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Correlated helium and lead isotope variations in Hawaiian lavas

JOHN M. EILER, KENNETH A. FARLEY, and EDWARD M. STOLPER

Division of Geologic and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA

(Received July7, 1997;accepted in revised form February13, 1998)

Abstract—Variations in3He/4He ratios among Hawaiian shield-building and pre-shield basalts are correlatedwith variations in208Pb/204Pb and206Pb/204Pb ratios. Using this correlation, the3He/4He ratio of Hawaiianlavas can be predicted to within 2.9 RA (mean deviation) between 7 and 32 RA based only upon the leadisotope composition. This level of prediction is as good as can be expected based upon the precision of leadisotope ratio measurements. This correlation demonstrates a coupling of volatile and nonvolatile elements inthe sources of Hawaiian basalts and allows the nonvolatile-element characteristics of the high-3He/4Hecomponent of the mantle sources of Hawaiian lavas to be defined. This result confirms and extends previousinferences based upon correlations between helium and strontium isotope ratios in individual suites ofHawaiian lavas. The source of high3He/4He ratios in Hawaiian lavas has a higher time-integrated Th/U ratiothan the sources of Pacific mid-ocean ridge basalts, consistent with it being a mixture containing primitivemantle or having differentiated in two or more stages from primitive mantle.Copyright © 1998 ElsevierScience Ltd

1. INTRODUCTION

Basalts with3He/4He ratios higher than typical mid-ocean ridgebasalts (MORB) are usually thought to have acquired theirhelium from regions of the mantle that have retained a largefraction of their primordial volatile inventory (Craig and Lup-ton, 1976; Kurz et al., 1982; 1983; Rison and Craig, 1983).Study of the major and trace element concentrations and iso-topic ratios of such lavas can provide insights into the chem-istry and history of the portions of the mantle hosting relativelyprimitive volatiles, which in turn can offer constraints upon theinteraction between such reservoirs, the atmosphere, and de-pleted upper-mantle sources of MORB (e.g., Alle`gre et al.,1987; Kellogg and Wasserburg, 1990).

Several studies have demonstrated similarities in the stron-tium, neodymium, and lead isotope ratios of lavas with high3He/4He ratios (Kurz et al., 1982; Zindler and Hart, 1986a;Farley et al., 1992; Hart et al., 1992; Graham et al., 1993).When compared with the array of87Sr/86Sr, 143Nd/144Nd, and206Pb/204Pb ratios for ocean island basalts, high-3He/4He lavasare internal to the well-defined DMM (i.e., MORB-like), EM1(low 143Nd/144Nd and206Pb/204Pb), EM2 (high87Sr/86Sr), andHIMU (high 206Pb/204Pb) endmembers (Zindler and Hart,1986a) and are similarly internal to these endmembers in theirlead isotope ratios (Hanan and Graham, 1996). In addition,some ocean island basalt suites form trends in87Sr/86Sr-143Nd/144Nd-206Pb/204Pb space that extend toward an internal com-ponent, and, in certain instances,3He/4He ratios increase as thiscomponent is approached (Hart et al., 1992; Farley et al., 1992).These observations, taken together, have led several authors topropose that there is a high-3He/4He mantle reservoir that ismoderately depleted (i.e., intermediate between bulk earth andMORB) in terms of its strontium and neodymium isotope ratios(PREMA of Zindler and Hart, 1986a; FOZO of Hart et al.,1992; PHEM of Farley et al., 1992).

However, correlations of strontium, neodymium, and lead

isotope ratios with helium isotope ratios are not always appar-ent, and even in cases such as lavas from Hawaii and Samoa,where helium and strontium isotope ratios are well correlated insome sample suites (Kurz et al., 1987; Kurz and Kammer,1991; Farley et al., 1992), large ranges in3He/4He ratio can beaccompanied by negligible variations in some nonvolatile-ele-ment isotope ratios (e.g.,3He/4He ranges between 8 and 32 RA

among Hawaiian lavas with87Sr/86Sr ; 0.7035; Kurz et al.,1983; Staudigel et al., 1984; Kurz et al., 1987). The lack of arecognized, unique geochemical signature for high-3He/4Hemantle sources of Hawaiian and other lavas has led to thesuggestion that noble gases (and perhaps other volatiles) aresomehow decoupled from nonvolatile elements in the mantle orcrust. Processes that have been proposed for this decouplinginclude separation of a vapor phase from the mantle prior to orduring melting and/or ascent of plumes (Anderson, 1985; Val-brecht et al., 1996) and magmatic degassing accompanying4Heingrowth during magma storage and/or assimilation (Zindlerand Hart, 1986a b; Hilton et al., 1995). Another possibility isthat there is a large contrast between the ratio of helium tononvolatile elements in high- and low-3He/4He sources, suchthat mixtures between such sources in plots of3He/4He vs.other isotope ratios are highly curved (Kurz et al., 1982; Al-legre et al., 1987). It is also possible that mixing of olivinephenocrysts (a commonly analyzed material in3He/4He deter-minations on basalts) between lavas derived from differentsources could confound correlations between helium and stron-tium, neodymium, and lead isotope ratios. If these variousfactors have influenced covariations of the isotope ratios ofhelium and other elements in mantle-derived basalts, then itwill likely be difficult to define precisely the isotopic andchemical composition of nonvolatile elements in the ultimatesources of high-3He/4He signatures.

The question of whether helium is coupled with nonvolatileelements during basalt petrogenesis is particularly significant inthe interpretation of correlations between3He/4He ratios and

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eruption ages of Hawaiian lavas (e.g., Kurz and Kammer, 1991;Kurz et al., 1995, 1996). If these variations reflect differentmixing proportions of high- and low-3He/4He silicate sourcesin the mantle, then temporal variations likely reflect the geom-etry and/or dynamics of the upwelling mantle (e.g., Kurz et al.,1995; Hauri et al., 1996). If such variations instead reflectdifferential sampling of primitive volatiles, decoupled fromtheir original silicate sources, then they instead record details ofthe processes of volatile transfer beneath and within the Ha-waiian volcanic edifice (e.g., Valbrecht et al., 1996).

Here we present data compiled from literature sources thatshow correlations between helium and lead isotope ratios inHawaiian lavas. These correlations indicate that high3He/4Heratios in Hawaiian lavas are carried by a distinctive mantlesource and that the characteristics of this source are not ob-scured. In addition, the lead isotope signature of this high-3He/4He source provides a new constraint on the origin of primitiverare-gas signatures in basalts.

2. DATA FOR HAWAIIAN LAVAS

We have compiled from the literature a data base of lead and heliumisotope compositions of Hawaiian lavas, including seventy-two forwhich the isotopic compositions of both elements were measured onthe same hand specimen or lava flow, and an additional ninety-six forwhich only helium or lead isotope ratios were measured (Table 1). Thelatter were compiled in order to calculate average isotope ratios foreight suites of closely related lavas for comparison with the seventy-two individual samples on which both isotope ratios were measured.

Most of the helium isotope ratios in Table 1 were determined bycrushing of olivine separates in vacuum. Where data from both fusionand crushing of samples were reported, data from crushing were usedbecause fusion analyses can include a contribution from post-eruptiveingrowth of radiogenic4He and/or production of cosmogenic3He (e.g.,Kurz et al., 1996). Reported precision for3He/4He ratios is as good as60.2 RA, but the reproducibility of replicate measurements of the samehand sample or lava flow among the data compiled for this study isgenerally worse than this, averaging60.8 RA (2s). Consequently, wetake the latter value to be the effective precision of the data. All leadisotope data included in this compilation were normalized to the NBS981 standard. The average reported 2s precisions on lead isotope

1978 J. M. Eiler, K. A. Farley, and E. M. Stolper

measurements are60.016 for206Pb/204Pb and60.054 for208Pb/204Pb;these values do not vary significantly among the data sets used. How-ever, due to the difficulty in correcting for mass fractionation, theseerrors may underestimate inaccuracies in lead isotope data. We havenot attempted to cull the data on these grounds, but have notedanomalous lead isotope data where they could indicate an analyticalartifact.

In our treatment of the isotopic data for Hawaiian lavas, we distin-guish three stages of volcanism: pre-shield (i.e., Loihi lavas), shield(tholeiitic lavas making up most of the known volume of Hawaiianvolcanoes other than Loihi), and post-shield (alkalic lavas erupted atthe end of shield building and during the post-erosional or rejuvenatedperiod of volcanism that follows;1–2 Ma after shield building;Clague, 1987). We consider only the first two of these stages in ouranalysis. Post-shield lavas are virtually invariant in their3He/4He ratios(averaging 8.06 0.2 R9A

IT). In addition, strontium isotope ratios in theselavas are below the ranges for shield-building and pre-shield lavas(Hegner et al., 1986; Stille et al., 1986; Kurz et al., 1987). This has ledto the suggestion that the sources of post-shield lavas include a MORB-like component (i.e., lithospheric mantle and/or a local MORB source;Chen and Frey, 1983; Kurz et al., 1987; Kurz and Kammer, 1991) inaddition to pre-shield and shield-building sources. Recent (#3 ka)Mauna Loa lavas also have a narrow range of3He/4He ratios (average:8.56 0.4 R9A

IT). This has been interpreted as an early sign of thetransition from plume to nonplume sources in Mauna Loa, and it is alsorecorded in subtle shifts in the strontium isotope ratios of these lavas(Kurz and Kammer, 1991). This subset of samples (post-shield andrecent Mauna Loa lavas) are compiled in Table 1 and plotted in Fig. 1,but have not been included in our consideration of the properties ofhigh-3He/4He sources.

3. LEAD-HELIUM ISOTOPE SYSTEMATICS OFHAWAIIAN LAVAS

The principal difficulty in associating a distinctive nonvolatile-ele-ment isotopic signature with the sources of high3He/4He ratios inHawaiian lavas is that lavas from Loihi (the high extreme in3He/4Hein Hawaii and among terrestrial lavas generally) are internal to theHawaiian field in most two-dimensional representations of strontium-neodymium-lead-oxygen isotopic systems (Staudigel et al., 1984; Stilleet al., 1986; West et al., 1987; Eiler et al., 1996). An exception to this

is a plot of 208Pb/204Pb vs.206Pb/204Pb (Fig. 1), in which the Loihisamples are at an extreme. One way to determine whether helium andother isotope ratios are correlated in Hawaiian lavas is thus to deter-mine whether a sample’s location in Fig. 1 is predictive of its3He/4Heratio. We have demonstrated such a correlation in the following ways:

We determined the two principal components (y1, y2) of variation inthe array of data in Fig. 1. The results of this exercise are shown bylines of constanty2 in Fig. 1. For a particular sample,y2 is proportionalto the deviation in208Pb/204Pb from the best fit trend of the Hawaiianarray at the sample’s206Pb/204Pb, and thus Loihi lavas, with theirthorogenic Pb, all have high values of this index. This index is con-ceptually similar toD208 of Hart (1984), but is relative to the Hawaiianfield rather than the Northern Hemisphere reference line. This treatmentof the lead isotope variability is unfamiliar in comparison to othersimilar indices (e.g.,D208, 208*/206*), but it is more effective forillustrating the covariation of helium and lead isotope ratios in Hawai-ian lavas. The comparison of3He/4He ratios and values ofy2 isillustrated in Fig. 2. For most shield-building and pre-shield lavas(fifty-seven of the sixty-one such points on this figure), this plot showsa continuous, gently curved trend, and the Loihi samples are at one endof this trend. The uncertainty in eachy2 score is60.6 (2s) based uponthe propagated uncertainties in measured lead isotope ratios; this is onthe order of the width in the horizontal dimension of the band ofwell-correlated data in Fig. 2, and thus these data demonstrate asignificant correlation of lead and helium isotopic compositions. Theexceptions to this trend include two points for Koolau and two sub-marine Mauna Loa lavas. These points could reflect a decoupling ofhelium from lead or that the sources of these lavas incorporate morethan two components (such that simple trends are not found in two-dimensional mixing diagrams; this is our preferred explanation for theKoolau samples; see below). However, we note that the two excep-tional submarine Mauna Loa samples are similar in all other isotopicratios to related lavas that do closely conform to the main trend of thedata. Their deviation from the main data trend could thus be the resultof larger errors and/or inadequate correction for mass fractionation inthe lead isotope ratios. Supporting this possibility is the fact that thesetwo measurements are outside the range of all other Hawaiian lavas inthe plot of206Pb/204Pb vs.208Pb/204Pb (Fig. 1) and lie on an extrapo-lation of the lead-isotope mass fractionation line from other Mauna Loadata.

Eiler et al. (1996) presented a mixing model for Hawaiian shield-

Fig. 1. Plot of208Pb/204Pb vs.206Pb/204Pb for Hawaiian lavas. Data for Loihi are enclosed in a shaded field. Parallel linesmarkedy2 are contours of an index of the lead isotope variations explained in the text and the caption to Fig. 2. Loihi, Kea,and Koolau labels mark the locations of mixing components (Table 2). The field for northern Pacific MORB is from Ito etal. (1987). Values ofy2 can be calculated as follows:y2 5 208Pb/204Pb z 12.435-206Pb/204Pb z 7.351- 337.768.

1979Helium and lead in Hawaiian lavas

building and pre-shield lavas based upon a principal component anal-ysis of strontium, neodymium, lead (206Pb/204Pb, 207Pb/204Pb, and208Pb/204Pb), oxygen, and helium isotope ratios. The results led to thedefinition of three components (Koolau, Loihi, and Kea, listed in Table2) that could be mixed in varying proportions to account simulta-neously for most of the variability in Hawaiian lavas (excluding post-shield lavas) to within a small multiple of analytical precision for all ofthe isotope systems. The Loihi component they defined is characterizedby strongly elevated3He/4He ratios, moderate206Pb/204Pb ratios, andhigh 208Pb/204Pb ratios (not surprisingly, these are precisely the char-acteristics of the Loihi end of the data trends in Figs. 1 and 2). Incontrast, the Kea component is characterized by3He/4He ratios approx-imately equal to MORB, high206Pb/204Pb ratios, and moderate208Pb/204Pb ratios, and the Koolau component is characterized by moderate3He/4He ratios and nonradiogenic lead. We have tested whether themixing proportions of these components that are required to explain asample’s lead isotope ratios can also explain its helium isotope ratio.

The proportions of the three components present in the source ofeach sample (or average) listed in Table 1 were calculated based onlyupon the208Pb/204Pb and206Pb/204Pb ratios of the samples and com-ponents. The proportions of lead from each of the three componentscontributing to the lead in a given sample are described by the follow-ing equations, which were derived by manipulation of the equationsdescribing the isotopic composition of mixtures in a three-componentsystem:

Xkoo 5RS

208 2 Rkea208 2 U z (RS

206 2 Rkea206)

Rkoo208 2 Rkea

208 2 U z (Rkoo206 2 Rkea

206)

Xloihi 5RS

206 2 Xkoo z Rkoo206 2 Rkea

206 1 Xkoo z Rkea206

Rloihi206 2 Rkea

206

Xkea 5 1 2 Xkoo 2 Xloihi

where Xi is the fraction of lead from each component in the sample(s 5 sample, koo5 Koolau, loihi 5 Loihi, kea5 Kea in Table 2), Riis the isotope ratio normalized to the average and standard deviation ofthat ratio for all of the data (e.g., Rs

206 5 (206Pb/204Pbsample2 206Pb/204Pbaverage)/s(206Pb/204Pb), and

U 5Rloihi

208 2 Rkea208

Rloihi206 2 Rkea

206

The calculated proportions of the components were then used tocalculate the expected3He/4He ratio for each shield and pre-shield lavathrough the relationship:3He/4Heexpected5 Si(

3He/4He)i z Xi. Note thatthis calculation assumes that the4He/206Pb ratios of all three compo-nents are the same. This approximation was made because of theapproximately linear correlation of helium isotope and lead isotoperatios shown by most of the data in Fig. 2 and based upon previousobservations of approximately linear correlations among lead, oxygen,and helium isotope ratios for subsets of the Hawaiian lavas measuredfor these ratios (Eiler et al., 1996). The resulting prediction for eachsample is plotted against its measured3He/4He ratio in Fig. 3. Theuncertainty in predicted3He/4He ratio (62.6 RA) is based on theanalytical uncertainty in lead isotope ratio measurements and is largelycontrolled by the uncertainty in208Pb/204Pb. The level of agreementbetween calculation and data (2.9 RA, mean deviation) is approxi-mately as good as can be expected and confirms the visual evidence inFig. 2 for correlation of lead and helium isotope ratios. A large fractionof the mean error in the fit of this model is due to misfits of a smallnumber of samples that are either in lead or helium isotopes and otherisotopic systems similar to samples that are well fit by the calculation(e.g., the two anomalous submarine Mauna Loa samples discussed inreference to Fig. 2). If the five worst-fit samples are disregarded, themean deviation between this model and the data is reduced to 2.4 RA.The importance of analytical uncertainty in lead isotopes as a limitingfactor in our treatment of these data is illustrated in Fig. 3 by plottingthe averages for six suites in which lead isotope ratios are well con-strained and vary by little more than analytical precision (these datainclude the Loihi, HSDP Mauna Kea, subaerial Mauna Loa older than3 ka, submarine Mauna Loa, and shield-building Haleakala and Koolausuites). These suite averages are shown as unfilled squares in Fig. 3.The average3He/4He ratios measured for these suites and those pre-dicted by the three-component lead isotope mixing model agree towithin 1.6 RA (mean deviation).

Note that a somewhat different test of this three-component model ofthe isotopic variability of Hawaiian lavas was presented by Eiler et al.(1996). Each sample in a similarly large sample suite was fit by leastsquares as a mixture of the defined components using all isotopic data,including strontium, neodymium, oxygen, helium, and all three leadisotope ratios. In this case, the3He/4He ratio can be fit to within 1.3 RA(mean deviation) while simultaneously fitting other isotopic ratios towithin a small multiple of their analytical precisions (60.6 eNd,60.0000687Sr/86Sr,60.069206Pb/204Pb,60.006207Pb/204Pb,60.035208Pb/204Pb, 60.1‰ d18O; see Eiler et al., 1996). However, theircalculation differs from the one presented in Fig. 3 in that heliumisotopes were used with the other isotope ratios to constrain theproportions of the three components, whereas Fig. 3 shows the results

Fig. 2. An index of the lead isotope compositions has been calculatedto permit a two-dimensional view of the correlations between heliumand lead isotopes. The index,n2, is derived from principal componentanalysis of the208Pb/204Pb-206Pb/204Pb data in Table 1. It is analogousto D208 of Hart (1984). Lines of constanty2 are shown in Fig. 1.3He/4He is well correlated withy2 for ;95% of the data, showing thehighly coupled nature of helium and lead isotope variations. The fourexceptions to the main trend of the data on this figure are discussed inthe text. Post-shield and recent Mauna Loa lavas (essentially constantin their 3He/4He ratios at;8 RA) are thought to sample nonplumesources (Kurz and Kammer, 1991) and are excluded from this plot.


of a prediction of helium isotopes based only on a sample’s208Pb/204Pband206Pb/204Pb ratios.

It is significant that the correlations in Figs. 2 and 3 include a pointfor the Napali formation of Kauai, which is the only sequence ofHawaiian lavas other than Loihi known to have3He/4He ratios signif-icantly and consistently greater than 20 RA. Napali formation lavas arecharacterized by relatively thorogenic Pb (208Pb/204Pb ratio of 37.85-38.15, at a206Pb/204Pb ratio of 18.2–18.5; i.e., similar to Loihi; Park,1990) and are predicted by our model to have3He/4He ratios extendingto higher than any other Hawaiian shield-building lavas. The average3He/4He ratio for Kauai in Table 1 includes only the value of 246 3RA from Rison and Craig (1983), but recent analyses of3He/4He ratiosin Napali lavas have yielded values of 19–27 RA (N 5 11; Mukho-padhyay et al., 1996) and thus the value that we have used is repre-sentative. The existence of such high3He/4He ratios in lavas from theupper portions (i.e., above sea level) of a Hawaiian volcano arguesagainst the hypothesis that high-3He/4He volatiles are only sampled inthe pre-shield stage of volcanism (Valbrecht et al., 1996), but isconsistent with variable sampling of a high-3He/4He mantle source overthe lifetime of a Hawaiian volcano.

4. DISCUSSION

The main result of this study is that the isotopic compositionsof helium and lead covary in Hawaiian lavas, such that athree-component model of the sources of pre-shield and shield-building lavas can account simultaneously for their helium andlead isotope systematics. A similar result has previously beenshown to apply to strontium, neodymium, and oxygen isotopes(Eiler et al., 1996), and recent results suggest that it will beextended further to osmium isotopes (Bennett et al., 1996;

Hauri et al., 1996). This is strong evidence that helium is notdecoupled from the nonvolatile isotope systems, and it confirmsprevious inferences based upon correlations of helium andstrontium isotope ratios among individual suites of Hawaiianlavas (Kurz et al., 1987; 1996; Kurz and Kammer, 1991). Thedistinctively high 3He/4He ratios of Hawaiian lavas can beaccounted for by a single source component (the Loihi com-ponent; Table 2) that is also distinctive in its lead isotopecharacteristics. Previous investigators may have failed to rec-ognize the correlation between helium and lead isotopes de-scribed in this study because it is not generally possible todetect correlations among mixtures of three or more compo-nents using two-dimensional isotope covariation diagrams(e.g., 3He/4He vs. 206Pb/204Pb). We draw four principal con-clusions from these results:

1) The processes that have been proposed to decouple heliumfrom nonvolatile elements typically invoke a mobile fluidphase that is rich in helium relative to strontium, neody-mium, and lead (and, by extension, presumably relative tooxygen and osmium as well). Although we cannot rule outthe possibility that the distinctive, high-3He/4He Loihicomponent is a fluid of some sort, it is clear that heliumisotopes are not decoupled from the other isotopic systems;thus if it is a fluid, it must be rich in nonvolatile elementsas well as helium. It seems more plausible to us to supposethat, as is envisioned for the other Hawaiian source com-ponents, the Loihi component is a mafic or ultramafic rockvariably incorporated into the source regions of Hawaiianvolcanoes.

2) Poor correlations previously described between3He/4Heand87Sr/86Sr ratios have been ascribed to very high ratiosof helium to nonvolatile elements in high-3He/4He sourcesrelative to other mantle sources (i.e., such that mixingbetween high- and low-3He/4He sources is hyperbolic;Kurz et al., 1982; Alle`gre et al., 1987). However, ourcharacterization of the helium and lead isotope variationsin Hawaiian lavas argues against strongly hyperbolic mix-ing for these elements, and instead suggests that high-RA

and low-RA sources can have comparable He/Pb ratios.This result is apparent both from the approximately linearcovariation of helium and lead isotope compositionsshown in Fig. 2 and from the success of mixing modelsthat assume equal He/Pb ratios in all sources (Fig. 3; seealso Eiler et al., 1996). If mixing relationships amongisotopic end members reflect mixing of melts rather thanmixing of sources, it is possible that the apparently equalHe/Pb ratio of these components reflects a trade-off be-tween differences in the He/Pb ratios among sources and inthe fractionation of He/Pb during melting of those sources.However, if this were the case, this trade-off would have tobe sufficiently systematic to produce consistently the ap-pearance of approximately equal He/Pb among all threecomponents in nearly all pre-shield and shield-buildingsamples.

3) Coherent variations of3He/4He with time in some Hawai-ian volcanoes (e.g., Mauna Loa; Kurz and Kammer, 1991)reflect variations in the mixing proportions of the Loihicomponent and other sources within the region of meltingin the focused upwelling that feeds Hawaiian volcanoes.

Fig. 3. Comparison of measured3He/4He ratios in shield (lightshading) and pre-shield (dark shading) lavas vs. those predicted by athree-component mixing model using source components in Table 2.The proportion of each component in a given sample was calculatedbased only upon its208Pb/204Pb and206Pb/204Pb ratios. These propor-tions were then used with the helium isotope ratios of the componentsto calculate each sample’s expected3He/4He ratio. This mixing modelsuccessfully predicts the data to within 2.9 RA (mean deviation). Thepropagated uncertainty in the predicted3He/4He ratio based uponanalytical uncertainties in208Pb/204Pb is 2.6 RA. Unfilled, large boxesare average measured and predicted values for relatively well-definedand homogeneous suites, including: L (Loihi), aML (subaerial MaunaLoa), mML (submarine Mauna Loa), H (Haleakala), K (Koolau), andMK (HSDP Mauna Kea). These are fit to within a mean deviation of1.6 RA, illustrating the importance of uncertainty in lead isotope ratiosfor the fitting of individual samples.


Although our results do not constrain the mantle processesleading to these variations, they are consistent with recentmodels that interpret temporal variations in the isotopegeochemistry of Hawaiian lavas in terms of the motion ofthe edifice of each Hawaiian volcano with respect to azoned (or perhaps more complexly heterogeneous) Hawai-ian plume (e.g., Kurz et al., 1995; Hauri et al., 1996).

4) The relatively thorogenic lead of the Loihi componentindicates that it has a high time-integrated Th/U ratiorelative to the sources of Pacific MORB and the Keacomponent (Fig. 1; Table 2). The time integrated Th/Uratio of the Loihi component is comparable to that inferredfor the Koolau component (Table 2), although this char-acteristic may have different origins in these two compo-nents. High Th/U ratios are characteristic of crustal rocksand, when observed in ocean island lavas, have beeninterpreted as evidence of subducted sediments or basaltsin their mantle sources (Hart, 1988). This explanation isplausible in the case of the Koolau component, which isalso known to be high in La/Nb,d18O, and187Os/188Os (allindicative of crustally-derived sediments or crustal rocks;Roden et al., 1994; Eiler et al., 1996; Bennett et al., 1996;Hauri et al., 1996). There are no comparable indicators ofa crustal affinity for the Loihi component, and, further-more, crustal rocks and sediments are characteristicallylow in 3He/4He; recycled crustal rocks are therefore anunlikely source of high time-integrated Th/U of the Loihicomponent.

The estimated Th/U ratio of the bulk earth is also greaterthan that inferred for the sources of MORB (Galer andO’Nions, 1985; McDonough and Sun, 1995), and thus thehigh time-integrated Th/U ratio in the Loihi componentmight reflect its derivation from relatively primitive mantle(as is also inferred from the high3He/4He ratio of thiscomponent). The232Th/238U ratio of the Loihi component,based upon its208Pb*/206Pb* value and integrated over4.55 Ga, is 3.92: intermediate between those of the sourcesof Pacific MORB (3.7; Galer and O’Nions, 1985) andthose estimated for the primitive mantle (3.9–4.2; Galerand O’Nions, 1985; Alle`gre et al., 1986; McDonough andSun, 1995). Therefore, the lead isotope characteristics ofthe Loihi source could reflect mixing of relatively primi-tive mantle with the depleted, upper-mantle sources ofMORB. Such a process has been suggested previously toexplain the strontium and neodymium isotope compositionof high-3He/4He lavas (Zindler and Hart, 1986a; Alle`gre etal., 1987; Hart et al., 1992) and has been proposed as amechanism that supports the time-integrated Th/U ratio of3.7 inferred from the lead isotope composition of MORBdespite a value of 2.5 for this ratio in the modern MORBreservoir (Galer and O’Nions, 1985). An alternative expla-nation is that the time-integrated Th/U ratio of the Loihicomponent is derived from a primitive source through amulti-stage evolution, such as the two-stage models thatinterpret the sources of MORB as residual to melting of aprimitive source (e.g., DePaolo and Wasserburg, 1978;Jacobsen and Wasserburg, 1978). However, given that theTh/U ratio of garnet lherzolite decreases as melting pro-ceeds (LaTourette and Burnett, 1992; Beattie, 1993), sucha model for the Loihi component would require that its first

stage of melting (i.e., of the primitive mantle) occurredmore recently and/or to a lesser extent than the equivalentfirst stage in the production of the MORB source.

5. CONCLUSIONS

We have presented evidence that high3He/4He ratios inHawaiian shield-building and pre-shield lavas are well-corre-lated with their lead isotope ratios. Given the covariations instrontium, neodymium, lead, oxygen, and osmium isotopes inHawaiian shield-building lavas (Eiler et al., 1996; Bennett etal., 1996; Hauri et al., 1996), this constraint permits the defi-nition of the isotopic compositions of all these nonvolatileelements in the high-3He/4He Loihi source component in Ha-waii. The existence of correlations among the isotope ratios ofhelium and these nonvolatile elements indicates that processeswithin the zone of melting in the Hawaiian plume and withinthe volcanic edifice of the Hawaiian islands have not stronglydecoupled helium (and presumably other volatiles) from thenonvolatile elements. The high-3He/4He Loihi component inthe source of Hawaiian lavas has a higher time-integrated Th/Uratio than the sources of Pacific MORB, perhaps due to mixingwith or differentiation from primitive mantle.

The correlation between helium and lead isotopes observedfor Hawaiian shield-building lavas identified here has not, toour knowledge, been so clearly recognized elsewhere. Deter-mination of whether this result has significance beyond theHawaiian plume will probably require the definition of thenonvolatile element characteristics of high-3He/4He compo-nents in the sources of other ocean islands at a level of detailcomparable to that available for Hawaii. There are currently,however, currently insufficient published data to repeat thestatistical exercises we have performed for the Hawaiian dataon other hot spots at anything approaching the same level ofdetail. Existing data suggest, however, that there are severalocean islands where high3He/4He ratios are associated withmoderately thorogenic lead (i.e., similar to Loihi). These loca-tions might be fruitful places for future detailed work. The JuanFernandez islands, Galapagos islands, and Iceland are amongthe most promising of ocean islands in this respect (Shilling,1973, Sun et al., 1975; Kurz et al., 1982; Condomines et al.,1983; Gerlach et al., 1986; Poreda, 1986; Farley et al., 1993;Graham et al., 1993; White et al., 1993). Samoa and Heardisland also contain high-3He/4He lavas with relatively thoro-genic lead, but the sources of these islands are known to haveone or more enriched components (i.e., like the Koolau com-ponent in Hawaii) that may make more difficult any efforts tolink high 3He/4He ratios to high time-integrated Th/U ratios(Barling and Goldstein, 1990; Farley et al., 1992; Hilton et al.,1995).

Acknowledgments—We thank Don Porcelli and Don Anderson for theinsights they provided in discussion of this work. This paper wasimproved by helpful and detailed reviews by Mark Kurz and ananonymous reviewer. This work was funded by NSF grant EAR-9628142 to KAF and EMS and DOE grant DEFG03-85ER13445 toEMS.

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