186Os^187Os systematics of Hawaiian picrites

Alan D. Brandon a;*, Marc D. Norman b, Richard J. Walker a,John W. Morgan a;c

a Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742, USAb Centre for Ore Deposit Research, School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001,

Australiac Department of Earth Resources, Colorado State University, Fort Collins, CO 80523, USA

Received 29 March 1999; accepted 13 October 1999

Abstract

Primitive Hawaiian picrites have 187Os/188Os as high as V0.145 and are more radiogenic than the depleted uppermantle, reflecting a time-integrated suprachondritic Re/Os ratio. The high Re/Os may be explained either by an ancientrecycled crustal component or an evolved outer core component in the Hawaiian plume. New high precision 186Os/188Osmeasurements for these picrites, combined with previous analyses, show that the Hawaiian plume source has 186Os/188Os that range from chondritic mantle values of V0.119834 to more radiogenic values as high as V0.119848. Thehigher 186Os/188Os reflects long-term suprachondritic Pt/Os and is coupled with higher 187Os/188Os in all but the Koolaupicrites. The latter have near-chondritic 186Os/188Os but with radiogenic 187Os/188Os. The Pt/Re of crustal materials thatmay make up ancient recycled slabs ranges from V0.1 to 33. Recycled slab material with such Pt/Re ratios evolved for1^3 Ga and added to the plume source may explain the Koolau Os isotopic compositions. Pt/Re ratios of 88^100,however, are required for the ancient recycled crust to generate the coupled enrichments of 186Os/188Os and 187Os/188Osin picrites from Loihi and Hualalai. These high Pt/Re ratios do not occur in any known crustal materials, but they areconsistent with the observed partition coefficients for OssResPt, during metal crystallization from an initiallychondritic molten core (Pt/Re V21^24). Such partitioning may have produced an evolved outer core withsuprachondritic Pt/Os, Re/Os and Pt/Re, resulting in the production of suprachondritic 186Os/188Os and 187Os/188Osover time. Small amounts of outer core metal (9 1.2%) mixed into the Hawaiian plume source can explain the coupled186Os/188Os and 187Os/188Os enrichment in some of the Hawaiian picrites. In addition, the most radiogenic 186Os/188Osin the Hawaiian picrites is correlated with higher 3He/4He, consistent with an undegassed, and likely, lower mantlesource. These data provide compelling geochemical evidence that the Hawaiian plume was generated at the core-mantleboundary. ß 1999 Elsevier Science B.V. All rights reserved.

Keywords: Os-187/Os-186; picrite; Hawaii; mantle plumes

1. Introduction

The wide range in 187Os/188Os ratios of plume-derived magmas, including oceanic basalts (OIB),is now well documented [1^13]. The origins ofthese isotopic variations remain controversial,

0012-821X / 99 / $ ^ see front matter ß 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 2 5 1 - 4

* Corresponding author. Tel. : +1 301-405-0084;Fax: +1 301-314-9661; E-mail: brandon@geol.umd.edu

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with debate focusing primarily on the in£uence ofancient subducted crust in generating the compo-sitional variability in the deep mantle [2,4,8,10].Mantle plumes are generally considered to be gen-erated at boundary layers, such as the interfacebetween the upper and lower mantle (i.e. 670km discontinuity) or the DQ region at the core-mantle boundary (CMB), although there hasbeen little direct evidence for either location. Re-cent seismic evidence suggests that the roots of theIceland and Hawaiian plumes may reside at theCMB, supporting the view that some plumes mayoriginate at this interface [14^16]. Constraints onthe composition and location of the source re-gions that ultimately give rise to mantle plumeswould provide important information on thestructure of the deep mantle, timescales of mantlemixing and possibly the degree of interaction be-tween the mantle and the outer core [17^19].

Oceanic island basalts typically have 187Os/188Os isotopic ratios that are more radiogenicthan those for the generally-chondritic depletedmantle or primitive upper mantle (PUM), re£ect-ing long-term evolution of sources with supra-chondritic Re/Os ratios. Additional constraintson the history and depth of mantle plume sourcescan also potentially be obtained from the 186Os/188Os ratios of OIB [17]. 186Os is produced from190Pt by K decay, with a half-life of V469.3 Ga(V= 1.477U10312 a31, recalculated based on190Pt = 0.01296 atomic% [19]). Because 190Pt is aminor isotope of Pt in combination with the smalldecay constant, this system is very insensitive toperturbations by most geochemical processes andonly long-term and relatively large fractionationsof Pt from Os can produce measurable di¡erencesin 186Os/188Os ratios. Because of the di¡erences inthe geochemical behaviors of Pt and Re, this sys-tem is complementary to the 187Re^187Os isotopicsystem. The combined 186Os/188Os and 187Os/188Ossystematics provide constraints on time-integratedPt/Os, Re/Os and Pt/Re of mantle sources, there-by leading to a more thorough understanding ofprocesses that control the highly siderophile ele-ment budget of the mantle.

Walker et al. [17,19] recognized the potentialvalue of the combined Pt^Os and Re^Os isotopicsystems for constraining the depths of origin of

mantle plumes. They suggested that the outer coremay have developed higher Pt/Os and Re/Os rel-ative to chondrites, owing to the crystallization ofthe inner core. This was based on the assumptionthat these elements would partition between solidmetal and liquid metal in the Earth's core in asimilar manner to their partitioning behavior inasteroidal cores where DOs sDRe sDPt (D = solidmetal/liquid metal bulk distribution coe¤cient).From these arguments, Walker et al. [17] pro-posed that if chemical exchange occurs acrossthe CMB, the hypothesized long-term elevationin Pt/Os and Re/Os in the outer core would im-part a coupled enrichment of 186Os and 187Os,respectively, to mantle plumes derived from theDQ region. This isotopic ¢ngerprinting might beobserved in a plume whose source incorporated9 1% outer core metal, because the outer corelikely has approximately 300 times higher abun-dances of Os than the mantle (approximately1 part per million (ppm) versus 3 parts per billion(ppb), respectively). Such coupled enrichmentshave been con¢rmed via high precision isotopeanalyses of ¢ve Hawaiian picrites and one orerepresentative of the 251 Ma Siberian Flood Ba-salt Province [19,20]. These coupled Os isotopicenrichments were argued to be evidence for theorigins of these plumes at the CMB.

Some crustal rocks also have very high Pt/Osand Re/Os, however, and long-term decay of 190Ptand 187Re in these rocks also results in elevated186Os/188Os and 187Os/188Os, respectively. Conse-quently, entrainment of ancient recycled crust inplumes has been favored by some to explain theradiogenic 187Os/188Os ratios for some Hawaiianlavas and other OIB [4,11,12]. In particular, tho-leiites from the Koolau Volcano, with 187Os/188Osas high as 0.145, have N18O and Pb isotopic com-positions consistent with a component composedof a mixture of ancient recycled basalt (V2 Ga)plus a small fraction of pelagic sediment [10^12].The Hawaiian picrites from Loihi, Manua Loaand Koolau analyzed by Brandon et al. [20] had186Os/188Osv 0.119841, all signi¢cantly above theupper mantle value of 0.119834 [19]. The Koolaupicrite analyzed by Brandon et al. [20] had themost radiogenic 186Os/188Os and 187Os/188Os forthe Hawaiian samples yet measured. Mixing rela-

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tionships show that v 70% of a 2 Ga recycledcomponent would be needed in a mixture withperidotite to explain the 186Os enrichment [20].Because of the presence of olivine phenocrystswith Fo88ÿ89 cores in the Koolau picrites, such alarge amount of a basaltic component in the man-tle source region is considered petrologically un-reasonable [21]. Therefore, Brandon et al. [20] fa-vored core-mantle interaction to explain thesecoupled enrichments in 186Os/188Os and 187Os/188Os.

In this contribution, high precision 186Os/188Osand 187Os/188Os isotopic data for an additionaleight Hawaiian picrites are presented. Combinedwith the previous ¢ve analyses, these data spanthe range in variation in 187Os/188Os for the shieldbuilding stages for seven Hawaiian volcanoes. Inaddition, one Koolau sample from the previousstudy was re-analyzed here. These data con¢rmthe coupled enrichments in 186Os and 187Os inprimitive Hawaiian tholeiites previously reported[20] and the likely origin of these compositions asresulting from core-mantle interaction. An impor-tant ¢nding of this work is that the highly radio-genic 186Os/188Os composition of the Koolau tho-leiite is not supported by new analyses and thatan additional component in the Hawaiian plumeis required to explain the new Koolau data.

2. Samples and analytical techniques

The Hawaiian picrite samples analyzed in thisstudy are from Hawaiian Scienti¢c Drilling Proj-ect drilled cores from Mauna Kea [22], submarinedredge haul samples and subaerial £ows [21].These picrites represent samples of shield buildingtholeiites that span the end-member isotopic andtrace element compositions for Hawaiian tholei-ites. All of these samples consist of a ¢ne-grainedbasaltic matrix with abundant phenocrysts of oli-vine. Plagioclase and clinopyroxene phenocrystsare absent and these samples appear to representprimitive Hawaiian melts that accumulated varia-ble amounts of cogenetic olivine and have, there-fore, had a fairly simple petrogenesis [21].

Outer surfaces were ground o¡ with carborun-dum to remove any metal contaminants. The

samples were cleaned repeatedly in an ultrasonicbath for 30 min using ultrapure water, dried in anoven at 120³C and broken into cm size chips witha high-tensile strength alloy mortar and pestle.These chips were inspected for metal contamina-tion prior to grinding into a ¢ne powder with aceramic alumina shatterbox.

Samples were processed for mass spectrometricanalysis in two ways. In the ¢rst procedure usedfor determining Os concentrations, 2 g portions ofeach sample were spiked with 190Os and dissolvedin reverse aqua regia at 250³C in borosilicate Car-ius tubes to obtain sample-spike equilibration[23]. Osmium was puri¢ed using a carbon tetra-chloride solvent extraction technique [24], fol-lowed by microdistillation puri¢cation [25]. Pro-cedural blanks averaged 5.7 þ 0.8 pg for Os.Isotopic ratios were measured using negative ionmass spectrometry (NTIMS) as oxides [26]. TheOs isotopic ratios for the spiked analyses weremeasured on a single collector NBS style massspectrometer at the University of Maryland Iso-tope Geochemistry Laboratory (IGL).

In the second procedure for unspiked high pre-cision analyses of Os isotopes, 100^300 g of sam-ple was fused using the NiS ¢re assay method[27]. The fusions were performed at 1100³C for1.2 h using sodium tetraborate as £ux in a 1:1weight ratio with the sample powder and a sam-ple:nickel :sulfur weight ratio of 10:1:0.5, respec-tively. Osmium from the melted powder partitionsinto immiscible NiS blebs, which are extractedfrom the fusion glass after quenching. The NiSblebs are dissolved in 12 N HCl and insolubleOs sul¢de is trapped on cellulose paper by ¢lter-ing the HCl solution. The Os-bearing celluloseis dissolved in a Carius tube using reverse aquaregia and Os is puri¢ed using the procedureslisted above for the spiked samples. Two blanksmeasured were 0.44 and 1.0 pg/g fused samplewith 186Os/188Os = 0.1199 þ 2 and 187Os/188Os =0.1240 þ 5. The lowest Os concentration in thepicrites analyzed here was 465 pg/g (KOO-17A,Table 1). If the blank was as high as 1.5 pg/g(50% higher than the maximum observed) with186Os/188Os = 0.1201, a correction to 186Os/188Osfor this sample is 8.6U1038, well within the mea-sured precisions cited. Blank corrections for the

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Table 1Os and He (R/Ra) isotopic data for Hawaiian picrites

Sample Run type Os(ppt)

184Os/188Os 186Os/188Os 187Os/188Os QOs R/Ra

MKR211.6-212.1 Spiked 625 0.12945 (25) 1.93 7.1 (1.0)1

Unspiked Hi-P 0.001320 (5) 0.1198372 (61) 0.1295493 (80) n = 10.001328 (5) 0.1198343 (68) 0.1295403 (75)0.001317 (4) 0.1198337 (55) 0.1295452 (60)0.001319 (5) 0.1198337 (55) 0.1295369 (73)

Mean 0.001321 (4) 0.1198339 (28) 0.1295429 (54) 2.00MKR365.6.7-7.4 Spiked 1 024 0.12876 (13) 1.39 9.1 (0.6)1

Unspiked Hi-P 0.001314 (5) 0.1198354 (56) 0.1285652 (59) n = 10.001314 (4) 0.1198382 (55) 0.1284619 (72)0.001311 (4) 0.1198324 (61) 0.1285578 (69)0.001313 (4) 0.1198323 (48) 0.1285533 (64)

Mean 0.001313 (1) 0.1198346 (28) 0.1285598 (51) 1.23Kilauea 1-7 Spiked 639 0.13063 (23) 2.86 13.3 (0.5)4;7ÿ9

Unspiked Hi-P 0.001313 (4) 0.1198430 (58) 0.1303067 (65 n = 60.001317 (4) 0.1198368 (63) 0.1302977 (68)0.001320 (4) 0.1198334 (44) 0.1303030 (65)0.001317 (5) 0.1198340 (46) 0.1303023 (52)0.001309 (3) 0.1198352 (40) 0.1303019 (55)

Mean 0.001316 (5) 0.1198365 (35) 0.1303023 (29) 2.60Kilauea 1-18 Spiked 665 0.13219 (18) 4.09 13.3 (0.5)4;7ÿ9

Unspiked Hi-P 0.001315 (5) 0.1198407 (60) 0.1322051 (77) 4.10 n = 6Kilauea 1840 Spiked 911 0.12989 (16) 2.28 13.0 (0.5)4;7

621 0.12952 (14) 1.98 n = 3Unspiked Hi-P 0.001316 (7) 0.1198324 (76) 0.1305335 (91)

0.001325 (5) 0.1198375 (77) 0.1305339 (85)0.001332 (7) 0.1198379 (103) 0.1305303 (123)

Mean 0.001324 (9) 0.1198359 (35) 0.1305326 (23) 2.78ML-2-50 Spiked 792a 0.1361 (1)a 7.17 17.6 (1.0)5;6

Unspiked Hi-P 0.001322 (5) 0.1198428 (75) 0.1339040 (110) n = 130.001317 (4) 0.1198397 (47) 0.1338445 (70)0.001325 (5) 0.1198421 (58) 0.1338501 (60)0.001326 (6) 0.1198464 (83) 0.1339123 (112)

Mean 0.001322 (4) 0.1198425 (32)b 0.1338781 (35)b 5.41ML1868.9 Spiked 717b 0.13453 (14)b 5.93 8.0 (1.5)5

Unspiked Hi-P 0.001315 (4) 0.1198421 (62) 0.1354986 (79) n = 10.001320 (6) 0.1198397 (64) 0.1355016 (92)0.001334 (6) 0.1198410 (67) 0.1354976 (70)

Mean 0.001323 (11) 0.1198409 (12)b 0.1354993 (24)b 6.69LO-02-02 Spiked 863a 0.1355 (1)a 6.69 25 (2.1)1;3;4;9

Unspiked Hi-P 0.001314 (5) 0.1198424 (59) 0.1333320 (83) n = 120.001315 (5) 0.1198482 (52) 0.1333318 (67)0.001321 (4) 0.1198424 (44) 0.1338937 (46)0.001317 (4) 0.1198443 (58) 0.1338780 (80)

Mean 0.001317 (3) 0.1198443 (28)b 0.133606 (324)b 5.20LO-02-04 Spiked 1 083a 0.1347 (2)a 6.06 25 (2.1)1;3;4;9

Unspiked Hi-P 0.001409 (24) 0.1198441 (92) 0.1340490 (126) n = 120.001521 (27) 0.1198424 (93) 0.1344428 (130)

Mean 0.00146 (11) 0.1198433 (17)b 0.134266 (352)b 5.72KOH 1-28 Spiked 1 077 0.13167 (11) 3.68 10.5 (2.0)10

Unspiked Hi-P 0.001320 (5) 0.1198385 (62) 0.1326610 (87) n = 10.001322 (5) 0.1198417 (66) 0.1326633 (87)0.001323 (4) 0.1198363 (56) 0.1326630 (71)

Mean 0.001321 (2) 0.1198388 (31) 0.1326624 (21) 4.45

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samples with higher concentrations of Os are evensmaller. Therefore, the contribution of the blankto the measured 186Os/188Os in this study is negli-gible.

The high precision Os isotopic measurements

were performed on a 7 Faraday cup mass spec-trometer (VG Sector 54) in dynamic mode at theIGL. In-run fractionation corrections were madefor 186Os/188Os and 187Os/188Os using 189Os/188Os = 1.21978. Beam intensities of 80^120 mV

Table 1 (continued)

Sample Run type Os(ppt)

184Os/188Os 186Os/188Os 187Os/188Os QOs R/Ra

H-11 Spiked 1 008 0.13614 (18) 7.20 17.9 (0.1)3

Unspiked Hi-P 0.001316 (5) 0.1198476 (72) 0.1364687 (90) n = 10.001317 (5) 0.1198450 (72) 0.1364775 (75)0.001323 (5) 0.1198500 (68) 0.1364758 (95)

Mean 0.001318 (4) 0.1198475 (29) 0.1364740 (54) 7.46KOO-17A Spiked 465a 0.1426 (2)a 12.23 14.2 (0.2)2

Olivine 971 0.13865 (30) 9.17 n = 1Unspiked Hi-P 0.001339 (5) 0.1198556 (64) 0.1385001 (80)Analyzed 12/97 0.001352 (7) 0.1198497 (46) 0.1384925 (58)

0.001349 (7) 0.1198525 (90) 0.1385036 (111)Mean 0.001346 (8) 0.1198526 (34)b 0.1384988 (65)b 9.05Unspiked Hi-P 0.001317 (5) 0.1198368 (46) 0.1138510 (150)Analyzed 1/99 0.001312 (3) 0.1198329 (56) 0.1384988 (55)

0.001311 (4) 0.1198319 (44) 0.1385143 (61)0.001312 (3) 0.1198307 (38) 0.1385042 (50)0.001314 (4) 0.1198382 (46) 0.1385041 (52)

Mean 0.001313 (2) 0.1198343 (22) 0.1385063 (54) 9.06KOO-CF Spiked 522 0.13814 (30) 8.77 12.4 (1.5)2

Unspiked Hi-P 0.001323 (6) 0.1198378 (66) 0.1375549 (80) n = 60.001312 (5) 0.1198372 (54) 0.1375579 (41)0.001322 (5) 0.1198435 (52) 0.1375572 (85)

Mean 0.001319 (7) 0.1198395 (40) 0.1375567 (18) 8.31Johnson Matthey Os standard (3/97^1/99)Run 1 5 ng 0.001322 (4) 0.1198452 (48) 0.1138101 (57)Run 2 20 ng 0.001313 (3) 0.1198477 (48) 0.1137997 (61)Run 3 30 ng 0.001311 (4) 0.1198466 (55) 0.1138034 (71)Run 4 35 ng 0.001325 (6) 0.1198454 (70) 0.1137952 (84)Run 5 40 ng 0.001316 (3) 0.1198489 (36) 0.1138005 (45)Run 6 70 ng 0.001306 (4) 0.1198463 (43) 0.1138057 (48)Mean, runs 1^6 0.001315 (6) 0.1198467 (11) 0.1138024 (42)Mean, 28 runs 0.001314 (2) 0.1198464 (12) 0.1138067 (21)Standard solutions mixing test (7/19/99)Johnson Matthey standard 0.001323 (2) 0.1198480 (32) 0.1137820 (30)72.7227% J. Mat.+27.2773% JWM 0.001315 (2) 0.1198825 (36) 0.1299953 (44)49.1717% J. Mat.+50.8283% JWM 0.001321 (3) 0.1199133 (44) 0.1440759 (49)26.5312% J. Mat.+73.4688% JWM 0.001309 (3) 0.1199427 (34) 0.1577560 (54)JWM standard 0.001321 (3) 0.1199755 (48) 0.1739176 (69)

Samples are: MK, Mauna Kea; ML, Mauna Loa; LO, Loihi; KOH, Kohala; H, Hualalai ; K, Koolau. Within-run precisionsand 2c of the mean for the unspiked high precision (Hi-P) measurements are listed in parentheses for the last decimal place ofeach ratio. Helium data sources: 1 [68], 2 [69], 3 [51], 4 [70], 5 [71], 6 [67], 7 [50], 8 [49], 9 [45], 10 [72]. Data for the two MaunaKea samples are taken within 30 cm of core from where the Os data come from and are the same £ows. Other data are volcanoaverages, with the number of samples analyzed (n) listed behind the values, except Kilauea 1840, ML1868.9 and KOO-17A, forwhich the data are from the same £ows. He data chosen for ML-2-50 are the average of the Mauna Loa submarine samples.aFrom Bennett et al. [10].bFrom Brandon et al. [20].

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on mass 234 (186Os16O33 ) and 235 (187Os16O3

3 ) areneeded for 100 or more ratios to reach a desiredprecision of 50^80 ppm (2c) for 186Os/188Os and187Os/188Os. Multiple runs of each sample are re-quired in order to obtain an uncertainty of thepopulation of close to or better than those forthe long-term external precision of þ 40 ppm(2c) for the standard. In most cases, þ 40 ppmor better is obtained on the 2c deviation of thepopulation of three or more analyses (Table 1).

In order to test the accuracy of the high preci-sion measurements for 186Os/188Os, two standardsolutions, with di¡erent 186Os/188Os and 187Os/188Os and well determined Os concentrations (Ta-ble 1), were mixed together in variable amounts.The mixtures (Table 1) were then equilibrated inCarius tubes with aqua regia and processed withthe standard chemical procedures presentedabove. The two standards and the mixtures wererun in sequence on a single day (7/17/99). Themeasured 186Os/188Os ratios of the mixtures liewithin uncertainty of the calculated mixing linebetween the two standard solutions (Fig. 1), indi-cating that di¡erences of þ 40 ppm in 186Os/188Oscan be readily distinguished with the techniquesemployed.

Multiple runs are also made in order to mon-itor the presence of mass interferences which cana¡ect the interpretation of the data. Interferingions can be present over the mass range ofnegative OsO3

3 ions measured using NTIMSfrom masses 232^240 (184Os16O3

3 ^192Os16O33 ).

These interferences may include tungsten tri-oxide (184W16O3

3 , 186W16O33 ), platinum dioxide

(198Pt16O18O3) and barium chloride. These inter-ferences could potentially have a profound e¡ecton the measured 186Os/188Os. For instance, for asignal of 100 mV on mass 234, an interfering sig-nal of just 2U1032 mV could shift a measured186Os/188Os ratio from 0.119835 to 0.119859, ap-proximately equal to the observed range in Ha-waiian lavas. Such a signal would not be detect-able unless an independent mass can be used tomonitor another isotope of the same element orcomplex ion or it can be shown only to have ane¡ect on certain masses through replicate runs.

One of the more serious potential interferencesthat may be present on mass 234 (186Os16O3

3 ) is

186W16O33 . If 186W16O3

3 is present, an interferencewith nearly the same intensity (1.073 times larger)is present at mass 232 (184Os16O3

3 ) from184W16O3

3 . Therefore, the 184Os/188Os ratio maybe used as a monitor of tungsten trioxide interfer-ences for masses 232 and 234. In Table 1, isotopicdata for six runs of a Johnson Matthey Os stan-dard are shown using 5^70 ng of Os. The 184Os/188Os ratios of these runs vary from 0.001306 to0.001325 and they give identical 186Os/188Os ratioswithin uncertainty of the measurements. These184Os/188Os data for the standard are an indica-tion of how precisely this ratio can be measuredand show an external precision of þ 0.000012from the mean of 0.001314 during the analyticalperiod. This external precision, which is higherthan the internal precisions that are typicallyþ 0.000002^0.000006, results from the fact thatthe beam intensities for mass 232 are much lowerthan for the other Os trioxide masses. All of theruns were made with 80^120 mV beam intensitieson mass 234 and oxygen pressures of 1^4U1037

millibar to maximize beam intensities. Althoughthe temperature of the Pt ¢lament was not moni-tored, in order to achieve similar beam intensitiesfrom 5^70 ng runs, a higher temperature is re-quired. The fact that the measured 186Os/188Osratios of each run are within uncertainty for allsix runs and the mean of 28 runs indicates thatinterfering ions must be negligible during thestandard runs. Because 188Os and 184Os are notradiogenic daughter products, 184Os/188Os doesnot vary in nature. Therefore, the compositionof this standard can be used in comparison withthe 184Os/188Os ratios measured on the samplesduring the same period to check possible interfer-ences.

For sample measurements, a minimum of 30^50ng of Os is typically needed to obtain the requiredprecision [19,20]. Most of the 184Os/188Os ratiosmeasured for the Hawaiian picrites are statisti-cally indistinguishable from each other and fromthe standard value, given the external precisionlisted above (Table 1), and can therefore be con-sidered to be free of potential interferences. Thereare two exceptions however. For KOO-17a, amean for 186Os/188Os of 0.1198526 þ 34 was pre-viously obtained on three replicate runs of a single

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¢lament [20]. The mean for the measured 184Os/188Os for these analyses is 0.001346 þ 8, which issigni¢cantly higher than the 184Os/188Os for theother picrites and the standard (Table 1). Wetherefore chose to re-examine this sample in thepresent study. To obtain larger signals, muchgreater quantities of sample were processed. Thenew measurements of this sample run were withhigher signal intensities for mass 234 (186Os16O3

3 ,100 mV versus 40^50 mV previously) and give amean for 184Os/188Os of 0.001313 þ 2, similar tothe other samples and standard values, and a cor-responding lower value for 186Os/188Os of0.1198343 þ 22. These duplicates were run ontwo separate ¢laments. Assuming that the high184Os/188Os previously obtained was due to aninterference from 184W16O3

3 interferences, a cor-rection to the previous measurement gives a 186Os/188Os of 0.1198219 þ 127 (higher uncertainty isdue to error propagation for the correction). Weconclude that the previous measurements of186Os/188Os for KOO-17a were erroneously highand at least in part may have re£ected a tungstentrioxide interference. The previous data for KOO-17a are therefore discarded and will not be con-sidered further.

The second exceptional sample is the Loihi pic-rite LO-02-04. The mean 184Os/188Os for this sam-ple is 0.00146 for duplicate analyses, which is welloutside the range of the measured value of thestandard or the other picrites. Despite this high184Os/188Os, the mean on its 186Os/188Os ratio iswithin the error of the mean of 186Os/188Os for theother Loihi sample (LO-02-02), which is otherwisesimilar to LO-02-04 in virtually all respects [21].Replicates on LO-02-04 were run at much lowerbeam intensities than the other samples, with amass 234 beam intensity of 40^50 mV. This wasnecessary because smaller amounts of samplewere available for processing and hence, less Oswas loaded on the ¢laments. If the measured186Os/188Os ratio is corrected for tungsten triox-ide, it would have a value of 0.11969, which iswell below any previous high precision Os iso-topic measurement on any material [19,20,28](Table 1), including the estimate for the solar sys-tem initial of 0.119820 [19]. This would imply thatthe Loihi source region evolved with a stronglysubchondritic Pt/Os for greater than the age ofthe Earth, an inference that is unrealistic. Given

Fig. 2. 186Os/188Os versus QOs for the Hawaiian picrites. Alsoshown is the ¢eld for upper mantle data and a sample fromthe Siberian plume [17,20,28]. Error bars are 2c of the meanfor the multiple analyses. A regression line for 10 of the Ha-waiian picrites gives a r2 of 0.937.

Fig. 1. 186Os/188Os versus 187Os/188Os and %Johnson Matthey(JM) standard for the mixing texts listed in Table 1. The sol-id lines are for the calculated mixtures between the twostandards with known 186Os/188Os versus 187Os/188Os (Table1). Within-run 2c precision error bars are shown for eachmeasured data point.

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the similarity of 186Os/188Os for both Loihi sam-ples, we conclude that this very high 184Os/188Osmeasured for LO-02-04 was not a result of tung-sten trioxide interference, but was instead likely tohave been a 198Pt16O18O3 interference. Small plat-inum dioxide signals have been identi¢ed subse-quent to the analysis of this sample. The Pt diox-ide signals are not present at higher masses than232. These data demonstrate the need for carefulmonitoring of interferences and replication ofanalyses in order to determine the validity ofhigh precision measurements for Os isotopic ra-tios. The data reported in this paper suggest thatif beam intensities of v80 mV on mass 234 areachieved, these minute isobaric interferences be-come insigni¢cant in a¡ecting 186Os/188Os ratiosexcept in rare cases where tungsten oxide maybe present. In this case, monitoring 184Os/188Oscan be used to screen erroneous 186Os/188Os ra-tios.

3. Results

High precision Os isotopic measurementsfor picrites from seven Hawaiian volcanoes arelisted in Table 1, including data previously pub-

lished [20]. The 186Os/188Os ratios range from0.1198339 þ 28 to 0.1198475 þ 29 and 187Os/188Osratios range from 0.12856 to 0.13851. The 187Os/188Os for the individual samples from each volca-no plot within the range of values previouslyreported for similar samples from the same volca-noes [1,7,10^12]. One olivine separate, fromthe picrite KOO-17a, has a 187Os/188Os of0.13865 þ 30 and is indistinguishable from the val-ue of 0.138506 þ 5 and 0.138499 þ 6 measured forthe bulk rock (Table 1). These data are consistentwith other compositional data which suggest thatthe olivines in these picrites are cogenetic withtheir matrix [21].

In Fig. 2, 186Os/188Os versus QOs is shown, whereQOs is the percentage deviation of 187Os/188Os fromthe average chondritic value at a given time (thepresent day value is 0.1270, [29]). With the excep-tion of the two Koolau picrites (KOO-17a andKOO-CF) and a 19th century Mauna Loa £owpicrite (the 1868 £ow), the Hawaiian picrite dataconform to a line with a positive slope with a r2 of0.937. This line extends from a slightly positiveQOs of V+1^+2, similar to the estimated presentday PUM [30], and 186Os/188Os of V0.119834 forthe Mauna Kea samples to QOs of +7.5 and 186Os/188Os of 0.1198475 for the Hualalai picrite. A

Fig. 3. Pt^Os concentration data for materials that may be part of ancient recycled crust and the PUM. Data for picrites, ba-salts, MORB, metalliferous sediments and reducing sediments are from Ravizza and Pyle [31]. The average upper crust comesfrom Schmidt et al. [32], where the Pt concentration was calculated by interpolation from Pd and Ru.

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sample previously analyzed from the 251 Ma Si-berian Flood Basalt Province [19] also plots onthe line with a slightly more radiogenic 186Os/188Os of 0.119849 and a more positive QOs of +8.

The data from the Hawaiian and Siberianplumes show 186Os-187Os systematics that are dis-tinct from those of upper mantle samples. Sam-ples including Os-Ir alloys and chromitites fromophiolites and alpine peridotites worldwide [19,20]and abyssal peridotites from Leg 153 in the At-lantic Ocean [28] have been measured for highprecision 186Os/188Os. These materials plot witha restricted range in 186Os/188Os fromV0.119830 to 0.119838 with an average ofV0.119834 and QOs from +0.5 to 36 [19,20,28].The average 186Os/188Os of the present day uppermantle appears to be indistinguishable from the186Os/188Os of 0.119831 þ 60 measured for the Al-lende carbonaceous chondrite, implying that theupper mantle has evolved with approximatelychondritic Pt/Os over the age of the Earth[19,20,28].

At least three components with di¡erent 186Os-187Os systematics are required to explain the rangeof compositions observed for Hawaiian picrites.One component with near-chondritic 186Os/188Osand 187Os/188Os is present in the lavas fromMauna Kea (Fig. 2). A second component withsigni¢cantly higher 186Os/188Os (v 0.119850) andQOs (v+9) falls along the Hawaiian regressionline. This component is best expressed in the pic-rites from Loihi and Hualalai and in the Siberianplume, but is present to variable degrees in all ofthe picrites analyzed except those from MaunaKea and KOO-17a. A third component with ahigh QOs (v+9) and near-chondritic 186Os/188Osis required to account for the composition ofthe two picrites from Koolau and the MaunaLoa 1868 £ow, which all fall to the right of theregression line de¢ned by the other picrites.

4. Discussion

A key issue for the origin of the isotopic com-positions inferred for the Hawaiian plume is therelative roles of crustal recycling and core-mantleinteraction. Speci¢cally, can contributions from

these materials account for the radiogenic 186Os/188Os and 187Os/188Os values observed in the prim-itive Hawaiian tholeiites?

In order to explain the most radiogenic 186Os/188Os ratios by addition of ancient recycled crustor outer core material to the plume source region,several conditions must be met. First, the addedcomponent must have both time-integrated Pt/Osand Re/Os ratios higher than the chondritic man-tle in order to produce a radiogenic 186Os/188Osand 187Os/188Os, respectively, over a period oftime. This component must also have a Pt/Reratio that results in a coupled enrichment in186Os/188Os and 187Os/188Os over time that corre-sponds to the slope of the Hawaiian regressionline (Fig. 2). The radiogenic component mustalso have Pt and Re concentrations higher thanthe mantle. Higher Pt/Os and Re/Os ratios alonewill produce higher 186Os/188Os and 187Os/188Osover time. However, as shown below in the casefor ancient recycled crust, if the Pt concentrationis lower than the mantle Pt concentration, unrea-sonably large amounts of this component are re-quired by mass balance. Finally, those samples

Table 2Core crystallization models

Model 1, rapidcrystallizationto 4.3 Gaa

Model 2, constantcrystallization beginningat 4.0 Gab

DPt 2.9 2.9DRe 14 21DOs 19 33@ 5.5% crystallization and O Ma:Pt (ppm) 5.137 5.137Re 0.1223 0.08231Os 1.112 0.5039186Os/188Os 0.1198515 0.1198526190Pt/188Os 0.00427 0.00931QOs +7.95 +9.88187Re/188Os 0.5332 0.7923Pt/Re 42.0 62.0

See text for details.aStarting parameters at 4.5 Ga: Pt = 5.72 ppm, Os = 3.08ppm, Re = 0.2552 ppm, 186Os/188Os = 0.1198222, 190Pt/188Os =0.001695, QOs = 0, 187Re/188Os = 0.4018.bStarting parameters at 4.0 Ga: Pt = 5.72 ppm, Os = 3.08ppm, Re = 0.2552 ppm, 186Os/188Os = 0.1198235, 190Pt/188Os =0.001695, QOs = 0, 187Re/188Os = 0.4018.

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with the most radiogenic 186Os/188Os should havelithophile and noble gas isotopic compositionsthat are consistent with addition of either crustalor outer core components. The following sectionsaddress these issues and from this discussion, a

model for the origin of the Os isotopic variationin the Hawaiian shield building lavas is pro-posed.

4.1. Pt/Os and Pt/Re relationships

Fig. 3 shows Pt^Os concentration data forocean sediments, average upper continental crust,ma¢c lavas and the primitive mantle value [31,32].The primitive mantle has an Os concentration ofapproximately 3.3 ppb [33] and a chondritic Pt/Osratio of 1.984 [19]. These data correspond to a Ptabundance of 6.54 ppb for the primitive mantle,similar to the average of 6.5 þ 0.9 measured for 14peridotite samples from the Pyrenees [34]. Basalticmagmas tend to have Pt concentrations that aresimilar to or less than the primitive mantle. Atlow Os concentrations, ma¢c lavas can have Pt/Os ratios up to 20 times higher than the primitivemantle. Sediments can also have Pt/Os ratiosmuch higher than the primitive mantle and anestimate for the average upper continental crustgives a Pt/Os ratio V45 times that of the primi-tive mantle (Fig. 3). With the exception of certaintypes of metalliferous sediments, all of these ma-terials that may be recycled as crustal portions ofslabs have Pt concentrations that are generallylower than the upper mantle. Hence, the highPt/Os in these materials is primarily a functionof much lower Os concentrations relative to theprimitive mantle. Accordingly, mixtures betweenancient recycled crust and average upper mantlerequire large amounts of a crustal component toobtain the maximum 186Os/188Os observed in theHawaiian and Siberian plumes. For example,Brandon et al. [20] showed for a mixture of 2 Gaold recycled oceanic crust (basalt+pelagic sedi-ment) and upper mantle peridotite, at least70^95% must be recycled component to producethe maximum 186Os/188Os values of 0.119845^0.119849 in Hawaiian picrites. This would essen-tially require a portion of the Hawaiian plume tobe dominated by eclogite rather than lherzolite.Such a source would tend to generate primarilytonalitic magmas rather than primitive tholeiiticbasalts and picrites [21].

The combined 186Os/188Os and 187Os/188Os sys-tematics also show that mixtures of ancient re-

Fig. 4. Mantle^crust mixing relationships for 186Os/188Os ver-sus QOs. The ¢eld for the 10 Hawaiian picrites and the No-ril'sk (Siberian plume) sample that plot along the HRL (Ha-waiian regression line) is shown. The three samples that donot plot along the HRL are plotted separately. Mixing linesare shown for mixing between a peridotite having MaunaKea Os isotopic characteristics with 186Os/188Os = 0.119834and QOs = +1.2 and Os = 3.3 ppb with the crustal components.The crustal components were formed at 2 Ga with chondriticmantle 186Os/188Os = 0.1198286 and 187Os/188Os = 0.113385.These components evolved to the present day using the fol-lowing characteristics: Basalt: Os = 0.050 ppb, Re = 0.927ppb, Pt = 6.0 ppb, resulting in present day 186Os/188Os =0.1201809 and QOs = +2368. Upper crust: Os = 0.03 ppb,Re = 0.08 ppb, Pt = 2.69 ppb, resulting in present day 186Os/188Os = 0.1200919 and QOs = +331.4. Reducing sediments:Os = 0.244 ppb, Re = 40.26 ppb, Pt = 1.16 ppb, resulting inpresent day 186Os/188Os = 0.1198426 and QOs = +21161. Metal-liferous sediments: Os = 0.153 ppb, Re = 2.021 ppb, Pt = 7.41ppb, resulting in present day 186Os/188Os = 0.1199708 andQOs = +1684. The calculated present day components weremixed with peridotite to create the mixing lines with symbolsrepresenting 10% increments of the mixtures. Pt/Re ratios ofeach crustal component are shown along the mixing lines.Data sources: [31,32,35^37, 61^66].

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cycled crust with peridotite cannot explain thetrend of the Hawaiian regression line (Fig. 2).This is shown in Fig. 4, for mixing lines betweenperidotite with a QOs of +1.2, and 186Os/188Os of0.119834, and various possible ancient recycledoceanic crustal components (upper crust, basalt,reducing sediments and metalliferous sediments)formed at 2 Ga. The slopes of the lines corre-spond to Pt/Re ratios for each recycled compo-nent that range from 0.03 to 33.6. These slopes donot change for older or younger formation ages ofthe recycled components, nor if the Os concentra-tion of the recycled component changes, as longas the recycled components have the same Pt/Reratios. Only the proportions of the componentsneeded to generate a given value for 186Os/188Osand QOs along the line will change. That is, it takesless older material, than younger material, withthe same Pt/Re ratio to achieve a given composi-tion, because the older material will be more ra-diogenic. Also, a higher Os concentration for arecycled component of the same age would resultin a component with lower Pt/Os and Re/Os, suchthat the mixing relationships will be the same pro-vided that the recycled component has the samePt and Re concentrations. All of these mixinglines have less positive slopes than the Hawaiianregression line, producing more radiogenic 187Osin proportion to 186Os. This is a function of thelow Pt/Re in crustal materials and it demonstratesthat none of these components could produce theHawaiian compositions, even if they contributedpreferentially to the melt. A Pt/Re ratio of V88^100 for ancient recycled crust is required toroughly match the Hawaiian regression line (Fig.4). For example, if a 2 Ga recycled componenthas 50 parts per trillion (ppt) Os, 1 ppb Re anda Pt/Re of 88, then, 16% of this component mustbe added to the PUM to generate the most radio-genic Loihi and Hualalai 186Os/188Os and 187Os/188Os ratios. The Re and Os characteristics of thishypothetical recycled component are similar tothat used by Hauri et al. [11]. The componentconsists of 90% 1.8 Ga altered ocean basalt and10% pelagic sediment and addition of 6% of sucha component to PUM will generate Manua LoaPb and Os isotopic compositions [11]. While 1 ppbof Re is within the range measured for mid-ocean

ridge basalts [35,36], no materials yet measuredthat may make up portions of ancient recycledcrust have Pt/Re of 88^100 and as in this example,Pt of 88^100 ppb.

The Pt/Re ratios used to model radiogenic Osingrowth in crustal materials are based upon avery limited database. However, the Pt/Re ratiosused here for crustal materials are consistent withthe geochemical behavior for these elements basedon the data for peridotites. During melting ofperidotite, Pt is weakly compatible to weakly in-compatible [37], while Re is moderately incompat-

Fig. 5. (a) 3He/4He (R/Ra) versus QOs for the Hawaiian pic-rites. Error bars are 2c. He data sources are listed in Table1 and formation averages are plotted where no data exist forpicrites analyzed for Os in the present study. The ¢elds forthe three end-member components proposed by Hauri [40]and Hauri and Kurz [42] for the Hawaiian shield lavas areshown (Kea, Loihi, Koolau). (b) 3He/4He (R/Ra) versus186Os/188Os for the Hawaiian picrites.

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ible [33]. Hence, the Pt/Re ratio of the bulk crustshould be less than or equal to the initial Pt/Reratio of the PUM. Sediments derived from crustwill also have a lower Pt/Re ratio unless a processoccurs that fractionates this ratio to higher values.If the PUM has a Pt/Re ratio similar to chon-drites [33] as indicated by the 186Os/188Os of uppermantle materials, which range from V21 to 24[19,30,38,39], then, the Pt/Re ratios for crustalrocks should be similar to, or less than, such aratio. The Pt/Re ratios used in the modeling aretherefore consistent with these expectations. Anycombination of various components that maycomprise ancient recycled crust appears to be in-su¤cient to explain the high Pt/Re ratios (88^100)needed to produce the slope of the Hawaiian re-gression line. This conclusion is irrespective ofwhether a simple mixing model is used or whethera complex model that avoids mass balance con-straints is called upon.

Data for three picrites lie below the regressionline and require an additional explanation (Fig.4). This is especially the case for the two Koolausamples, which are well resolved from the regres-sion array towards more radiogenic values of187Os/188Os for a given 186Os/188Os. These twoKoolau picrites have Os isotopic compositionsthat fall along mixing lines between crustal andmantle values and could, therefore, re£ect a com-ponent of ancient recycled crust in the plume.Using the simple mass balance approach, for in-stance, 20% or less of 2 Ga basalt or 10% or lessof metalliferous sediment or reducing sedimentcould result in the shifts to the right of the mainregression line for these samples (Fig. 4). There-fore, it appears that at least three components arerequired to explain the 186Os/188Os and 187Os/188Os relationships for the Hawaiian picrites.

4.2. Os^He^Pb^O isotopic relationships

Previous work has shown that mixing betweenthree end-member isotopic components may ex-plain the isotopic compositions for the Hawaiianshield building lavas, the Kea, Koolau and Hua-lalai^Loihi components [40,41]. These three com-ponents are displayed for QOs^R/Ra (where R/Rais the 3He/4He ratio normalized to the atmospher-

ic ratio at 1.39U1036) in Fig. 5a [42]. Heliumisotopes are problematic because they may poten-tially be decoupled from Sr, Nd, Pb and Os iso-topes during melt migration in the mantle pro-vided that chromatographic e¡ects dominate.This is because the di¡usibility of He is 104 timeshigher than those for these other elements [42^44].Helium isotopic ratios may also be decoupled as aresult of late stage degassing of the lavas. How-ever, Hauri and Kurz [42] demonstrated thatchromatographic element fractionation was mini-mal during melt transport in the Hawaiian plumeand hence, the correlations between He and Sr,Nd, Pb, O and Os isotopes in the shield buildinglavas probably re£ect compositional heterogeneityin the plume itself. The picrite samples analyzedhere plot between the three end-member compo-nents previously proposed for Hawaii shield lavas[40,41]. Samples from Mauna Kea, Kilauea andKohala have lower values of QOs and come fromvolcanoes with low R/Ra lavas, while the picritesfrom Koolau, Hualalai and Mauna Loa generallyplot to higher values of QOs and come from volca-noes with intermediate R/Ra in their lavas. TheLoihi picrites are characterized by intermediateQOs and Loihi samples have some of the highestvalues of R/Ra measured for Hawaiian lavas (Fig.5). Fig. 5b shows that the picrites from volcanoesthat have the highest R/Ra (Loihi, Hualalai andone Mauna Loa sample) also have the highest186Os/188Os. Ancient recycled crust will have R/Ra much lower than the atmospheric value ofone [45], so the high 3He signal in these £owscannot re£ect a contribution from such a source.This observation is consistent with constraintsfrom the Pt/Re ratio of any ancient recycled crus-tal component being far too low to be the radio-genic end-member required to generate the Ha-waiian regression line in Fig. 2.

Osmium, lead and oxygen isotopic systematicsshed further light on the nature of the possibleend-member components for the Hawaiian sys-tem. Bennett et al. [10] showed that a correlationexists between QOs and Pb isotopes and arguedthat these relationships could be explained bymixing between two end-member components.This correlation is shown in Fig. 6a for QOs versus206Pb/204Pb. However, such a linear correlation is

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not present for 186Os/188Os versus 206Pb/204Pb(Fig. 6b). The highest values in 186Os/188Os lie atintermediate values for 206Pb/204Pb (Loihi, Huala-lai, Mauna Loa) and decrease to lower 186Os/188Os in the Koolau picrites with low 206Pb/204Pb and samples from Mauna Kea and Kilaueawith high 206Pb/204Pb. A linear correlation is alsopresent for these lavas between QOs and N18Oolivine,which may be interpreted as a two componentmixture [12], but this correlation is not presentfor 186Os/188Os versus N18Oolivine (Fig. 6c). Instead,a linear correlation exists from Mauna Kea iso-topic compositions to Loihi^Hualalai isotopiccompositions, with the Koolau samples fallingto the right of the trend. These data are consistentwith the interpretation of a mixture between atleast three isotopic components represented byLoihi^Hualalai, Koolau and Mauna Kea to ex-plain the isotopic heterogeneity of the Hawaiianshield building lavas.

4.3. Origin of 186Os-187Os isotopic variation forHawaiian shield lavas

From the previous two sections, constraints canbe placed on the three isotopic end-members re-quired to explain the Os isotopic variation in theHawaiian shield lavas. The Kea component lies atone end of the Hawaiian regression line withchondritic 186Os/188Os of V0.119834 and QOs of1^2. These characteristics are similar to the esti-mates for the Os isotopic composition of theupper mantle [28,30]. The fact that the most ra-diogenic abyssal peridotites [28] and unaltered sul-¢des from MORB [46] have QOs of V1^2, sup-ports the view that the upper mantle hasretained a primitive mantle-like Os isotopic com-position until today. The low R/Ra values ofV7^9 for Mauna Kea lavas are consistent witha mantle source that has undergone degassing,likely the result of melting, and the low N18O val-ues of +4.6^+5.1 in the Mauna Kea and Kilaueaolivines appear to require either late modi¢cationof the magmas by assimilation of local Paci¢ccrust [47] or entrainment of hydrothermally al-tered oceanic lithosphere in the mantle plume[12]. The QOs of Kea component lavas similar tothe present day upper mantle and their chondritic

186Os/188Os are consistent with this componentbeing derived from entrained asthenosphereunderneath Hawaii (i.e. [11,40]). In such a case,it is questionable whether processes that resultedin low R/Ra and N18O in these lavas had measur-able e¡ects on their Os isotopic compositions.

The component represented by Koolau has186Os/188Os ranging from 0.119834 to 0.119840that is also approximately chondritic to slightlysuprachondritic and has a QOs of v+8. The lowR/Ra and high N18O values for Koolau lavas areconsistent with the presence of ancient recycledcrust in this component [11,12,40]. The Pt/Re re-lationships described above are consistent withthese interpretations, where 20% or less of variousrecycled components added to a lava source gen-erated along the Hawaiian regression line can ex-plain the de£ections to higher values of QOs with-out a major e¡ect on 186Os/188Os (Fig. 4).Alternatively, the Koolau component could be amantle source that has been refertilized by meltsderived from a ma¢c source at some time in thepast [10]. The Os data presented here do not dis-tinguish between these two possibilities. Both sce-narios are likely to result in the same isotopice¡ects. Norman and Garcia [21] showed that theprimitive Koolau melts are in equilibrium withperidotite and have trace element ratios inconsis-tent with partial melts of recycled crust or withbulk addition of recycled crust into the Koolausource. If this scenario is valid, then, the metaso-matic model of Bennett et al. [10] may best ex-plain the Koolau isotopic characteristics for Os.Melt^rock interaction documented in abyssalperidotites [48] shows that only small increasesin Pt/Os occur as a consequence of this process(about a 30% maximum increase), such that thisprocess will have no measurable e¡ect on 186Os/188Os, even if the metasomatism occurred duringthe Archean. The Os isotopic data for the Koolaupicrites presented here are consistent with such amodel.

Finally, the component that lies to the radio-genic end of the Hawaiian regression line (Loihiand Hualalai) must have a minimum 186Os/188Osof V0.119852, a QOs of +8^+9 and a Pt/Re ratiomuch higher than those for the upper mantle orcrustal materials (Fig. 4). The Loihi and Hualalai

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picrites are from volcanoes with the highest R/Rameasured for the Hawaiian lavas and have beeninterpreted to re£ect undegassed lower mantle[45,49^51]. This is consistent with the previousinterpretation of the coupled enrichment of186Os/188Os and QOs resulting from addition of

evolved outer core into the lower mantle[17,19,20]

4.4. Isotopic composition of the core andentrainment into plumes

Walker et al. [17] used a model of rapid crys-tallization of the inner core to produce supra-chondritic Re/Os and Pt/Os in the outer core. Inthis model, Walker et al. [17] used solid metal/liquid metal partition coe¤cients (D) for Pt of2.9, for Re of 14 and for Os of 19. These Dswere obtained from regression of the composi-tions of group IIA iron meteorites [17,52], assum-ing a DIr of 14 [53,54]. Group IIA meteorites arethought to represent the products of simple frac-tional crystallization of an asteroidal core similarin bulk composition to that estimated for theEarth's core [52,55]. If it is assumed that the ini-tial Pt/Os and Pt/Re for the bulk core are chon-dritic and Pt, Re and Os concentrations in thecore are similar to chondritic metal [17], then, apresent day 186Os/188Os of 0.1198515 and QOs of+7.95 is generated by rapid, complete crystalliza-tion (5.5% of the total core) of the inner core by4.3 Ga (model 1, Table 2). This model results in aPt/Re ratio of the outer core of V42. Addition of0.8% of this component into a plume source withan upper mantle Os concentration of 3 ppb [33], a186Os/188Os of 0.119834 and a QOs of +1.2 (i.e.Mauna Kea characteristics) can generate a com-position similar to those for the Loihi and theHualalai picrites with a 186Os/188Os of 0.1198471and a QOs of +6.25.

Fig. 6. (a) 206Pb/204Pb versus QOs for the Hawaiian picrites.Symbols as in Fig. 5. (b) 206Pb/204Pb versus 186Os/188Os forthe Hawaiian picrites. Error bars are 2c. The Pb isotopedata for most of these picrites are from Norman and Garcia[21]. The Pb data for ML1868.9 are from the same £ow [67].For the two Mauna Kea samples, the Pb isotopic data arefrom Lassiter et al. [12,73], which are for samples takenwithin 30 cm in the drill core from where the Os data in thisstudy come from and are from the same £ow units. (c)N18Oolivine versus 186Os/188Os for the Hawaiian picrites. Errorbars are 2c. The oxygen isotope data on olivines are forma-tion averages from Eiler et al. [47], except for the twoMauna Kea samples, where N18Oolivine data are used from thedrilled core samples analyzed by Lassiter et al. for Os andPb isotopes [12,73,74]).6

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Alternatively, the inner core may have crystal-lized at a signi¢cantly slower rate. Stacey and Lo-per [56], for instance, presented a cooling modelfor the core and the mantle that results in con-stant growth of the inner core to the presentday. In this model, a present day 186Os/188Os of0.1198526 and QOs of +9.88 can be generated usingthe same DPt of 2.9, with a DOs of 33 and DRe of21 (model 2, Table 2). These D values are consis-tent with the observed partitioning behavior ofthese elements obtained in experiments on iron-sulfur-nickel systems thought to be similar incomposition to the Earth's core (e.g. Lauer andJones [57]). This model results in a present day Pt/Re of 62. Addition of 1.2% of this componentinto a plume source with Mauna Kea Os charac-teristics can generate a 186Os/188Os of 0.1198465and a QOs of +6.70.

Although the behavior of Pt, Re and Os duringcrystallization of the inner core is unknown, thecalculations in the preceding paragraphs demon-strate two important points. First, the Pt/Re ra-tios needed to generate a component in the outercore with a coupled enrichment in 186Os and 187Osthat lies along the Hawaiian regression line mustbe higher than the chondritic ratio of V21^24. Inorder to generate a minimum coupled enrichmentfor 186Os/188Os of 0.119852 and for QOs of +9 re-quires a Pt/Re ratio of V42^62, depending on thetimescales for growth of the inner core. Second,this requirement for a high Pt/Re ratio is consis-tent with the known behavior of Pt, Re and Osduring metal crystallization in asteroidal cores[17,57,58] and not consistent with the behaviorfor these elements during melting in the silicatemantle. Therefore, we conclude that the best mod-el for the origin of the Os isotopic componentthat is represented by the Loihi and Hualalailavas is the deep lower mantle that has been in-¢ltrated by small amounts (91.2%) of evolvedouter core metal as previously proposed [17].

The upper limit of 1.2% core added to the Loihiand Hualalai portions of the Hawaiian plume isdependent on the plume having upper mantle Osconcentrations (i.e. 3 ppb). Core formation wouldleave the silicate Earth strongly depleted in Pt, Reand Os. Virtually all of the amounts of these ele-ments in the mantle at present may have been

supplied by a late veneer of chondritic materialafter core formation [33]. It is possible that thelate veneer of chondritic material was addedonly to the upper mantle [18,33]. In such a case,the lower mantle would still have a very low Osconcentration and the amount of outer core re-quired to cause the coupled 186Os and 187Os en-richments in the Hawaiian lavas would be sub-stantially less. For instance, using a proposedestimate for the lower mantle Os concentrationof 0.05 ppb [18], the addition of less than 0.2%of evolved outer core would completely controlthe Os isotopic composition of the plume forthe two models presented in Table 2. This scenar-io is appealing because even 1.2% of added corematerial may have noticeable e¡ects on the con-centration of siderophile elements in the mantle[8]. Additional constraints on the early dynamichistory of the Earth, in conjunction with highpressure experiments, are required to further de-lineate between these models and whether a lateveneer of chondritic material had any e¡ect on thelower mantle Pt, Re and Os concentrations.

The addition of such small amounts of outercore material into the plume source is likely tohave little e¡ect on the bulk composition of thelithophile isotopic compositions. Geochemicaland cosmochemical considerations indicate thatvirtually no Nd or Sr or other lithophile elementsreside in the core [59], such that addition of outercore metal would not a¡ect the budget of theseelements in the plume source. The concentrationof Pb in the core is likely to be similar to theconcentration of Pb in the mantle or less (e.g.0.4 ppm versus 0.15 ppm, respectively [59]). Thisis an upper estimate of the Pb concentration inthe core, because Pb is a highly volatile elementand was likely strongly depleted prior to accretionas indicated by the 98% depletion of the less vol-atile element, K [8,60]. Because of the low concen-tration of Pb in the outer core, small amountsadded to the plume source will have no discerni-ble e¡ect on the Pb isotopic composition [8]. In-stead, the Sr, Nd and Pb budget of plumes ismore likely to be controlled by materials that con-tain high relative concentrations of these ele-ments, such as recycled ma¢c materials or sedi-ments [61].

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5. Conclusions

High precision Os isotopic analysis of 13 pic-rites from seven Hawaiian volcanoes show that atleast three end-member components are needed toexplain the variation in 186Os/188Os and 187Os/188Os. These components generally correspondto previously proposed end-members [40]. Onecomponent represented by lavas from MaunaKea and Kilauea have Os isotopic characteristicssimilar to the present day PUM, despite havingevidence for melt depletion from lithophile andnoble gas isotopes. If this component is represen-tative of the entrained asthenospheric upper man-tle in this region, then, these data show that melt-ing, and possibly other processes, such as melt^rock interaction, has had little e¡ect on the Osisotopic evolution of the bulk upper mantle.

A second component represented by picritesfrom Loihi and Hualalai has the most radiogenic186Os/188Os and the highest R/Ra of the Hawaiianlavas. These characteristics are consistent withthis component originating at the CMB, whereundegassed lower mantle may exist and wherecore-mantle chemical exchange may occur. Thesedata are the strongest geochemical evidence yetthat some plumes may come for the CMB. Koo-lau lavas have radiogenic 187Os/188Os but they arenot enriched in 186Os/188Os. These characteristics,combined with a lower R/Ra than the Loihi com-ponent and a distinct Pb isotopic composition,may be consistent with the addition of ancientrecycled crust to the plume source or metasoma-tism by melts derived from ma¢c reservoirs.

These high precision Os isotopic data for theHawaiian plume system demonstrate the advan-tages of having two independent Os isotopic sys-tems. Application of the coupled systems has re-vealed the complex nature of the source regions ofoceanic island lavas and how Os isotopes may beused to distinguish between an upper versus alower mantle origin of plumes. Continued workon Hawaiian lavas and on other plume sys-tems should further enhance our knowledge ofmantle dynamics and compositional variabilityof the sources for oceanic lavas in both upperand lower mantle and the source regions of man-tle plumes.

Acknowledgements

This work was supported by NSF Grants EAR968703, EAR 970986, EAR 9804909, EAR9711454 and by the Australian Research Council.We thank the Hawaiian Scienti¢c Drilling Projectand Mike Baker for the Mauna Kea samples.Harry Becker, Vickie Bennett, John Chesley,Robert Fei, Erik Hauri, Munir Humayun, JohnLassiter and Mukul Sharma are thanked for nu-merous enlightening discussions on Os isotopesand PGE analytical techniques and geochemistry.Mark Rehkamper and Elizabeth Widom arethanked for thorough journal reviews.

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