate mineralsand glass adhering to the oxide grains, are antici-pated to contain sub-blank concentrations of Reand Os, and therefore only dilute the Re and Osconcentrations.

3.2. Analytical methods

Re^Os isotope analyses were done at the De-partment of Terrestrial Magnetism, Carnegie In-stitution of Washington (DTM). All samples werespiked with separate 190Os- and 185Re-enrichedsolutions. Fe^Ti oxides (1^2 g of granular ali-quots) were digested using a modi¢ed two-stageCarius tube technique, which provides complete

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Table1

Chemical

andisotopicdata

forLVC,associated

oldercenters,andoldercrustalrocks

Sample

Sequence

age

Sequence/

volcanic

center

87Sr/86Sr

ONd

206Pb/

204Pb

207Pb/

204Pb

208Pb/

204Pb

Re

Os

187Re/

188Os

187Os/

188Os

meas.

2SE

meas.

187Os/

188Os

corr.

Blank

corr.

error

QOs

(ka)

(ppt)

(ppt)

(%)

(%)

LC84-443

100^0

EaglePeak

0.70410

3.80

18.949

15.605

38.587

274

40.8

32.75

0.2213

0.09

0.2233

+0.97/3

0.45

75LC83-360

100^0

EaglePeak

0.70390

3.57

18.951

15.612

38.616

190

20.7

44.75

0.2357

0.31

0.2410

+2.65/3

1.14

89LC81-706

250^200

Bum

pass

0.70396

3.32

18.977

15.624

38.668

326

22.8

71.27

0.3715

0.30

0.3886

+5.29/3

2.31

205

LC84-541

250^200

Bum

pass

0.70412

3.32

18.943

15.606

38.589

386

8.9

218.3

0.3621

0.40

0.4040

+16.6/3

5.73

217

LC81-659

400?

Loomis?

0.70402

3.53

18.946

15.604

38.591

253

30.2

41.80

0.3979

0.30

0.4127

+4.10/3

1.85

224

LM80-899

400

Loomis

0.70402

3.26

18.947

15.604

38.591

343

7.9

217.8

0.3381

0.42

0.3764

+16.7/3

5.64

195

LC82-194

470^400

BV

0.70420

3.20

18.595

15.611

38.621

270

51.0

25.89

0.2197

0.47

0.2212

+0.73/3

0.35

73LM80-824

600^470

BV

0.70418

2.56

18.954

15.626

38.657

619

169

17.88

0.2032

0.09

0.2034

+0.13/3

0.07

59LM80-854

600^470

BV

0.70370

3.96

18.874

15.595

38.498

1390

124

54.14

0.1578

0.20

0.1574

+0.12/3

0.23

23LC88-1392

V1200

MVC

0.70418

2.22

18.990

15.613

38.645

789

64.8

59.07

0.1755

0.17

0.1753

+0.05/3

0.09

37BR-1

130Ma

SN3

27.5

0.50

0.3249

154

BR-1

OL

130Ma

SN12.1

0.5765

350

Z-11

85Ma

SN7

3.7

9.44

2.368

1750

Z-11OL

85Ma

SN2.2

2.021

1479

MG-5

OL

85Ma

SN212

3.8

272.4

1.886

1373

92TF105OL

85Ma

SN261

12.7

100.3

0.8477

562

PFP-1

OL

85Ma

SN89

16.2

26.54

0.9362

631

UW-1

63900

18500

16.76

0.2779

117

Samplenames,ages,andSr,Nd,

andPbisotopecompositionsfrom

[18,28,31,38].Samplelocationsandadditional

chem

ical

data

canbe

foundin

thesereferences.

ReandOsconcentrations

and

187Os/188Osratios

arefrom

magnetite

concentrates

(unlessnoted),andarecorrectedforchem

ical

procedural

blankandformass

fractionation(192Os/188Os=

0.30826).In

thisstudy,

OsandReblankvalues

were

62pg

and

68pg,respectively,with

187Os/188Os=

0.1805

(W9).Standard

errors

arebasedon

2cvariations

ofuncorrecteddata.Blank-corrected

errors

(in%)arebasedon

variations

ofcorrected

187Os/188Osvalues

with4pg

and1pg

blanklev-

els,thus

allowingfortheuncertaintyof

theblanklevelsto

beassessed.Sincetheblankisrelatively

unradiogenic

( QOs=41),

QOsvalues

fortherocksof

thisstudy

representminimum

values,except

where

theQOsvalues

arealreadylow.BV=Brokeo¡

Volcano

;MVC=Maidu

VolcanicCenter.SN

=Sierra

Nevadacrustalrocks

provided

byR.Kistler

andA.Glazner.187Os/188Osratios

forSN

rocksarepresent-dayvalues.AllSN

magnetite

was

hand-pickedto

s99%

purity.Oxalic

acid

leachused

toremovepotentialoxidecoatings

containing

ReandOs.UW-1=metal

fragmentsfrom

jawcrusherat

UW-M

adison.OL=oxalicleach.

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dissolution of magnetite and spike equilibration[40]. The two-stage Carius tube technique consistsof an initial acid dissolution stage of V2^3 mlconcentrated HCl at 220‡C for V12^15 h, fol-lowed by a highly oxidizing stage of mixedHCl^HNO3 (obtained by adding V5^6 ml con-centrated HNO3) at 220‡C for V12^15 h. Onpure magnetite samples, this two-stage processproduced clear dissolutions of magnetite that oth-erwise would not have been dissolved using thesingle-stage method. Because this technique re-quires sealing the Carius tubes twice, care mustbe taken to ensure enough neck material remainson the tube for proper seals. Osmium was ex-tracted using the solvent-extraction method ofCohen and Waters [41], and a microdistillationprocess using concentrated HBr and chromicacid. Re was separated by anion exchange chro-matography [40,42].Isotopic ratios were measured by negative ther-

mal ionization mass spectrometry, where Os wasmeasured as OsO3

3 and Re as ReO34 . Reproduc-

ibility of an in-house DTM Os standard solutionyielded 187Os/188Os better than 0.2%. During thecourse of this study Os and Re blanks wereV2 pgand V8 pg, respectively; the blank 187Os/188Os=0.1805W 9. Osmium concentrations in the samplesreported here are generally 10^50 ppt, and uncer-tainties in blank corrections are therefore the larg-est contributions to the total uncertainties in Osisotope compositions (Table 1). The QOs valuespresented here generally represent minimum val-ues because the blank is relatively unradiogenic.

4. Results

The intermediate- to silicic-composition lavasfrom LVC have radiogenic QOs values, rangingfrom +23 to +224 (Table 1), signi¢cantly higherthan those of MORB and OIB [14,43]. Sampleswithin individual eruptive sequences have similarOs isotope compositions (Fig. 3), indicating bothhomogeneity on the sample to sample scale and

Fig. 3. Stratigraphic variations for SiO2 (wt%), 87Sr/86Sr, ONd, Os content, and QOs variations for LVC and regional ma¢c lavas.The shaded boxes re£ect the range in values for a given sequence, center, or group. The symbols are samples from this studychosen to be representative of the sequence, center, or group. Data for regional ma¢c lavas from [16]. Osmium contents and QOs

values are from magnetite concentrates. The increase in QOs from the Brokeo¡ stage to the Loomis and Bumpass sequences is at-tributed to interaction with a more radiogenic lower ma¢c crust. The strong decrease in QOs values for the Eagle Peak Sequencemay re£ect mixing with high-Os content (low-QOs) primitive basaltic magmas, which has been previously suggested based on thepresence of forsteritic olivine xenocrysts [18,39].

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that Os analyses on magnetite approximate theisotopic composition of these young lava £ows.In situ age corrections on the magnetite concen-trates are insigni¢cant because the samples areless than 0.6 Myr old (except LC88-1392) andthe parent/daughter ratios are low (Table 1).The Re contents of the magnetite concentratesvary from V190 to 1400 ppt, and the Os contentsrange from V8 to 170 ppt, similar to slightlylower than the Os content (11^370 ppt) of mostma¢c whole-rocks from the Lassen region [16].Whole-rock Os contents of evolved rocks are be-low blank levels (V2 ppt), based on reconnais-sance work of granitic [36] and silicic rocks (un-published data; G.L. Hart, C.M. Johnson, S.B.Shirey, W. Hildreth, R.L. Christiansen).The Brokeo¡ Volcano and Maidu samples have

lower QOs values (Figs. 3 and 4), similar to thoseof regional ma¢c lavas. The Os contents of theBrokeo¡ Volcano samples decrease and the QOs

values increase with SiO2. The stage III sequences(Loomis, Bumpass, and Eagle Peak) shift to moreevolved compositions and re£ect similar degreesof overall di¡erentiation as shown, for example,by the similarity of Ba and Th contents [31]. TheLoomis and Bumpass sequences have high-QOsvalues, ranging from +195 to +224, whereas thevalues of the younger Eagle Peak Sequence dropsigni¢cantly (QOs =+75 to +89), with no apparentchanges in the isotopic compositions of other el-ements (Figs. 3 and 4). The 87Sr/86Sr ratios andONd values for all three sequences are similar,ranging from 0.7037 to 0.7042 and +2.6 to +4.2,respectively [31].

5. Role of the crust

The radiogenic nature of Os in the silicic Lassenlavas is similar to that found in other arcs. Forexample, QOs values of ma¢c- to silicic-composi-tion rocks from Java vary from +88 to +2804[33], which are much higher than values observedin this study of the southern Cascade arc. Alves etal. [33] interpret the elevated QOs values from Javato re£ect a variable subduction-derived contami-nant. The inferred subduction component in Javahas much higher QOs values than those inferred for

the Lassen region, where Borg et al. [16] interpretthe subduction component to have QOs values ofV+120. However, because the volumetricallydominant type of basalt from the arc axis in theLassen region has low-QOs values [16], which isinterpreted to re£ect little Os in£uence from sub-duction-derived material, an explanation otherthan a subducted sediment or £uid component isneeded for the high-QOs silicic rocks at LVC. Wepropose below that intra-crustal magmatic pro-cesses that occurred during magma transportand emplacement within the lower crust exertedthe major control on Os isotope compositions ofthe evolved rocks at LVC.Con¢ning the source of radiogenic Os to the

arc crust in the Lassen region places speci¢c iso-topic constraints on the nature of the crust in-volved, and requires involvement of crustal mate-rial that has high-QOs values but sub-arc mantle-like Sr, Nd, and Pb isotope compositions. Suchmaterial must be present within the crustal col-umn below LVC where it may interact with as-cending and evolving magma bodies through as-similation and/or crustal melting processes.Potential components of the crustal column belowLVC include Paleozoic and Mesozoic igneous andaccreted rocks, and both primitive and fractionat-ed arc basalts that were derived from the mantlewedge. These components will be discussed below

Fig. 4. QOs vs. 87Sr/86Sr for the silicic volcanic rocks from theLassen region. Data are from magnetite separates. Sequencesare in stratigraphic order with the youngest at the top. Ba-salt ¢eld is whole-rock data from [16]. The solid ¢eld repre-sents the volumetrically dominant parental basalts, like thosefrom the arc axis, and the dashed ¢eld represents basaltsfrom more toward the forearc. See Fig. 3 and text for morediscussion.

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in terms of their ability to generate the observedOs, Sr, Nd, and Pb isotope ratios in the evolvedvolcanic rocks through processes within the crust.

5.1. High-QOs values in evolved rocks at LVC

The Loomis and Bumpass eruptive sequencesof the LVC have QOs values that are V200 QOs

units higher than those for primitive basalts andV125^175 QOs units higher than other eruptivesequences at LVC. Such high values suggest ahigh-QOs component to the magmas that has notbeen reset to mantle values by disaggregated xeno-crystic material. Crustal components in the Las-sen region that may have high-QOs values includeSierra Nevada and Klamath batholithic rocks,and the associated Paleozoic and Mesozoic wall-rocks that underlie LVC [44]. These componentsare su⁄ciently old enough that elevated QOs valuesare likely to have developed by in situ decay of187Re. However, these older crustal componentsalso have present-day 87Sr/86Sr ratios that are sig-ni¢cantly higher than those of LVC rocks(s 0.7054W 7; n=94, 2c ; [7,28,31,45^47]) aswell as low Os contents (Table 1; [36]). Involve-

ment of this high-87Sr/86Sr crustal material (‘Crust1’, Fig. 5), either through mixing or assimilation,does not produce QOs^87Sr/86Sr variations thatmatch the LVC data, assuming that the parentalbasaltic compositions are equal to those of thevolumetrically dominant type of arc axis basaltin the Lassen region.An additional source of high-QOs material can

be found in the primitive basalts that have QOs

values extending to +120 [16]. This potential com-ponent is unlikely to have contributed to the high-QOs values of the evolved LVC rocks because thehigh-QOs basalts are volumetrically insigni¢cant inthe arc axis and have low Os abundances. More-over, the 87Sr/86Sr ratios of the high-QOs basaltsare far too unradiogenic to have been involvedin the LVC magmas, and no mixing or assimila-tion models reproduce the observed LVC data(‘Crust 3’, Fig. 5).We conclude, therefore, that the high-QOs values

of the evolved volcanic rocks at the LVC cannotbe explained through incorporation of older base-ment rocks or primitive high-QOs forearc basalts.The high-QOs values must re£ect an additionalcrustal component, such as young fractionated

Fig. 5. Results of mixing and fractional crystallization models for LVC. Fields and symbols are as in Fig. 4. Data represent anal-yses on magnetite concentrates. The modeled basalt has 87Sr/86Sr= 0.7038, Sr= 370 ppm, QOs = 10, and Os= 200 ppt. The crustalcomponents were chosen to represent the possible range of compositions present in the Lassen region. Crust 1 has 87Sr/86Sr = 0.7055, Sr= 250 ppm, QOs = 225, and Os=30 ppt; Crust 2 has 87Sr/86Sr = 0.7038, Sr= 250 ppm, QOs = 225, and Os=30 ppt;and Crust 3 has 87Sr/86Sr= 0.7031, Sr= 250 ppm, QOs = 115, and Os=30 ppt. Crust 1 represents older Sierra Nevada/Klamathrocks, Crust 3 represents ma¢c arc basalts (regional basalts), and Crust 2 represents the type of crust that must have interactedwith the evolving magmas. (a) Simple mixing models of a basaltic magma with various crustal components. Hatch marks indicatepercentages of end-member components in the mixture. (b) Assimilation and fractional crystallization models for a basaltic mag-ma with various crustal components. An R value of 0.5 was used, and DOs = 1 and DSr = 3; various D values were tried, but donot change the general shape of the curves. The hatch marks indicate the percentage of melt left in the system (‘F’ value).

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lower crustal basalts, that had high-QOs values but87Sr/86Sr ratios that were similar to those of themain arc axis parental basalts. Using such a com-position, both simple mixing and assimilation/fractional crystallization processes ¢t the observedQOs^87Sr/86Sr variations for the LVC data (‘Crust2’, Fig. 5).

5.2. Shifts in QOs values of evolved LVC volcanicrocks

The large contrast in Os contents between siliciclavas and mantle-derived basalts raises the possi-bility that late-stage, pre-eruptive mixing in mag-ma chambers may signi¢cantly modify the QOs val-ues of silicic magmas. For example, the QOs valuesof the Eagle Peak Sequence drop V120 QOs unitsfrom the previous two eruptive sequences (Figs. 3and 4). The large decrease in QOs values couldoccur by incorporation/mixing of as little as 2%ma¢c magmas (Os = 300 ppt; QOs =+10) intoevolved magmas during crustal melting, or duringlater stages of magmatic evolution, assuming thesilicic magmas had 1 ppt Os; higher percentagesof 10^20% are allowed if the Os abundance of thema¢c magma is decreased. There is therefore noreason to suspect that the evolved Lassen rocksthat have ‘intermediate’ QOs values (QOs =+30 to+90) re£ect signi¢cantly less crustal involvementor interaction with another type of crust than thatinvolved in the higher-QOs samples. The e¡ects ofmixing even small amounts of high-Os ma¢c com-ponents into silicic magmas suggest that the mea-sured QOs values of silicic volcanic rocks may onlyprovide minimum estimates for the QOs values ofthe crustal component. At LVC, late-stage mixingis consistent with the presence of mantle olivine,undercooled inclusions, and disaggregated xeno-liths [18,19,39].

6. A model for Re^Os isotope evolution in thelower arc crust

The high-QOs values in intermediate- to silicic-composition rocks from the LVC suggest thepresence of a high-QOs crustal component otherthan Paleozoic and Mesozoic crust and forearc

primitive basalts, and a 5^10 Ma ma¢c lowercrustal material is the most likely candidate forsuch a component. The large contrasts in Reand Os Kd values in ma¢c magmas [48^52] canproduce high Re/Os ratios in even moderatelyevolved magmas [12^16,33,53]. These high 187Re/188Os ratios will produce very radiogenic compo-sitions in only a few Myr solely by radiogenicdecay (Fig. 6). Such radiogenic crust may thenbe involved in succeeding magmatic events suchas those that lead to the formation of the evolvedrocks at LVC [13]. We note that because the Re/Os ratios of the primitive arc basalts are quite low[16], these rocks cannot produce su⁄ciently radio-genic Os isotope compositions in time scales of6 100 Myr.Lower crustal material is not exposed as out-

crop or xenoliths at LVC but the isotopic compo-sition of the lower crust can be modeled as itwould have been produced by variably fractionat-ed mantle-derived magmas emplaced prior to de-velopment of the LVC. We develop a new math-ematical model (Fig. 6) that accounts for the Reand Os concentrations in an evolving crustal col-umn during magmatic activity. The model pre-sented here is developed independently of thedata and is intended to illustrate the range ofpotential crustal compositions that may becomecomponents or sources of younger magmatism;the model is not intended to predict or describethe isotopic nature of ma¢c magmas that havereached the surface.The lower arc crust is modeled by ¢ve compo-

sitions (evolved basalt to rhyolite) that representcrystallization ranges of mantle-derived magmathat may have been emplaced into the crustal col-umn beneath LVC in the Pliocene or Late Mio-cene (Fig. 6). Each gradient-shaded ‘box’ repre-sents the sum of all magmas derived by a givenrange of fractional crystallization. Because Re andOs Kd values di¡er so much, the magmatic com-positions, total Re and Os contents, and Os iso-tope composition of each crustal column (‘box’)cannot be obtained by a simple average, but in-stead can only be calculated by a mass- and con-centration-weighted integration of the entire col-umn (see appendix II in the Background Data Set1

for calculations). Due to a lack of consensus on

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distribution coe⁄cients and the very low Os con-tents in the rocks, Re and Os Kd values (Table 2)are inferred from the excellent correlations be-tween 187Re/188Os ratios and Os contents for ba-

saltic (and some evolved) lavas throughout theworld, regardless of their tectonic setting or pet-rologic a⁄nity, assuming that this range is largelydue to crystallization.In the center of Fig. 6, the integrated mass- and

concentration-weighted elemental and isotopiccompositions are shown for each crustal ‘box’.The age dependent QOs values for 1, 5, 10, and20 Myr of isotopic evolution are plotted at thetop of the ¢gure for each integrated ‘box’. Thegray band illustrates the range of isotopic compo-sitions that have been measured for silicic lavasfrom the Lassen region, which are signi¢cantlymore radiogenic than the mantle (QOs = 0). It isapparent that large QOs values require at leastsome aging of the ma¢c lower crust.The model is most robust for the range of 0^

60% crystallization, over which the Kd values (Ta-ble 2) are best constrained as inferred from mea-sured Re and Os contents in rocks of basalt- tobasaltic-andesite composition [14,53]. The rangeof 0^60% crystallization is similar to the rangeestimated for the majority of Brokeo¡ Volcanolavas of LVC [18,19]. The modeled wt% SiO2

and MgO contents for 60% crystallization varyfrom 47.5 to 52.6 and 9.4 to 5.7, respectively,using crystal fractionation models developed forsouthern Cascade volcanoes based on phaseassemblages and modal abundances [18,54,55](Figs. 6 and 7). Crystal cumulates are not in-cluded in the evolved (integrated) crustal columns,because these would be refractory and not likelyto participate in melting or assimilation.The last four crystallization ranges of the model

(Fig. 6) show changes in 187Re/188Os ratios and Oscontents as the magma evolves from basaltic-an-desite to rhyolite. There is less certainty in extra-polating the model to these intermediate and siliciccompositions, largely because appropriate bulk Dvalues are poorly known, causing the model tobecome highly parameter dependent, and becauseOs isotope analyses were only done on magnetiteseparates. With increasing wt% SiO2 the D valueshave been adjusted (Table 2) to keep the modelwithin the range of Os and Re contents measuredfor crustal granitic rocks and subduction zone set-tings [33,36], as well as those suggested by exper-imental work [52]. For the more evolved inte-

Fig. 6. A model for predicting the integrated Os contentsand isotope compositions of young arc crust, calculated for¢ve compositions that re£ect a range of fractional crystalliza-tion of mantle-derived basalt magma that has a starting com-position of 187Re/188Os= 9.6, ppt Os= 200, wt% SiO2 = 47.5.Each shaded ‘box’ represents all residual liquids producedduring the range of crystallization indicated (relative size ofboxes to scale). Integrated crustal compositions for these‘boxes’ are shown in the middle of the ¢gure. Isotopic evolu-tion of the crustal compositions is shown at the top for 1, 5,10, and 20 Myr periods. The gray band shows the range ofmeasured QOs values for evolved rocks at LVC. See text fordiscussion.

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grated crustal compositions the Os contents areless than 1 ppt and the Re contents are s 4000ppt, which results in very high Re/Os ratios, andhighlights the compatible nature of Os in fraction-ing mineral assemblages as the main control ondeveloping such high parent/daughter ratios.The increase in 187Re/188Os ratios, and the de-

cline in Os contents, are less extreme for the mass-and concentration-weighted crystal fractionation(MCWCF) models as compared to Rayleigh frac-tionation models (Fig. 7), because the MCWCFmodels continuously re-integrate the mass of thesystem during magmatic evolution; this methodmimics the crustal section beneath the arc andwould include all crystallized material. In con-trast, Os depletion and 187Re/188Os enrichmentare more extreme for pure Raleigh fractionationmodels, which represent the composition of dis-crete batches of magma that have evolved to spe-ci¢c ‘F’ values, rather than an integrated compo-sition over the range of ‘F’. Conceptually, theintegrated crustal model presented here could rep-resent minimum 187Os/188Os ratios that may bedeveloped in young arc crust, whereas Rayleighfractionation likely re£ects maximum expected187Os/188Os ratios.

7. Implications for crustal growth

The potential for high-QOs values to develop inyoung ma¢c plutons coupled with unradiogenic Srhas implications for the nature of the material inthe crustal column, its age, and the interaction ofevolving arc lavas with it. Quaternary magmatismat LVC may represent signi¢cant additions to thecrustal column, but the radiogenic Os isotopecompositions of the evolved rocks of the Loomisand Bumpass sequences, as well as simple mass-balance calculations, suggest that signi¢cantly

Table 2Parameters for forward AFC modeling of magma reservoirs

Stage 1 Stage 2 Stage 3 Stage 4 Stage 5

F (incremental) 1.0^0.4 1.0^0.75 1.0^0.6 1.0^0.72 1.0^0.62F (total) 1.0^0.4 0.4^0.3 0.3^0.18 0.18^0.13 0.13^0.08DRe 0.5 0.8 0.85 0.9 0.95DOs 7 1.5 1.4 1.3 1.2Re ppt 400 632.5 669.9 723.3 747.2Os ppt 200 0.82 0.71 0.58 0.52

Partition coe⁄cients are based upon Os concentrations and 187Re/188Os data from the Lassen region and Java (see text for refer-ences), and used in the Rayleigh fractionation model. The stage 1 coe⁄cients are the most robust since they control most of thefractionation path and are constrained by data. Coe⁄cients for stages 2^5 are allowed to change as needed to keep the fractiona-tion model reasonable. It should be noted that the stages are decoupled from each other, thus making the Re and Os concentra-tions valid only for a package of rocks of a given SiO2 content, and are not meant to model the integrated mass^concentrationweighted values of Fig. 6.

Fig. 7. Relative mass of magma remaining (F) versus 187Re/188Os (solid lines) and Os content (dashed lines) calculatedusing a Rayleigh crystal fractionation model (RF) and an in-tegrated MCWCF model for basalt. The integrated mass-and concentration-weighted model line is only shown for the¢rst 60% crystallization because this is where the model de-couples the calculations of Os content and isotope composi-tion from the next range of crystallization. Compared to RFmodels, the changes in Os contents and 187Re/188Os ratiosare less extreme for MCWCF models, because they continu-ously re-integrate the mass of the system during magmaticevolution. The more extreme pure RF models represent thecomposition of discrete batches of magma that have evolvedto speci¢c ‘F’ values. The circles represent the integratedcrustal 187Re/188Os ratios for the last four crystallizationranges of the model (Fig. 6) at an averaged ‘F’ value.

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larger crustal additions may have occurred in thePliocene (or Late Miocene). Thus, in the Lassenregion it seems likely that the contribution fromthis older pulse of magmatism to overall crustalgrowth may be signi¢cant. This conclusion is sup-ported by Gu¡anti et al. [54] where they predictedan additional volume of basalt in£ux to the lowercrust was necessary to overcome the heat budgetconstraints of their petrologic model. More con-tinuous magma production models fail to gener-ate the volume of fractionated magmas needed tocontribute radiogenic Os to later magma pulses.The architecture of the crustal and mantle sec-

tion beneath the LVC, as discussed and modeledabove, is illustrated in Fig. 8. Sur¢cial exposuresindicate that evolved rocks dominate in the arcaxis, and more ma¢c rocks prevail along the mar-gins of the arc axis [27]. Based on the location andvolume of the evolved rocks it is inferred that alarge £ux of variably fractionated mantle-derivedmagma must have been emplaced into the crustalcolumn beneath the evolved volcanic centers [54],forming a zone of extensive mixing and crustalinteraction (cross-hatch pattern in Fig. 8) [8].This magma would have high Re/Os ratios, low

Os contents, and, in the case of the arc axis mag-mas in the Lassen region, would contain little Osin£uence from material derived from the slab.In summary, provided that subducted compo-

nents and old crust may be eliminated as sourcesfor high-QOs values in arc magmas, evolved vol-canic rocks that have high-QOs values suggest thatlarge volumes of mantle-derived magma musthave been emplaced at an earlier time, and thatthis magma must have undergone extensive frac-tionation within the crust. In a ‘modern’ arc set-ting, the timing of such an earlier magmatic eventis likely to have been far shorter than that whichwould produce measurable isotopic evolution ofSr, Nd, or Pb.

8. Preservation of high-QQOs magmas in a MASHenvironment

8.1. Addition of a ma¢c component

Mass-balance calculations suggest that magmaswhich have elevated QOs values and low Os con-tents must become isolated from further interac-

Fig. 8. Schematic west^east cross-section of the crustal section and subduction zone through the LVC. The relative volume of ba-saltic vs. evolved rocks located in the crustal column is schematically shown at the top of the ¢gure, indicating an increased pro-portion of evolved compositions at the arc axis, and more ma¢c composition away from the axis. The presence of more evolvedcompositions along the arc axis suggests the predominance of fractionated magmas with high Re/Os ratios in the crustal column.Over time, such crust could develop radiogenic Os isotope compositions that could later be incorporated into ascending magmasthat produced the LVC. See text for further discussion. Modi¢ed from [27].

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tion with more primitive, high-Os, mantle-derivedmagmas because even small amounts of mixingwith primitive magmas would strongly shift theQOs values toward that of the mantle. AfterV30^40% crystal fractionation, a mantle-derivedmagma with 200 ppt Os might drop to V10^20ppt Os, based on observed variations in basalts.The addition of an equal volume of primitivemantle-derived magma, for example, would pro-duce a mixed magma that contained s 90% man-tle Os. It therefore seems likely that for domainsof contrasting Os isotope compositions and lowOs abundances to be preserved within the crustalcolumn, fractionated low Os magmas and plutonsmust be segregated or shielded from any furthermantle in£uence by primitive basalts, or if suchinteraction did occur, the evolved lavas place aminimum constraint on incorporation of olderarc crust.Fractional crystallization models of evolving

magma bodies indicate that silicic rocks, such asthose found in continental cratons, would havevery high Re and very low Os contents. Yet, theOs content data for the evolved rocks at LVC(especially Eagle Peak and Maidu rocks) haveOs contents substantially higher than predictedby models. This may suggest that many evolvedmagmas have interacted with small amounts ofless evolved material in the lower crust that hadhigher Os concentrations. The details of such in-teraction would depend on the degree of fraction-ation, Os concentration, and age of the materialinvolved.

8.2. MASH zone processes

The wide range in Os isotope variations inevolved arc rocks, especially as seen at Lassen,as well as their generally radiogenic nature, im-plies a di¡erent view on communication betweenprimitive basalts, evolved rocks, and crust thanthat gleaned using the more common isotopic sys-tems such as O, Sr, Nd, and Pb. The ‘MASH’model of arc magma evolution, as originally pro-posed by Hildreth and Moorbath [8], works wellin explaining isotopic variations and elementalabundances for elements where their relativeabundances are not greatly di¡erent from each

Fig. 9. Schematic representation of the proposed model forformation and preservation of high-QOs magmas by intra-crustal processes. Hachure pattern represents the heteroge-neous pre-Quaternary lower arc crust, which has experiencedmillions of years of magmatism. The gradient-¢lled domainsrepresent post-Quaternary magma bodies emplaced into thelower arc crust, with the heavy outline denoting the most re-cent magmatic events. The gradient-¢ll illustrates the hetero-geneity of these domains in terms of QOs values, Re/Os ratios,and Re and Os contents. These heterogeneities occur due tovariable amounts of melting and mixing of older, isotopicallyaged (in terms of Os) lower crust, diagrammatically shownby the various included material in the most recent magmabodies. In order for these heterogeneities to persist, themixed magma must become physically isolated from furthermantle input, either by some physical or chemical barrier,because any mixing between evolved (low-Os) and ma¢c(high-Os) components will rapidly move the evolved magmato mantle-like Os contents, Re/Os ratios, and Os isotopecompositions. The O, Sr, Nd, and Pb isotope compositionsof all these plutons and magma bodies, however, would beessentially identical because of the homogenizing nature ofthe MASH zone processes, the insu⁄cient parent/daughterratios to become radiogenic, and the generally young natureof the crust. These isolated domains may become incorpo-rated into younger magmatic events, and, perhaps, mostreadily preserved in the evolved rocks at volcanic centers.The scale bar for Os contents and QOs values is based onRayleigh fractionation models (DOs = 0.5, DRe = 7, t=5 Myr)and represents maximum values.

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other among the reservoirs involved [56,57]. How-ever, in the case of Os, the widely variable Oscontents and Re/Os ratios produced during mag-matic di¡erentiation will result in domains of arccrust that are extremely variable in terms of ele-mental abundances and isotopic compositions forOs, as compared to other isotopic systems. Thevariable Os isotope compositions measured in thisstudy suggest four possible conclusions regardingMASH zone processes: (1) some batches of mag-ma and/or plutons are apparently physically iso-lated from the homogenization processes in theMASH zone, (2) the MASH zone processes oper-ate on fractionated (low-Os) magmas only, (3)MASH zone processes are insu⁄cient to com-pletely homogenize components that are very het-erogenous in terms of Os contents, and (4) prim-itive magmas do not interact readily with evolvedmagmas. These scenarios most likely operate si-multaneously with each other on a larger regionalscale, but independently and separately on smallerscales. Factors which may contribute to incom-plete homogenization of various magmas includethermal, geometric, viscosity, and density barriers[58^61], which restrict the likelihood of mixingbetween, for example, primitive (high-Os) andevolved (low-Os) magmas.By Quaternary time at the LVC, it is envisioned

that the lower crust had already experienced mil-lions of years of magmatism and consisted of anamalgamation of overlapping plutons that variedindividually in terms of their Os contents and Osisotope compositions (Fig. 9), and yet were quitehomogenous in terms of their Sr, Nd, and Pbisotope compositions because of the relativelyyoung age of the crust and ‘MASH-type’ process-es. As younger Quaternary-age, mantle-derivedmagmas rose through this heterogenous (in termsof Os) crust (Fig. 9) they evolved to low Os con-tents and became isolated from further mantleinput by crystallization or sequestered long-livedmagma chambers, preserving individual domainsthat, over short periods of time, isotopicallyevolve to high-QOs values. The QOs values reportedfor the evolved rocks at LVC in this study may beconsidered minimum values because of the ex-ceedingly low potential for preservation of mag-mas with extremely high QOs values within the

MASH zone of the lower crust. Even slight inter-actions with primitive (high-Os) basalts, whichBorg et al. [16] establish are present at the surface,would erase the high-QOs crustal signature.

9. Conclusion

Application of high-precision Re^Os isotopeanalyses to problems in crustal evolution usingintermediate- to silicic-composition rocks is possi-ble using Fe^Ti oxides (primarily magnetite),which appear to be the major repository for Reand Os and to approximate whole-rock Os iso-tope compositions in evolved rocks. Osmium iso-tope compositions for evolved rocks from theQuaternary LVC are much more radiogenicthan other mantle-derived rocks, indicating in-volvement of a high-QOs component. The sourceof the radiogenic Os must be di¡erent than thatdescribed for other subduction zones (e.g. Java)and for regional forearc basalts of the Lassen re-gion, where high-QOs values are attributed to sedi-ment and slab-£uid contamination. In the Lassenregion, sediment and slab contamination is re-stricted to the forearc, with little in£uence at thearc axis, where this study has found radiogenic Osisotope compositions. The radiogenic Os source islikely to be young (5^10 Ma) lower crust becauseSr, Nd, and Pb isotope compositions of the silicicrocks are mantle-like, and overlap those of coevalbasalts, eliminating older (Mesozoic) crust as apotential contributor. A model for calculating ra-diogenic Os isotope evolution in young arc crustis consistent with assimilation of substantialamounts of earlier ma¢c crust because large con-trasts in Re and Os partitioning during crystalfractionation of basaltic magmas produce highRe/Os ratios after even modest degrees of crystal-lization. These results suggest that beneath youngarcs, such as in Cascade arc, early magmatic ad-dition and signi¢cant lower crustal recycling mayoccur which is only detected by Os isotopes, andnot isotopic systems such as O, Sr, Nd, and Pb.The occurrence of radiogenic Os in LVC indicatesa period of earlier (5^10 Ma) magmatism, thee¡ects of which may be underestimated basedon more traditional isotopic systems, due to lack

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of isotopic resolution. Furthermore, the presenceof radiogenic Os at LVC indicates that portionsof the lower arc crust must be resistant to or iso-lated from homogenization processes in theMASH zone and from further input of mantle-derived magmas, suggesting a magma evolutionmodel that preserves individual plutons withinthe crustal column.

Acknowledgements

We thank L. Borg, C. Hawkesworth, G. Pear-son, V. Salters, and anonymous reviewers forcomments on earlier versions of this manuscript.This project has been supported by Grants fromSigma Xi, GSA, and NSF (EAR-9980512).[AH]

References

[1] S.R. Taylor, Island arc models and the composition of thecontinental crust, Am. Geophys. Union Maurice EwingSer. 1 (1977) 323^335.

[2] S.R. Taylor, S.M. McLennan, The Continental Crust: ItsComposition and Evolution, Blackwell Scienti¢c, London,1985, 312 pp.

[3] R.W. Kay, S.M. Kay, Creation and destruction of lowercontinental crust, Geol. Rundsch. 80 (1991) 259^278.

[4] J. Gill, Orogenic Andesites and Plate Tectonics, Springer,Berlin, 1981, 390 pp.

[5] R.L. Rudnick, D.M. Fountain, Nature and compositionof the continental crust: A lower crustal perspective, Rev.Geophys. 33 (1995) 267^309.

[6] H.P. Taylor Jr., The oxygen isotope geochemistry of igne-ous rocks, Contrib. Mineral. Petrol. 19 (1968) 1^71.

[7] R.W. Kistler, Z.E. Peterman, Variations in Sr, Rb, K, Na,and initial 87Sr/86Sr in Mesozoic granitic rocks and in-truded wall rocks in Central California, Geol. Soc. Am.Bull. 84 (1973) 3489^3512.

[8] W. Hildreth, S. Moorbath, Crustal contributions to arcmagmatism in the Andes of central Chile, Contrib. Min-eral. Petrol. 98 (1988) 455^489.

[9] R.E. Zartman, S.M. Haines, The plumbotectonic modelfor Pb isotopic systematics among major terrestrial reser-voirs ^ a case for bi-directional transport, Geochim. Cos-mochim. Acta 52 (1988) 1327^1339.

[10] D.J. DePaolo, A.M. Linn, G. Schubert, The continentalcrustal age distribution: Methods of determining mantleseparation ages from Sm^Nd isotopic data and applica-tions to the southwestern United States, J. Geophys. Res.96 (1991) 2071^2088.

[11] C.M. Johnson, Mesozoic and Cenozoic contributions to

crustal growth in the southwestern United States, EarthPlanet. Sci. Lett. 118 (1993) 75^89.

[12] E. Widom, S.B. Shirey, Os isotopic systematics of theAzores: Implications for mantle plume sources, EarthPlanet. Sci. Lett. 142 (1996) 451^465.

[13] W.K. Hart, R.W. Carlson, S.B. Shirey, Radiogenic Os inprimitive basalts from the northwestern U.S.A.: implica-tions for petrogenesis, Earth Planet. Sci. Lett. 150 (1997)103^116.

[14] P. Schiano, J.-L. Birck, C.J. Alle'gre, Osmium^strontium^neodymium^lead isotopic covariations in mid-ocean ridgebasalt glasses and the heterogeneity of the upper mantle,Earth Planet. Sci. Lett. 150 (1997) 363^379.

[15] J.T. Chesley, J. Ruiz, Crust^mantle interaction in largeigneous provinces: implications for the Re^Os isotopesystematics of the Columbia River £ood basalts, EarthPlanet. Sci. Lett. 154 (1998) 1^11.

[16] L.E. Borg, A.D. Brandon, M.A. Clynne, R.J. Walker,Re^Os isotopic systematics of primitive lavas from theLassen region of the Cascade arc, California, Earth Plan-et. Sci. Lett. 177 (2000) 301^317.

[17] D.A. Swanson, K.A. Cameron, R.C. Evarts, P.T. Pringle,J.A. Vance, Cenozoic volcanism in the Cascade Rangeand Columbia Plateau, southern Washington and north-ernmost Oregon, Mem.-NM Bur. Mines Min. Res. 47(1989) 1^50.

[18] M.A. Clynne, Geologic studies of the Lassen VolcanicCenter, Cascade Range, California, Ph.D. Thesis, Univer-sity of California, Santa Cruz, CA, 1993, 404 pp.

[19] M.A. Clynne, Stratigraphic, lithologic, and major elementgeochemical constraints on magmatic evolution at LassenVolcanic Center, California, J. Geophys. Res. 95 (1990)19651^19669.

[20] C.R. Stern, F.A. Frey, K. Futa, R.E. Zartman, Z. Peng,T.K. Kyser, Trace-element and Sr, Nd, Pb, and O isotopiccomposition of Pliocene and Quaternary alkali basalts ofthe Patagonian Plateau lavas of southernmost SouthAmerica, Contrib. Mineral. Petrol. 104 (1990) 294^308.

[21] P.N. Lin, R.J. Stern, J. Morris, S.H. Bloomer, Nd- andSr-isotopic compositions of lavas from the northern Ma-riana and southern Volcano arcs: Implications for theorigin of island arc melts, Contrib. Mineral. Petrol. 105(1990) 381^392.

[22] J. Gamble, J. Woodhead, I. Wright, I. Smith, Basalt andsediment geochemistry and magma petrogenesis in a tran-sect from oceanic island arc to rifted continental marginarc: The Kermadec-Hikurangi margin, SW Paci¢c, J. Pet-rol. 37 (1996) 1523^1546.

[23] E. Hegner, M. Tatsumoto, Pb, Sr, and Nd isotopes inbasalts and sul¢des from the Juan de Fuca Ridge, J. Geo-phys. Res. 92 (1987) 11380^11386.

[24] W.M. White, A.W. Hofmann, H. Puchelt, Isotope geo-chemistry of Paci¢c mid-ocean ridge basalt, J. Geophys.Res. 92 (1987) 4881^4893.

[25] C.R. Bacon, P.E. Bruggman, R.L. Christiansen, M.A.Clynne, J.M. Donnelly-Nolan, W. Hildreth, Primitivemagmas at ¢ve Cascade volcanic ¢elds: melts from hot,

EPSL 6191 21-5-02

G.L. Hart et al. / Earth and Planetary Science Letters 199 (2002) 269^285 283

heterogeneous sub-arc mantle, Can. Mineral. 35 (1997)397^423.

[26] W.P. Leeman, D.R. Smith, W. Hildreth, Z. Palacz, N.Rogers, Compositional diversity of late Cenozoic basaltsin a transect across the southern Washington Cascades:implications for subduction zone magmatism, J. Geophys.Res. 95 (1990) 19561^19582.

[27] L.E. Borg, M.A. Clynne, T.D. Bullen, The variable role ofslab-derived £uids in the generation of a suite of primitivecalc-alkaline lavas from the southernmost Cascades, Cal-ifornia, Can. Mineral. 35 (1997) 425^452.

[28] L.E. Borg, M.A. Clynne, The petrogenesis of felsic calc-alkaline magmas from the southernmost Cascades, Cali-fornia: Origin by partial melting of basaltic lower crust,J. Petrol. 39 (1998) 1197^1222.

[29] S.-S. Sun, W.F. McDonough, Chemical and isotopic sys-tematics of oceanic basalts: implications for mantle com-position and processes, in: A.D. Saunders, M.J. Norry(Eds.), Magmatism in the Ocean Basins, Blackwell Scien-ti¢c, London, 1989, pp. 313^345.

[30] M.A. Clynne, L.E. Borg, Olivine and chromian spinel inprimitive calcalkaline and tholeiitic lavas from the south-ernmost Cascade Range, California; a re£ection of rela-tive fertility of the source, Can. Mineral. 35 (1997) 453^472.

[31] T.D. Bullen, M.A. Clynne, Trace element and isotopicconstraints on magmatic evolution at Lassen VolcanicCenter, J. Geophys. Res. 95 (1990) 19671^19691.

[32] A.D. Brandon, R. Creaser, S.B. Shirey, R.W. Carlson, Osrecycling in subduction zones, Science 272 (1996) 861^864.

[33] S. Alves, P. Schiano, C.J. Alle'gre, Rhenium^osmium iso-topic investigation of Java subduction zone lavas, EarthPlanet. Sci. Lett. 168 (1999) 65^77.

[34] K. Righter, Radiogenic Os in arc basalt: £uid or crust?Eleventh Annual V.M. Goldschmidt Conf. LPI contrib.1088, 2001, CD ROM #3657.

[35] H.J. Stein, J.W. Morgan, R.J. Walker, M.F. Horan, Rhe-nium^osmium data for sul¢des and oxides from climax-type granite-molybdenum systems: Mt. Emmons, Colora-do, Geol. Soc. Am. Ab. Prog. 24, 1992, A144.

[36] C.M. Johnson, S.B. Shirey, K.M. Barovich, New ap-proaches to crustal evolution studies and the origin ofgranitic rocks through the Lu^Hf and Re^Os isotope sys-tems, Trans. R. Soc. Edinburgh Earth Sci. 87 (1996) 339^352.

[37] C.M. Brauns, J.M. Hergt, J.D. Woodhead, R. Maas, Osisotopes and the origin of the Tasmanian Dolerites,J. Petrol. 41 (2000) 905^918.

[38] L.E. Borg, The origin and evolution of magmas from theLassen Region of the southernmost Cascades, Ph.D. The-sis, University of Texas, Austin, TX, 1995, 228 pp.

[39] M.A. Clynne, A complex magma mixing origin for rockserupted in 1915, Lassen Peak, California, J. Petrol. 40(1999) 105^132.

[40] S.B. Shirey, R.J. Walker, Carius tube digestion for low-blank rhenium^osmium analysis, Anal. Chem. 67 (1995)2136^2141.

[41] A.S. Cohen, F.G. Waters, Separation of osmium fromgeologic materials by solvent extraction for analysis byTIMS, Anal. Chim. Acta 332 (1996) 269^275.

[42] R.J. Walker, Low-blank chemical separation of rheniumand osmium from gram quantities of silicate rock formeasurements by resonance ionization mass spectrometry,Anal. Chem. 58 (1988) 2923^2927.

[43] E.H. Hauri, S.R. Hart, Rhenium abundances and system-atics in oceanic basalts, Chem. Geol. 139 (1997) 185^205.

[44] W.D. Stanley, W.D. Mooney, G.S. Fuis, Deep crustalstructure of the Cascade range and surrounding regionsfrom seismic refraction and magnetotelluric data, J. Pet-rol. 95 (1990) 19419^19438.

[45] C.G. Barnes, C.M. Allen, J.D. Hoover, R.H. Brigham,Magmatic components of a tilted plutonic system, Kla-math Mountains, California, in: J.L. Anderson (Ed.), TheNature and Origin of Cordilleran Magmatism, Geol. Soc.Am. Mem. 174, 1990, pp. 331^346.

[46] C.G. Barnes, S.W. Petersen, R.W. Kistler, T. Prestvik, B.Sundvoll, Tectonic implications of isotopic variationamong Jurassic and Early Cretaceous plutons, KlamathMountains, Geol. Soc. Am. Bull. 104 (1992) 117^126.

[47] C.G. Barnes, S.W. Petersen, R.W. Kistler, R.W. Murray,M.A. Kays, Source and tectonic implications of tonalite-trondhjemite magmatism in the Klamath Mountains,Contrib. Mineral. Petrol. 123 (1996) 40^60.

[48] C.E. Martin, R.W. Carlson, S.B. Shirey, F.A. Frey, C.-Y.Chen, Os isotopic variation in basalts from HaleakalaVolcano, Maui, Hawaii: A record of magmatic processesin oceanic mantle and crust, Earth Planet. Sci. Lett. 128(1994) 287^301.

[49] S.R. Hart, G.E. Ravizza, Os partitioning between phasesin lherzolite and basalt, in: A. Basu, S.R. Hart (Eds.),Earth Processes: Reading the Isotopic Code, Geophys.Monogr. 95, 1996, pp. 123^134.

[50] G.A Gaetani, T.L. Grove, Partitioning of moderatelysiderophile elements among olivine, silicate melt, and sul-¢de melt: constraints on core formation in the Earth andMars, Geochim. Cosmochim. Acta 61 (1997) 1829^1846.

[51] W. Roy-Barman, G.J. Wasserburg, D.A. Papanastassiou,M. Chaussidon, Osmium isotopic compositions and Re^Os concentrations in sul¢de globules from basaltic glasses,Earth Planet. Sci. Lett. 154 (1998) 331^347.

[52] K. Righter, J.T. Chesley, D. Geist, J. Ruiz, Behavior ofRe during magma fractionation: an example from VolcanAlcedo, Galapagos, J. Petrol. 39 (1998) 785^795.

[53] E. Widom, K. Hoernle, S.B. Shirey, H.-U. Schmincke, Osisotope systematics of the Canaries: Implications for crus-tal and lithospheric mantle contamination, J. Petrol. 40(1999) 279^296.

[54] M. Gu¡anti, M.A. Clynne, L.J.P. Mu¥er, Thermal andmass implications of magmatic evolution in the Lassenvolcanic region, California, and minimum constraints onbasalt in£ux to the lower crust, J. Geophys. Res. 101(1996) 3003^3013.

[55] T.A. Zeichert, C.M. Johnson, R.L. Christiansen, Sr, Nd

EPSL 6191 21-5-02

G.L. Hart et al. / Earth and Planetary Science Letters 199 (2002) 269^285284

and Pb isotope variations at Mount Shasta, California,Geol. Sci. Am. Ab. Prog. 29, 1997, p. 76.

[56] G. Wo«rner, S. Moorbath, S. Horn, J. Entenmann, R.S.Harmon, J.P. Davidson, L. Lopez-Escobar, Large- and¢ne-scale geochemical variations along the Andean arcof northern Chile (17.5^22‡S), in: K.-J. Reutter, E.Scheuber, P.J. Wigger (Eds.), Tectonics of the SouthernCentral Andes, Springer, New York, 1993, pp. 77^92.

[57] S.M. Kay, J.M. Abbruzzi, Magmatic evidence for Neo-gene lithospheric evolution of the central Andean ‘£at-slab’ between 30‡S and 32‡S, Tectonophysics 259 (1996)15^28.

[58] R.S.J. Sparks, L.A. Marshall, Thermal and mechanicalconstraints on mixing between ma¢c and silicic magmas,J. Volcananol. Geotherm. Res. 29 (1986) 99^124.

[59] C.M. Oldenburg, F.J. Spera, D.A. Yuen, G. Sewell, Dy-namic mixing in magma bodies: theory, simulations, andimplications, J. Geophys. Res. 94 (1989) 9215^9236.

[60] A.R. Cruden, H. Koyi, H. Schmeling, Diapiric basal en-trainment of ma¢c into felsic magma, Earth Planet. Sci.Lett. 131 (1995) 321^340.

[61] N. Laube, J. Springer, Crustal melting by ponding ofma¢c magmas: a numerical model, J. Volcanol. Geo-therm. Res. 81 (1998) 19^35.

EPSL 6191 21-5-02

G.L. Hart et al. / Earth and Planetary Science Letters 199 (2002) 269^285 285

App

endi

x I

Maj

or a

nd tr

ace

elem

ent d

ata

for L

asse

n ar

ea s

ilici

c ro

cks

of th

is s

tudy

Ele

men

tLC

84-4

43LC

83-3

60LC

81-7

06LC

84-5

41LC

81-6

59LM

80-8

99LC

82-1

94LM

80-8

24LM

80-8

54LC

88-1

392

SiO

269

.81

70.8

267

.31

64.2

173

.49

69.4

262

.96

60.2

260

.174

.99

TiO

20.

350.

330.

440.

560.

280.

450.

720.

80.

620.

25A

l 2O3

15.6

15.4

216

.416

.93

13.9

115

.78

16.8

17.2

418

.05

13.3

4Fe

2O3

0.56

0.48

0.72

0.92

0.43

0.58

1.06

1.24

1.07

0.28

FeO

2.02

1.75

2.6

3.31

1.56

2.1

3.81

4.49

3.86

1.01

MnO

0.06

0.05

0.06

0.09

0.05

0.05

0.08

0.1

0.08

0.02

MgO

1.19

3.31

4.29

2.34

0.87

1.11

2.8

3.62

4.01

0.43

CaO

3.31

2.69

4.43

5.15

1.96

2.88

5.41

6.24

7.27

1.32

Na 2

O4.

294.

434.

374.

173.

984.

463.

853.

723.

63.

77K

2O2.

642.

862.

522.

13.

353.

032.

242.

061.

154.

53P

2O5

0.12

0.06

0.12

0.18

0.08

0.1

0.21

0.21

0.16

0.05

Tota

l99

.68

99.2

999

.88

99.9

199

.87

99.7

299

.87

99.7

899

.86

LOI

0.76

0.54

0.2

0.34

0.51

0.66

0.66

0.14

0.26

2.86

Mg-

no.

51.2

50.5

53.2

55.7

5048

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.658

.964

.943

.3R

b71

7667

5088

7962

5123

129

Ba

804

813

729

672

849

888

715

666

289

Zr13

713

913

915

713

519

917

117

512

114

3S

r33

132

737

145

322

034

041

345

979

912

9Y

1315

1417

1520

2121

1516

Nb

88

64

811

76

2P

b12

.913

.211

.19.

214

.813

.38.

98.

53.

9C

r11

4.8

9.6

215.

7645

.654

.958

Cs

3.79

4.09

2.6

0.93

4.35

1.8

1.5

0.3

Hf

3.43

4.03

3.6

44.

784.

14.

12.

4Th

10.3

11.2

8.9

7.3

107.

66

2.1

U3.

373.

432.

92.

433.

932.

41.

90.

8La

23.6

2421

22.8

22.9

2322

1121

.3S

m2.

742.

772.

33.

723.

444.

24.

42.

72.

39E

u0.

640.

641

0.68

0.87

0.78

20.

951

0.8

0.43

Yb

1.24

1.4

1.4

1.82

1.91

1.8

21.

21.

64M

ajor

ele

men

ts a

nd t

race

ele

men

ts a

re in

wt.%

and

ppm

, res

pect

ivel

y. D

ata

from

[18,

28,3

1,38

].

1

Appendix II Isotope evolution in systems with hugely varying parent-daughter ratios cannot be modeled by simple averaging of fractionated magmas. Rather, magmatic compositions, elemental contents, and isotopic compositions must be obtained by a mass- and concentration-weighted integration of the fractionated magmas in the crustal column. This new modeling method more accurately represents the range of chemical and isotopic compositions of material in the lower crust. The following equations calculate the concentration- and mass-weighted isotopic compositions of the crust based on parent/daughter fractionation due to Rayleigh fractionation during crystallization of magmas. Variations in Os isotope compositions are due to radioactive decay of the 187Re to 187Os in the Re-Os isotope system. To describe the isotopic variations due to radioactive decay, we define Rmeas as the measured isotope ratio 187Os/188Os and Ri as the initial 187Os/188Os a rock had at some time in the past. We also define λ = decay constant and t = time (in years).

(1) )1(188

187

−+= timeas e

OsReRR λ

Because ex ≈ 1 + x for x << 1, (1) becomes:

(2) tOsReRR imeas λ188

187

+=

Assuming a constant conversion factor k between 187Re/188Os and the [Re]/[Os] wt. ratio produces:

(3) tkOsReRR imeas λ

][][+=

Because Re and Os concentration variations in the Earth are fundamentally due to magmatic crystallization, which can be described by the Rayleigh fractionation model, Re and Os variations will follow: (4) )1(

0][][ −= ReDFReRe and )1(0][][ −= OsDFOsOs

where F is the fraction of magma remaining during the crystallization of a magma (from 0 to 100% crystallized, F goes from 1 to 0), and DRe and DOs are the bulk crystal-liquid distribution coefficients. Combining produces:

(5) )1(

00 ][)

][1(][

][][ −= γγ Os

OsRe

OsRe

where ]1

[1-D

D

Os

Re −=γ , which may be considered a constant. Equation (3) then becomes:

(6) tOsOs

RekRR imeas λγγ )1(

00 ][)

][1(][ −+=

Solving for [Os] produces:

(7) ]

)1(1[

00

}][][

][{][ −−

= γγ

λOs

tRekRR

Os imeas

2

We further define:

(8) )1(

1,,[

)]([

0

0

−===

γλ

γ

CARBtRe]k

OsA i

Equation (7) then becomes: (9) )(][][ RfBAROs C

meas =−= The concentration-weighted average isotope ratio R can be calculated using the formula for the center of mass of a rod:

(10) dRRfdRRfRR AvgC )(

)(∫∫=−

Recast in terms of equation (9), this becomes:

(11) meas

Cmeas

measC

measmeasAvgC dRBAR

dRBARRR][

][−∫

−∫=−

Integration within the limits Min

measR and MaxmeasR produces:

(12) MaxmeasMinmeas

MaxmeasMinmeas

RR

RR

AvgCn

mR

|

|=− where

)2)(1(

])1([][2

)1(

++++−

=+

CCABCARBARm meas

Cmeas

and

)1(][ )1(

+−

=+

CABARn

Cmeas

Equation 12 will produce the concentration-weighted isotope ratio R that reflects the range of Rmeas that is produced over a continuous crystallization interval F, followed by isotope evolution over time t. Equation 12 assumes that each crystallization interval is equally represented in the crust. Crystal cumulates are removed from the system in this model, reflecting the fact that such cumulates will either be too refractory to participate in later magmatic processes, or will, by definition, lie below the crust-mantle boundary. Equation 12, however, does not account for the mass of magmas associated with early crystallization that will represent larger volumes than the magmas that remain after extensive crystallization. A mass- and concentration- weighted average isotope composition R is calculated below to account for the decreasing mass contribution to the crust of magmas that have undergone greater extents of crystallization. We define a mass-weighted [Os] as (13) ][][ OsFOs MW =

3

where F is as defined in equation (4). Substituting equation (9) produces (14) C

measMW BARFOs )(][ −= Recall that (15) C

measD BARFOsOs Os )(][][ )1(

0 −== − solving for F produces:

(16) ]

)1(1[

0

]][

)([ −−

= OsDC

meas

OsBARF

Equation (14) then becomes:

(17) ]

)1([

00 ]

][)(

[][][ −−= Os

Os

DDC

measMW Os

BAROsOs

Following equation (10), the mass- and concentration-weighted isotope composition R is defined as:

(18) measMW

measMWmeasAvgCM dROs

dROsRR][

][∫

∫=−−

Substitution of equation (17), followed by integration within the limits Min

measR and MaxmeasR , produces:

(19) ( )( ) Max

measMinmeas

MaxmeasMinmeas

RRr

q

RRp

o

AvgCMR|

|=−− where

}]1[]1[{})][

][){()(1(][

])1(

[

00 measOsOsOs

DDC

measmeasOs RCDDADB

OsBARBARDOso Os

Os

+−+−⋅−

−−= −

)2]2)([1(2 −++−= OsOsOS DCCDDAp

})][

][){((])[1(

])1(

[

00

−−−−= Os

Os

DDC

measmeasOs Os

BARBAROsDq

)1( OsOs CDDAr +−=