Correlated helium and lead isotope variations in Hawaiian lavas

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<ul><li><p>PII S0016-7037(98)00113-6</p><p>Correlated helium and lead isotope variations in Hawaiian lavas</p><p>JOHN M. EILER, KENNETH A. FARLEY, and EDWARD M. STOLPER</p><p>Division of Geologic and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA</p><p>(Received July 7, 1997; accepted in revised form February 13, 1998)</p><p>AbstractVariations in 3He/4He ratios among Hawaiian shield-building and pre-shield basalts are correlatedwith variations in 208Pb/204Pb and 206Pb/204Pb ratios. Using this correlation, the 3He/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 high 3He/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</p><p>1. INTRODUCTION</p><p>Basalts with 3He/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).</p><p>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 of 87Sr/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 and 206Pb/204Pb), EM2 (high 87Sr/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 in 87Sr/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).</p><p>However, correlations of strontium, neodymium, and lead</p><p>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 in 3He/4He ratio can beaccompanied by negligible variations in some nonvolatile-ele-ment isotope ratios (e.g., 3He/4He ranges between 8 and 32 RAamong Hawaiian lavas with 87Sr/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 accompanying 4Heingrowth 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 of 3He/4He vs.other isotope ratios are highly curved (Kurz et al., 1982; Al-le`gre et al., 1987). It is also possible that mixing of olivinephenocrysts (a commonly analyzed material in 3He/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.</p><p>The question of whether helium is coupled with nonvolatileelements during basalt petrogenesis is particularly significant inthe interpretation of correlations between 3He/4He ratios and</p><p>PergamonGeochimica et Cosmochimica Acta, Vol. 62, No. 11, pp. 19771984, 1998</p><p>Copyright 1998 Elsevier Science LtdPrinted in the USA. All rights reserved</p><p>0016-7037/98 $19.00 1 .00</p><p>1977</p></li><li><p>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).</p><p>Here we present data compiled from literature sources thatshow correlations between helium and lead isotope ratios inHawaiian lavas. These correlations indicate that high 3He/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.</p><p>2. DATA FOR HAWAIIAN LAVAS</p><p>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.</p><p>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 radiogenic 4He and/or production of cosmogenic 3He (e.g.,Kurz et al., 1996). Reported precision for 3He/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, averaging 60.8 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</p><p>1978 J. M. Eiler, K. A. Farley, and E. M. Stolper</p></li><li><p>measurements are 60.016 for 206Pb/204Pb and 60.054 for 208Pb/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.</p><p>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 ;12 Ma after shield building;Clague, 1987). We consider only the first two of these stages in ouranalysis. Post-shield lavas are virtually invariant in their 3He/4He ratios(averaging 8.0 6 0.2 R9AIT). 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 of 3He/4He ratios (average:8.5 6 0.4 R9AIT). 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.</p><p>3. LEAD-HELIUM ISOTOPE SYSTEMATICS OFHAWAIIAN LAVAS</p><p>The principal difficulty in associating a distinctive nonvolatile-ele-ment isotopic signature with the sources of high 3He/4He ratios inHawaiian lavas is that lavas from Loihi (the high extreme in 3He/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</p><p>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 samples location in Fig. 1 is predictive of its 3He/4Heratio. We have demonstrated such a correlation in the following ways:</p><p>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 constant y2 in Fig. 1. For a particular sample, y2 is proportionalto the deviation in 208Pb/204Pb from the best fit trend of the Hawaiianarray at the samples 206Pb/204Pb, and thus Loihi lavas, with theirthorogenic Pb, all have high values of this index. This index is con-ceptually similar to D208 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 of 3He/4He ratios and values of y2 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 each y2 score is 60.6 (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 of 206Pb/204Pb vs. 208Pb/204Pb (Fig. 1) and lie on an extrapo-lation of the lead-isotope mass fractionation line from other Mauna Loadata.</p><p>Eiler et al. (1996) presented a mixing model for Hawaiian shield-</p><p>Fig. 1. Plot of 208Pb/204Pb vs. 206Pb/204Pb for Hawaiian lavas. Data for Loihi are enclosed in a shaded field. Parallel linesmarked y2 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 of y2 can be calculated as follows: y2 5 208Pb/204Pb z 12.435- 206Pb/204Pb z 7.351- 337.768.</p><p>1979Helium and lead in Hawaiian lavas</p></li><li><p>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 elevated 3He/4He ratios, moderate 206Pb/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 by 3He/4He ratios approx-imately equ...</p></li></ul>