Hf–Nd isotope decoupling in the oceanic lithosphere: constraints from spinel peridotites from...

Hf^Nd isotope decoupling in the oceanic lithosphere:constraints from spinel peridotites from Oahu, Hawaii§

Michael Bizimis a;b;�, Gautam Sen a, Vincent J.M. Salters b

a Department of Earth Sciences, Florida International University, Miami, FL 33199, USAb National High Magnetic Field Laboratory, Isotope Geochemistry, and Department of Geological Sciences, Florida State University,

1800 E. Paul Dirac Drive, Tallahassee, FL 32306, USA

Received 14 May 2003; received in revised form 1 October 2003; accepted 17 October 2003

Abstract

We present a detailed geochemical investigation on the Hf, Nd and Sr isotope compositions and trace and majorelement contents of clinopyroxene mineral separates from spinel lherzolite xenoliths from the island of Oahu, Hawaii.These peridotites are believed to represent the depleted oceanic lithosphere beneath Oahu, which is a residue of aMORB-related melting event some 80^100 Ma ago at a mid-ocean ridge. Clinopyroxenes from peridotites from theSalt Lake Crater (SLC) show a large range of Hf isotopic compositions, from OHf = 12.2 (similar to the Honoluluvolcanics series) to extremely radiogenic, OHf = 65, at nearly constant 143Nd/144Nd ratios (ONd = 7^8). None of thesesamples show any isotopic evidence for interaction with Koolau-type melts. A single xenolith from the Pali vent is theonly sample with Hf and Nd isotopic compositions that falls within the MORB field. The Hf isotopes correlatepositively with the degree of depletion in the clinopyroxene (e.g. increasing Mg#, Cr#, decreasing Ti and heavy REEcontents), but also with increasing Zr and Hf depletions relative to the adjacent REE in a compatibility diagram. TheLu/Hf isotope systematics of the SLC clinopyroxenes define apparent ages of 500 Ma or older and these compositionscannot be explained by mixing between any type of Hawaiian melts and the depleted Pacific lithosphere.Metasomatism of an ancient (e.g. 1 Ga or older) depleted peridotite protolith can, in principle, explain these apparentages and the Nd^Hf isotope decoupling, but requires that the most depleted samples were subject to the least amountof metasomatism. Alternatively, the combined isotope, trace and major element compositions of these clinopyroxenesare best described by metasomatism of the 80^100 Ma depleted oceanic lithosphere by melts products of extensivemantle^melt interaction between Honolulu Volcanics-type melts and the depleted lithosphere.C 2003 Elsevier B.V. All rights reserved.

Keywords: Hawaiian plume; xenoliths; spinel peridotite; hafnium; isotopes

1. Introduction

The Emperor^Hawaiian volcanic chain isthought to represent the surface expression of amantle plume that has erupted through the Paci¢clithosphere over the last V80 Myr. The locationof the Hawaiian islands, in the middle of the Pa-

0012-821X / 03 / $ ^ see front matter C 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0012-821X(03)00598-3

* Corresponding author. Tel. : +1-850-644-2263;Fax: +1-850-644-0827.E-mail address: [email protected] (M. Bizimis).

§ Supplementary data associated with this article can befound at doi:10.1016/S0012-821X(03)00598-3

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Earth and Planetary Science Letters 217 (2003) 43^58

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ci¢c oceanic plate and away from the continents,provides a well-suited tectonic setting for docu-menting and understanding the behavior of man-tle plumes. Consequently, a great amount of re-search has focused on the chemical compositionof the Hawaiian volcanoes. A rare feature of Ha-waiian volcanism is the abundance of mantle xe-noliths (most recovered from the island of Oahu)brought to the surface by the post-erosional alkalilavas of the rejuvenated stage of the Hawaiianvolcanism [1^3]. While erupting lavas usually rep-resent an ‘average’ composition of melts that arepooled and mixed from di¡erent depths, these xe-noliths provide a ‘snapshot’ of the mantle beneathOahu, and give a unique insight into the processes(e.g. melting, metasomatism, melt migration) thattake place within the mantle. As such, they pro-vide an invaluable addition to the studies of lavasas well as geophysical studies, in constraining thegeochemical and geophysical dynamics of plume^lithosphere interaction beneath Hawaii. Previousstudies on the Oahu xenoliths have already placedconstraints on the melting rates of mid-oceanridge basalts (MORBs) [4], metasomatism in theoceanic lithosphere [5^8] and generation of Ha-waiian volcanism [9^12].The mantle xenoliths from the Honolulu Vol-

canics (HV) in Oahu have been generally classi¢edas dunite, peridotite, and pyroxenite suites [1,3,9,12,13]. Spinel lherzolites are the most abundantxenolith type and are thought to represent frag-ments of the oceanic lithosphere formed some80^100 Ma ago at a mid-oceanic ridge [4,14], andwere subsequently metasomatized by alkalic meltsassociated with the Hawaiian plume [8]. The sim-ilarity in Nd and Sr isotopes between the spinelperidotites and the HV lavas [7,15,16] and theirgenerally enriched light rare earth element (REE)contents [8] have been cited as evidence favoring agenetic relationship between the two. The Hf iso-tope study by Salters and Zindler [6] howevershowed that the spinel peridotites can only berelated to the HV through a more complex pro-cess involving mantle^melt interaction betweenHV and the MORB-depleted lithosphere. Thesedata also left open the possibility that these peri-dotites could be ancient (s 1 Ga old) fragmentsof a recycled lithosphere (the depleted peridotite

portion) that was either brought up by the plumefrom the deep mantle or rafted within the asthe-nospheric upper mantle. Transfer of materialfrom the lower mantle by the Hawaiian plumehas recently been restated by the discovery of ma-jorite garnets in garnet pyroxenites [17] and theunradiogenic 187Os/188Os compositions of someSalt Lake Crater (SLC) spinel peridotites thathave been interpreted as ancient recycled litho-spheric mantle [18].Here, we present the ¢rst comprehensive study

on the major and trace elements contents, Hf, Ndand Sr isotopic compositions of clinopyroxenemineral separates from spinel lherzolites fromSLC, Pali and Kaau vents from the island ofOahu, Hawaii. Our results show signi¢cant decou-pling of Hf from Nd isotopes in the oceanic lith-osphere beneath Oahu, towards extremely radio-genic Hf compositions at nearly constant Nd. Thecorrelations between Hf isotopes and certain traceand major elements allow us to better constrainthe origin of this extreme isotope decoupling, andits implications for plume^lithosphere interactionand melt migration in the context of the Hawaiianvolcanism.

2. Samples and analytical techniques

All samples examined in this study are spinellherzolites (7^18% modal clinopyroxene) fromthe personal collection of D.C. Presnall, nowhoused at Florida International University (FIU).Eight samples (pre¢x 77SLC) are from the SLCvent, one from Pali (Pa27) and one from Kaau(KAPS 36) vents. Clinopyroxene (cpx) has thehighest concentration of incompatible trace ele-ments in peridotites [19], and in the case of xeno-liths which are often impregnated by the lava car-rier, cpx separates provide the most unambiguousevidence for investigating the isotope systematicsof mantle peridotites [6,20]. Major element anal-yses on the cpxs were performed at the FCAEM,FIU, using standard procedures [17]. The concen-trations are reported in Table 11 and are the aver-

1 See online version of this article.


M. Bizimis et al. / Earth and Planetary Science Letters 217 (2003) 43^5844

age of 10^20 points per sample. All other samplepreparation and analyses were performed at theNHMFL/Florida State University. 200^300 mg ofoptically pure cpx was handpicked under a binoc-ular microscope and cleaned following establishedprocedures [21]. For internal consistency, all sub-sequent analyses were performed on splits fromthe same batch of acid-leached cpx. Trace elementconcentrations were determined by solution ICP-MS [22] using USGS basalt BIR-1 as an externalstandard and the concentrations reported by [23]and [24] for Zr, Hf, Nb, Ta. The trace elementconcentrations of the cpx separates are reportedin Table 1 in the supplementary material, alongwith repeated measurements of the BHVO-1 andBCR-1 basalt standards for comparison. The Hfisotope compositions were determined with theHOT-SIMS technique [6,22,25,26] using the sepa-ration chemistry from [27]. Lu/Hf ratios were de-termined on a 10^15 mg cpx split spiked with175Lu and 179Hf [22], and they agree to betterthan 5% with the ICP-MS measurements, exceptfor sample 341. Nd and Sr isotope compositionswere determined either on a 20^40 mg cpx split,or from the fraction containing the bulk samplewashed out from the ¢rst Hf column. In this casethe sample was treated with 1 ml HClO4 to oxi-dize the ascorbic acid used in the separation, andwas further processed with conventional tech-

niques. Further analytical details are given in Ta-ble 2.

3. Results

3.1. Major and trace element compositions

Previous studies have shown that spinel perido-tites from the Kaau, Pali and Kalihi (KPK) vents(proximal to the Koolau shield caldera) and SLC(a distal vent) are distinct in terms of Mg# andNa2O but similar in Cr# [3,12]. Both suites areless depleted relatively to abyssal peridotites interms of Mg#; however, their Cr#, which is usu-ally taken as an indicator of relative depletion,overlap. We note that within the SLC and KPKperidotite suites the Mg# and Cr# of cpx andcoexisting orthopyroxene (opx) and spinel corre-late well [12], suggesting that melt depletion is thedominant mechanism for the major element var-iation in these cpxs. Our new data show excellentcorrelation between Mg# and Cr# but scatter in aNa2O vs. Cr# plot (Fig. 1a,b). The samples fromKPK have lower Cr# and Na2O contents than theSLC suggesting lower degrees of depletion, withsome falling within the abyssal peridotite ¢eld.Interestingly, some of our SLC samples fall withinthe KPK ¢eld. One SLC sample (470, Fig. 1) is

Table 2Sr, Nd and Hf isotope compositions and Lu/Hf ratios of cpx

Sample 87Sr/86Sr 143Nd/144Nd ONd176Hf/177Hf OHf

176Lu/177Hf

KAPS 36 0.703219U 12 0.513064U 15 8.3 0.283118U 20 12.2 0.0449KAPS 36 (d) 0.703229U 12 0.513070U 13 8.4 nd ndPa27 0.703234U 9 0.513180U 8 10.6 0.283319U 23 19.3 0.044277SLC326 0.703422U 15 0.513066U 10 8.3 0.284630U 16 65.7 0.079377SLC328 0.703294U 9 0.513036U 10 7.8 0.283207U 10 15.4 0.011577SLC341 0.703249U 7 0.513035U 6 7.7 0.283472U 19 24.8 0.029177SLC466 0.703285U 7 0.513034U 8 7.7 0.283329U 15 19.7 0.022977SLC470 0.703290U 8 0.513019U 10 7.4 0.284409U 27 57.9 0.047477SLC319 0.703230U 8 0.513049U 13 8.0 0.283326U 13 19.6 0.014777SLC320 0.703302U 8 0.513051U 10 8.1 nd nd77SLC405 0.703337U 10 0.513045U 14 7.9 0.283528U 29 26.7 0.0387

Errors on measurements are 2cO. ONd is calculated with 143Nd/144NdCHUR =0.512638 and OHf with 176Hf/177Hf= 0.282772 forpresent-day chondritic earth. Sr measurements are reported relative to the EpA Sr standard of 0.708003U 15 (2 S.D., n=10) andNd relative to LaJolla= 0.511847U 14 (2 S.D., n=11). The JMC 475 Hf standard was measured at 0.282199U 22 (2 S.D., n=15),and the values are reported relative to the accepted value of 0.28216. Hf blank 6 20 pg. nd: not determined, (d): duplicate, in-cluding mineral picking and dissolution.


M. Bizimis et al. / Earth and Planetary Science Letters 217 (2003) 43^58 45

more depleted than both KPK and most abyssalperidotites in terms of Cr# and Mg#. Based onthe elevated Na contents of the SLC peridotites(SLCPs), Sen [12] suggested that the majority ofthe Oahu lherzolites are not simple residues ofMORB melting but have been metasomatized bymelts associated with the Hawaiian volcanism.The cpx chondrite-normalized REE patterns

are shown in Fig. 2a. All SLCP cpxs from thisstudy have characteristic LREE enrichments withconvex-upward REE patterns with samples 326and 470 having more £at middle to heavy REE

slopes that the other SLCPs. The Kaau and Palisamples have characteristic ‘spoon-shaped’ REEpatterns, (not observed in the SLCPs) and gener-ally higher HREE contents than the SLCP sam-ples, in agreement with previous data [8]. Thevariable and enriched LREE patterns have beenpreviously explained as the result of metasoma-tism of the sub-Oahu lithosphere by post-erosion-al melts, with the KPK and SLC vents samplingportions of the lithosphere with di¡erent metaso-matic histories [8,12].The extended trace element patterns of the cpx

separates are shown in Fig. 2b. Most sampleshave characteristic Hf, Zr and Ti depletions, sim-ilar to other SLC samples [6] and other peridotitesworld-wide [28]. Nb (and Ta for the Kaau andPali samples) is also strongly depleted relative toU and La and the Nb/Ta ratios are sub-chondritic(3.2^11, except the Kaau sample with Nb/Ta=45). The lack of correlation between Nb (andTa) and Ti depletions (not shown) rules out theinvolvement of titanite or ilmenite as the source ofthese depletions.

Fig. 1. (a) Mg# (Mg/Mg+FeT) vs. Cr# (Cr/Cr+Al) and (b)Cr# vs. Na2O in cpxs from this study (symbols). Fields forabyssal peridotites (AP) from [19], SLC and KPK spinelperidotite ¢elds from [12,49,50]. Sample 470 is pointed forclarity.

Fig. 2. (a) Chondrite-normalized concentrations of cpx(chondritic values from [51]). (b) Primitive mantle-normalizedconcentrations of cpx (normalization values from [52]).



3.2. Sr, Nd and Hf isotope compositions

The Sr, Nd and Hf isotopic compositions and176Lu/177Hf ratios of the cpx separates are re-ported in Table 2. The Nd and Sr isotope compo-sitions of the SLC and Kaau samples fall withinthe range of previously published data for SLCPs[7,11,16] and within the range of the HV, at theisotopically depleted end of the Hawaiian lavas(Fig. 3). Both the SLCPs and HV lavas form anear-horizontal array in Nd^Sr isotope space,with almost constant 143Nd/144Nd ratios, similar

to the least radiogenic Paci¢c MORBs and theyoung Paci¢c seamounts [29], but shifted tomore radiogenic Sr compositions. The Pali samplehas a more radiogenic 143Nd/144Nd ratio than theSLCPs and HV, similar to the plagioclase perido-tites from Pali [14], and as radiogenic as the mostradiogenic Paci¢c MORB samples (Fig. 3). ThePali samples (both with and without plagioclase)show much larger variation in Nd isotopes thanthe SLCPs supporting the hypothesis that the twovents have sampled di¡erent portions of the lith-osphere [8,12].The Nd^Hf isotope compositions of the cpx

separates are plotted in Fig. 4. The SLC (andKaau) samples form a vertical array in Nd^Hfspace from HV-like to extremely radiogenic176Hf/177Hf ratios, at nearly constant 143Nd/144Nd. Samples 470 and 326 fall within the dataof [6], con¢rming the existence of extremely radio-genic Hf in the oceanic lithosphere beneath Oahu.Our new data however show that SLC lherzoliteshave a continuum of Hf isotope compositions,from similar to the HV to extremely radiogenic.

Fig. 3. Nd vs. Sr isotope compositions of cpxs. (a) Symbolsas in Fig. 1. Circles: SLC xenolith data from [7,11,16]. Tri-angles: Pali peridotite data from [14]; the cpxs from plagio-clase peridotites are pointed separately. HV ¢eld is shownfor comparison. (b) Comparison of the SLC and Pali perido-tite ¢elds with the Paci¢c MORB and Paci¢c Seamount dataand the rest of the Hawaiian lava ¢eld. Names along the Ha-waii ¢eld show the approximate locus of the data from otherHawaiian volcanoes. Note two peridotite samples plottingnear the Koolau ¢eld [7,16]. All basalt data from the compi-lation in [53].

Fig. 4. Hf vs. Nd isotope compositions of cpx. Symbols asin Fig. 1. Errors are smaller than the symbol size. Samples326 and 470 with the most radiogenic Hf are labeled forclarity. Crosses with error bars from [6]. MORB and OIB¢elds as in Fig. 3. The HV, Koolau and the rest of Hawaiilava ¢elds are shown for comparison. Note the steeper slopeof the HV ¢eld relative to the slope of the mantle array andthe Hawaiian lavas (see text for discussion).



Such an extreme decoupling of Hf from Nd (andSr) isotopes in the sub-Oahu lithosphere (repre-sented by these xenoliths) has not been observedin any terrestrial magma sources, where Hf andNd isotopes are well correlated. Only one of the11 available Hf isotope analyses of the SLCPsfalls within the 176Hf/177Hf range of HV [30],while eight out of the 11 samples have more ra-diogenic 176Hf/177Hf than any known MORB.The Pali sample is the only sample falling withinthe MORB ¢eld, suggesting that it may representlargely unmetasomatized oceanic lithosphere. TheKaau sample has a lower 176Hf/177Hf ratio (for agiven Nd) than both the Pali and SLC peridotitesand the HVs, but still with the range of Hawaiianlavas and MORB.Although the data are still limited, they show

that peridotites with highly radiogenic Hf compo-sitions are the rule rather than the exception inthe sub-Oahu lithosphere. As such, this portion ofthe oceanic mantle lithosphere cannot signi¢-cantly contribute to any type of Hawaiian volca-nism. At the same time, these cpxs could not haveequilibrated with HV or any other known Hawai-ian magma.

3.3. Lu/Hf systematics

The 176Lu/177Hf vs. 176Hf/177Hf compositions ofthe cpxs are shown in Fig. 5a and are comparedwith the average HV and Koolau lavas (essen-tially covering the total 176Hf/177Hf range of Ha-waiian lavas) and the calculated present-day Hfisotope composition of the variably depleted 100Ma old mantle lithosphere beneath Oahu (see Sec-tion 4.1.2 and Appendix1 for model details). TheSLCPs have more radiogenic Hf isotope compo-sitions and lower Lu/Hf ratios than those ex-pected for a 100 Ma depleted peridotite residueof MORB melting (i.e. the lithosphere beneathOahu). The six SLCPs with the least radiogenic176Hf/177Hf (¢ve from this study and one from [6])de¢ne a 500 Ma ‘errorchron’, while the more ra-diogenic samples de¢ne much older ages (1^2.8Ga, assuming that the least and more radiogenicsamples are co-genetic). The Kaau and Pali sam-ples fall below this trend with lower 176Hf/177Hfratios (for a given Lu/Hf ratio) than the SLCPs,

and can be interpreted as largely unmodi¢edMORB residues. These ancient ages de¢ned bythe SLCPs by the Lu/Hf isotopic system aremuch older than the ages previously determined

Fig. 5. Lu/Hf systematics in cpx. Symbols as in Fig. 4 withsamples 326, 470, Pali (P) and Kaau (K) labeled for clarity.Error bars for the [6] data assume 10% error on the Lu/Hfratio. HV : average HV (Table 3), Koolau : average from theliterature, with vertical error bars showing their ranges in176Hf/177Hf. 100 Ma RMC : present-day composition of cpxresidue of variable degrees of MORB melting 100 Ma agowith numbers along the curve depicting degree of melting.(a) 1 Ga depleted peridotite : present-day composition of cpxresidue of variable degrees of MORB melting at 1 Ga ago.‘500 Ma’ line is the regression line (R2 = 0.94) through thesix SLCPs with the lowest 176Hf/177Hf (see text for discus-sion). (b) Composition of MMI melts with tick marks alongsolid line curves showing percentage of melt mass remaining.MMI 312% residue : MMI melts, products of the MMI reac-tion between an AOB melt and 12% melt-depleted 100 Maold peridotite used for modeling sample 326 (see text andTable 3). The slope of this curve corresponds to a 1.05 Gaage. Note the change of the slope as the melt mass decreases.MMI 317% residue : MMI melts, products of the reactionbetween the AOB melt and a 17% melt-depleted 100 Ma oldperidotite. The more depleted the peridotite reactant, thesteeper the slope of the MMI melts.



on SLCPs (80 Ma: U/Pb in cpx [7]) and Paliperidotites (62 Ma, Sm/Nd in cpx [14]). Notethat at least eight out of these 11 SLCPs cannotbe products of simple metasomatism (i.e. mixing),between a 100 Ma lithosphere residue of MORBmelting and an HV (or any other Hawaiian) mag-ma. This is because any line connecting these sam-ples with an HV or even Koolau melt (i.e. mixingline) has a slope greater than a 100 Ma and itdoes not intersect the depleted 100 Ma old litho-sphere.

3.4. Correlations between isotope and trace andmajor elements

Fig. 6 shows some correlations between Hf iso-topes and selected major and trace elements. Thesamples with the most radiogenic 176Hf/177Hf tendto have the highest Mg# (and Cr#, not shown),the lowest Ti, Zr contents, the largest Zr deple-tions and the highest Lu/Hf ratios (Fig. 5a). Qual-itatively, these correlations are compatible withthese peridotites being residues of an earlier de-

Fig. 6. OHf vs. (a) Mg#, (b) Zr/Zr*, (c) Nd/YbðnÞ, (d) Ti, (e) Zr and (f) Yb concentrations in cpx. Symbols as in Fig. 4. Samples326 and 470 are pointed in panel a for clarity (similar in the other panels). Zr/Zr* is de¢ned as in [22]. (n) = chondrite-normalizedconcentrations. Dashed lines show the present-day composition of cpx residue of MORB melting at 100 Ma ago, at increasingdegrees of depletion. Tick marks with numbers are % of melting. Dashed arrow in panel c (similar in the other panels) points to-wards the direction of increasing degree of depletion. Note in panels a, b, d, f that Hf isotopes correlate with increasing degreeof depletion. Solid lines show the composition of a selected MMI model (used to model sample 326, see Table 3 and text for de-tails), with tick marks showing percentage of remaining melt mass at 10% increments. Solid arrow in panel c points to the direc-tion of decreasing melt mass during MMI (similar in the other panels).



pletion event: since Hf is more incompatible thanLu in the peridotitic mineralogy then, with pro-gressive melting, the Lu/Hf ratio of the residueincreases (together with increasing Mg# andCr# and decreasing Ti, Zr and HREE contents),which leads to radiogenic 176Hf/177Hf ratios overtime. However, melting alone cannot explain thehigh absolute trace element contents and enrichedtrace element patterns of these cpxs (e.g. high Zrcontents in Fig. 6e, high Nd/Yb ratios in Fig. 6c).No signi¢cant correlations are observed betweenNd and Sr isotopes and trace or major elementsmostly due to the limited Nd and Sr isotopic var-iation of these samples.

4. Discussion

Before examining in detail the origin of thechemical and isotope characteristics of thesespinel lherzolites and their implications, we sum-marize some of the constraints directly imposedby the data so far.1. Based on the Lu/Hf isotope systematics (Fig.5a) and the enriched trace element patterns(Fig. 2), the SLCPs do not represent an un-modi¢ed MORB-depleted lithosphere formednear a paleo-ridge some 80^100 Ma ago.

2. The SLCPs are not products of simple mixingbetween HV (or any other Hawaiian) melts andan 80^100 Ma depleted lithosphere (Fig. 5a).

3. The SLCPs can be interpreted as ancient(s 500 Ma old) melt residues that have beenrecently metasomatized by HV-type melts (Fig.5a). This would imply that at least some of theSLCPs could be fragments of an ancient de-pleted lithosphere, either brought up by theHawaiian plume as recycled material or‘rafted’ within the convective asthenosphere,and was accidentally sampled by the post-ero-sional lavas.

4. The ¢nal model needs to explain the extremerange of Hf isotopes at nearly constant Ndisotopes (Fig. 4), the correlation between Hfisotopes and with the degree of depletion(Fig. 6), as well as the LREE element enrich-ments and the relative Zr, Hf and Ti depletions(Figs. 2 and 6).

4.1. Origin of the SLC peridotites

Salters and Zindler [6] presented two di¡erentmodels to explain the decoupling of Hf from Ndisotopes, the LREE enrichments and the Ti, Zrand Hf depletions of the SLCPs: (1) recent meta-somatism of an ancient depleted peridotite by HVmelts, or (2) mantle^melt interaction between anascending HV-type melt and depleted 100 Ma oldlithosphere. Our new and more complete data seton the SLC and Pali and Kaau peridotites makesit possible to reexamine these models and betterconstrain the evolution of the lithosphere beneathOahu and the processes of plume^lithosphere in-teraction.

4.1.1. Ancient melt depletion followed by recentmetasomatism

The success of this model lies on the long-termstorage (s 1 Ga) of a depleted peridotite proto-lith somewhere in the sublithospheric mantle,which allows for the development of extreme Hf(and Nd) isotopic compositions [6]. This depletedperidotite is subsequently metasomatized by a Hf-, Zr- and Ti-depleted melt with ‘normal’ Hf andNd isotope compositions, i.e. from within themantle array in Nd^Hf isotope space [6]. Thismodel requires that either the depleted protolithhas remained chemically isolated in a less transi-ent part of the mantle (e.g. mid-lower mantle) andthen brought to the upper mantle by the plume orlarge-scale mantle convection, or that it actually‘£oated’ in the upper mantle and was accidentallybrought up by the post-erosional lavas at Oahu.The recent report of some SLCPs with highlyunradiogenic Os compositions [18] suggests thatindeed some of these xenoliths may representancient (1^2 Ga old) melt-depleted residues. Sim-ilar unradiogenic 187Os/188Os ratios have also beenreported in harzburgites from the Izu^Bonin fore-arc and abyssal peridotites from the Kane frac-ture zone and have been explained as blocks ofancient depleted lithosphere that resisted mixingback into the convective mantle [31,32].The Lu/Hf systematics of the SLCPs deter-

mined here agree, in principle, with the recentmetasomatism of an ancient depleted peridotiteby an HV melt. The 500 Ma errorchron de¢ned



by six of the SLCPs (Fig. 5) can be interpreted,for example, as a mixing line between a 1 Ga olddepleted protolith and an average HV melt whilethe more radiogenic samples can be explained asmixtures of HV and older (2^3 Ga) depleted peri-dotite.Fig. 7a shows the Nd^Hf isotope systematics of

a 1 Ga old depleted peridotite (residue of variabledegrees of melting) that has been metasomatizedby an HV melt. This metasomatism results in sig-ni¢cant decoupling between Nd and Hf isotopestowards radiogenic 176Hf/177Hf, similar to theSLCP compositions, because of the greater Nd/Hf ratio in the melt relative to that of a depletedperidotite. The mixing lines however are concaveupwards and do not generate the vertical distri-bution of the SLCPs in Nd^Hf space (Fig. 7a).Furthermore, the variable degrees of depletionobserved in the SLCPs (Fig. 1a) require a variablydepleted protolith. In order then to generate thevertical distribution of the SLCPs in Nd^Hf spaceby metasomatizing an ancient and variably de-pleted peridotite, the least depleted peridotitemust have had the largest melt addition, and themost depleted peridotite the least melt addition.We are not aware of any mechanism that canexplain such a fortuitous metasomatic scenario.Also, such a metasomatic process cannot generatethe characteristic Ti, Zr and Hf depletions in thesecpxs, unless the metasomatic melt was alreadysigni¢cantly depleted in these elements. Becauseof that, Salters and Zindler [6] proposed that asuch metasomatic agent must resemble a carbona-tite. Carbonatites however have very strong Nband Ba enrichments [22], opposite to the patternsobserved in these cpxs (Fig. 2b). Based on theseobservations we suggest that the combined iso-tope and trace element characteristics of theSLCP cpx cannot be explained by recent metaso-matism of an ancient depleted protolith. Instead amore complex mantle^melt interaction process isrequired.

4.1.2. Mantle-melt interaction (MMI)The mantle lithosphere beneath Oahu was

formed some 80^100 Ma ago as a residue ofMORB melting at a paleo-ridge. Salters and Zin-dler [6] estimated the present-day Nd and Hf iso-

topic composition and trace element concentra-tions of this 80^100 Ma old lithosphere from thecpx compositions of abyssal peridotites, which arethought to represent residues of MORB melting[19,33,34]. Here we take a more general approachand calculate the composition of such depleted100 Ma old lithosphere using a forward incremen-tal melting model [35] where a peridotite withMORB source composition melts at the ridgesome 100 Ma ago and is allowed to age to thepresent day. The lithosphere residue of MORBmelting is expected to be vertically strati¢edwith respect to the degree of depletion, i.e. morefertile at depth and more depleted at shallowerlevels [4,36]. We therefore calculate the trace ele-ment and isotopic composition of such a strati¢edresidual mantle column (RMC [36]) residue ofvariable degrees of near fractional melting, thatrepresents the composition of the lithosphere be-neath Oahu, before the interaction with melts as-sociated with the Hawaiian plume. Since Hf andNd are more incompatible than Lu and Sm in theperidotitic mantle, increasing degrees of meltingproduce a residue with progressively higher Lu/Hf and Sm/Nd ratios and, in time, radiogenicNd and Hf isotope compositions that fall at theextension and above the mantle array in Nd^Hfspace (Fig. 7b). In principle then, the 100 Ma oldMORB-depleted lithosphere beneath Oahu can bethe source of the radiogenic 176Hf/177Hf observedin the SLCPs, but not the source for their Ndisotopes since it will be too radiogenic in 143Nd/144Nd compared to the SLCPs.We envision an HV-type melt that ascends

through the lithosphere and reacts at di¡erentlevels with the variably depleted RMC peridotite(Table 3). The choice of an HV melt is justi¢ed bythe similarity of the Nd and Sr isotope composi-tions between HV and SLCPs and the fact thatthe HV carry these xenoliths to the surface. Weuse the assimilation^fractional crystallization(AFC) equations from [37] (equations 6a and15a) to model this MMI process, assuming thatduring the reaction the melt consumes cpx andcrystallizes opx [38^40], at decreasing melt mass(i.e. mass of crystallizationsmass of assimila-tion). We assume that 2^3% of modal cpx is dis-solved and an equal amount of opx is crystallized.



This reaction creates the largest decoupling of Hffrom Nd isotopes while generating relative Hf, Zrand Ti depletions in the emerging melts. We as-sume that a single pulse of melt impregnates aperidotite and we calculate the composition ofthe exiting melts at di¡erent amounts of remain-ing melt mass. This explicitly translates to a var-iable rock/melt ratio, or, in turn, to a variableporosity. For example, for a mass of assimila-tion/mass of crystallization ratio of 0.99, anMMI melt emerging through the reaction at20% of its original mass is equivalent to a reactionwith a melt/rock ratio of 80, or 1.5% volume po-rosity in the peridotite.The target of this model is to generate the Hf

and Nd isotope and the trace element composi-tions of samples 326 and 470 (highest 176Hf/177Hfand largest Ti, Zr and Hf depletions), and sample466 (highest LREE contents, large Ti, Zr, Hf de-pletions and ‘normal’ Hf isotopes) thus coveringthe full range of these SLCP cpx compositions.Our numerical tests show that a peridotite residueof 7^14% melting best reproduces the target sam-ples. Such degrees of depletion are considered rea-sonable because of the similarity in Mg# and Cr#between the SLCPs and abyssal peridotites (Fig.1a). These compositions ‘work best’ because sucha peridotite will develop over 100 Myr highly ra-diogenic 176Hf/177Hf ratios, while still retainingenough Hf in the residue to leverage against theHf content of the melt. We assume that the meltemerging from the MMI process metasomatizes(through bulk addition) a peridotite of the samecomposition as the reactant peridotite, by adding3^5% of this melt to the peridotite. The traceelement contents and Nd and Hf isotope compo-sitions of the metasomatic cpx are calculated bymass balance with the other mineral phases basedon the respective partition coe⁄cients (Table 3and Table A11).The Nd vs. Hf isotope compositions of melts

emerging from this MMI process (at decreasingmelt mass), have a vertical distribution relativeto the mantle array on a Nd^Hf isotope plot,identical to the SLCP trend (Fig. 7b). This Nd^Hf isotope decoupling is the result of both thecombined cpx assimilation/opx crystallization,the relative partition coe⁄cients of Nd and Hf

Fig. 7. Nd vs. Hf isotope compositions of model melts. Sym-bols and ¢elds as in Fig. 4. (a) Metasomatism (modeled asbinary mixing) of a 1 Ga old cpx residue of variable degreesof MORB melting by the average HV melt. ONd = 7 andOHf = 15 in the HV so that the mixing lines are forced topass through the SLCP Nd compositions. Three mixing linesare shown for comparison: average HV mixing with 7%,10%, and 14% melt-depleted 1 Ga old cpx. Tick marks withnumbers on the mixing lines show % of melt addition to thecpx. Inset ¢gure shows an expanded view of the mixing lines.(b) MMI between HV melts and 7%, 12% and 14% melt-de-pleted 100 Ma old peridotite (RMC). Three MMI curves areshown, used for modeling samples 326, 466 and 470 (Table3). Tick marks along these curves show % of remaining meltmass. As the mass of the melt decreases (or the rock/melt ra-tio increases) the MMI melts evolve vertical to the OIB arraysimilar to the compositions of the SLCPs. The MMI curvesend at 0.5% melt mass remaining. Inset ¢gure shows an ex-panded view of the model and the present-day compositionof the 100 Ma old RMC.



in opx, and the relative Nd and Hf concentrationsbetween melt and peridotite. Since any HV melt isby far more enriched over a depleted peridotite inNd than in Hf, the 143Nd/144Nd ratio of an emerg-ing MMI melt is essentially bu¡ered by the 143Nd/144Nd ratio of the input melt. In other words, theHf isotope compositions of the MMI meltschange well before that of the Nd, resulting inthe decoupling of Hf from Nd isotopes. Theseresults are little sensitive to the exact compositionof the melt, as long as it is relatively enriched (i.e.higher Nd/Hf ratio) over the peridotite. For ex-ample, using either the least enriched or the aver-age HV lava (Table 3) as an input melt, this AFCwill produce essentially identical curves in Fig. 7b.Note that MMI melts emerging from a less de-pleted peridotite (7% melt residue) will too evolvetowards higher 176Hf/177Hf but their 143Nd/144Ndratios will change earlier than melts emergingfrom a more depleted peridotite (12^14% meltingresidue), because the less depleted peridotite isrelatively more enriched in Nd than the depletedone.

In summary, mantle^melt interaction betweenHV-type melts and a 100 Ma old MORB-depletedlithosphere can generate signi¢cant decoupling be-tween Hf and Nd isotopes, similar to the compo-sitions observed in the SLCPs. The same decou-pling is produced between Sr and Hf isotopes (notshown), since Sr is even more incompatible thanNd and more enriched in the melt over the peri-dotite. The MMI melts are bu¡ered in their 87Sr/86Sr ratio because of the high Sr contents of themelt, while Hf isotopes become increasingly ra-diogenic at decreasing melt mass. Similar decou-pling is expected between Pb and Hf isotopesbased on the similarity of Pb isotopes in theSLCPs and HV [7].Fig. 8a shows the trace element patterns of se-

lected melts from the MMI process. As the extentof reaction between the melt and the peridotiteincreases (i.e. decreasing melt mass or increasingrock/melt ratio) the emerging melts become pro-gressively depleted in Zr, Hf and Ti, the MREEto HREE slope becomes more £at and the LREE/HREE remain high (e.g. Nd/Yb in Fig. 6c), sim-

Table 3Mantle^melt interaction parameters and results

Meltsa 100 Ma lithosphere (cpx)b Successful MMI meltsc Model cpxd

HV AOB 7% DM 12% DM 14% DM 326 melt(17%^118)

466 melt(83%^103)

470 melt(17%^15.5)

326 (5%melt added)

466 (6%melt added)

470 (3%melt added)

Ce 113.4 39.9 0.001 5.3E-07 4.5E-09 113.1 116.0 124.4 81.94 61.23 42.1Nd 52.1 22.4 0.036 0.0016 2.2E-04 18.78 44.58 23.86 12.71 26.23 11.32Zr 180.3 96.8 0.470 0.0234 0.00549 25.66 164.5 36.72 7.07 49.03 6.99Hf 3.8 2.5 0.025 0.0022 0.0006 0.63 3.31 0.71 0.28 1.48 0.21Sm 11.7 5.4 0.044 0.0056 0.0016 2.32 9.00 2.49 1.63 5.62 1.19Eu 3.6 1.8 0.025 0.0044 0.0015 0.74 2.67 0.59 0.51 1.79 0.30Ti 11095 12350 355 188.2 140 3136 6629 2420 1514 2974 991Dy 6.3 4.2 0.255 0.1007 0.0595 3.92 5.48 2.43 3.05 4.58 1.49Er 2.4 1.8 0.203 0.0914 0.0592 2.70 2.99 1.81 1.96 2.62 1.06Yb 1.7 1.5 0.263 0.1461 0.1091 2.58 2.84 1.96 1.64 2.21 1.07Lu 0.3 0.2 0.044 0.0266 0.0209 0.38 0.43 0.30 0.23 0.30 0.16176Hf/177Hf 0.28324 0.28324 0.28375 0.28666 0.29291 0.28457 0.28330 0.28421 0.28470 0.28334 0.28441143Nd/144Nd 0.51303 0.51303 0.51358 0.51452 0.51586 0.51306 0.51304 0.51304 0.51306 0.51304 0.51304

a HV : average HV basalt (average from the GEOROC (http://georoc.mpch-mainz.gwdg.de) database and [42]). 176Hf/177Hf istaken as the most radiogenic value from [30]. AOB : least trace element-enriched alkali olivine basalt 69KAL-1 [42].b Composition of cpx in the depleted lithosphere, residue of 7%, 12%, 14% MORB melting.c Composition of the successful MMI melts used for modeling samples 326, 466, 470. Numbers in % are the amount of the meltmass remaining and the number after the dash is the corresponding rock/melt ratio. 326 melt is the MMI reaction product be-tween AOB and 12% DM, 466 melt is the MMI reaction product between HV and 7% DM, 470 melt is the reaction product be-tween AOB and 14% DM.d Calculated cpx compositions for samples 326, 466, 470. These compositions are derived by the addition of 5%, 6%, and 3% ofthe ‘Successful MMI melts’ to the respective depleted peridotite compositions (b) and then calculating the concentration of cpxby mass balance with the other mineral phases. All isotope ratios are present-day values.



http://georoc.mpch-mainz.gwdg.de

ilar to the compositions of the SLCPs. Fig. 8b,ccompare the calculated and measured trace ele-ment composition of the targeted cpxs (samples326, 470 and 466). The calculated and observedtrace element patterns match well, except for thegenerally higher Ce contents in the calculated cpx.Increasing the DCeopx and/or D

Cecpx can partially com-

pensate the high Ce contents and the overall ¢t intrace elements can be further improved by slightlyadjusting the chosen partition coe⁄cients. How-ever, the overall good agreement between mea-sured and calculated trace element patterns andisotope compositions points to the plausibility ofthis MMI model.The key parameter for the success of this MMI

model is that the ratio of assimilated/crystallizedmass is less than 1 (note that we could only ¢ndsolutions that match both the trace element andisotopes of the SLCPs if this ratio is s 0.985 and6 1). Therefore, as the melt reacts with a largermass of peridotite (or increasing rock/melt ratio),it becomes exceedingly small in volume, while ac-quiring the radiogenic Hf composition of the peri-dotite. It is possible then that after extensive re-action, such small volume melts will ‘freeze’ orexperience ‘thermal death’ in the lithosphere.This can explain the absence of such radiogenicHf isotopic compositions in the erupted lavas,when instead such signatures are being recordedin the lithosphere. If, however, such melts do es-cape to the surface, then they must be volumetri-cally minor relative to other ‘normal’ melts andtheir radiogenic Hf isotope signature is lost bydilution.An important consequence of this MMI model

is that the emerging melts have Lu/Hf isotopesystematics similar to the SLCPs. In Fig. 5b, theMMI melt curve that reproduces sample 326(product of the interaction between the alkali ol-ivine basalt (AOB) melt and a 12% depleted 100Ma old peridotite, Table 3), de¢nes a slope withan age of ca. 1.05 Ga. Furthermore, the slope ofthis curve changes, from shallow (similar to thatde¢ned by the six least radiogenic samples) tosteep, as the melt mass decreases. This is becauseDLusDHf and the Lu in the MMI melts reachesa steady state with the reacting mantle at smallerextents of reaction than Hf (Fig. 8a). Reaction

between this melt and a more depleted peridotiteproduces an MMI curve with a steeper slope thatde¢nes even older ages (2.5 Ga in Fig. 5b). Notealso that mixing between any of these MMI meltsand the 100 Ma old depleted lithosphere will gen-erate compositions that fall to the right of theseMMI curves, creating apparent ages younger than1 Ga (but older than 100 Ma), similar to the 500Ma age de¢ned by six of the SLCP samples.The above calculations are non-unique and de-

Fig. 8. (a) Chondrite-normalized trace element concentrationsof selected model MMI melts produced by the reaction ofAOB melt and peridotite residue of 12% MORB melting(Table 3). Di¡erent symbols with numbers in % show theMMI melt compositions at decreasing (% remaining) meltmass. (b) Model (dashed lines) and measured cpx composi-tion of samples 466 and 470. (c) Model (dashed line) andmeasured cpx composition of sample SLC 326.



pend on the composition of the input melt, thedegree of depletion of the peridotite, the type ofreaction and the chosen partition coe⁄cients. Forexample, by choosing between the average andleast enriched HV lavas we can leverage on thein£uence of the melt in the resulting cpx traceelement composition, with a minimum e¡ect onthe isotopes. It is also known that cpx partitioncoe⁄cients depend on the composition of the sys-tem (e.g. [41] and references therein). If the dimin-ishing in volume MMI melts also change in majorelement composition (as in their trace elementcontents) then the relevant partition coe⁄cientsare also expected to change during reaction.Also, the actual melts that react with the litho-sphere at depth may have di¡erent compositionthan the compositions used in our modeling.For example, sample 328 has signi¢cantly smallerHf and Zr depletions compared to HV lavas[2,42], making any of these melts an unlikelymetasomatic agent. Our numerical experimentshowever show that in order to recreate both the

trace element and Nd^Hf isotope decoupling ofthe SLCPs, it is the degree of depletion of theperidotite and the type of reaction that have thelargest leverage on the results.This MMI model can also explain the correla-

tions between Hf isotopes and degree of depletionin the SLCPs. The variable and refractory majorelement composition of the SLCP cpxs suggeststhat they are residues of variable degrees of melt-ing and represent di¡erent portions of the litho-sphere (assuming that metasomatism did notsigni¢cantly overprint their major element compo-sitions). We envision that an HV melt ascendsthrough, in¢ltrates and metasomatizes the varia-bly depleted lithosphere through the MMI process(Fig. 9). Depending on the degree of depletion ofthe di¡erent parts of the lithosphere and extent ofthe reaction, the MMI melts acquire distinct traceelement (e.g. high Nd/Yb ratios, Zr and Ti deple-tions) and isotope characteristics (radiogenic Hf)and metasomatize the in¢ltrated peridotite. In thisrespect, each peridotite sample re£ects the compo-

Fig. 9. Schematic representation of the MMI model. The 100 Ma old RMC (panel b, only the spinel lherzolite ¢eld is shown) isstrati¢ed with respect to the degree of depletion (dashed arrows and lines in panel c) and has developed radiogenic Nd and Hfisotope compositions (panel a). The ascending HV melt metasomatizes the surrounding wallrock at di¡erent levels, with each lev-el representing a distinct domain (boxes in panel b, solid lines in panel c). Metasomatized peridotites from shallower (and moredepleted) levels are more likely to retain and record their radiogenic Hf isotope signature, than the deeper, less depleted and lessradiogenic samples (panel d). The Pali sample comes from a part of the RMC that was less extensively metasomatized than theSLC samples.



sition of a single ‘domain’ of the lithosphericmantle beneath Oahu, with a unique history ofdepletion and metasomatism. In this model, themost depleted (and thereby most radiogenic) por-tions of the lithosphere are more likely to recordhighly radiogenic Hf compositions than the lessdepleted and less radiogenic parts.

5. Summary and implications

The combination of the Hf and Nd isotopesystematics and the trace and major element com-positions of cpxs from spinel lherzolite xenolithsfrom Oahu reveals extensive interaction betweenHV melts and the depleted Paci¢c mantle litho-sphere. This interaction appears more pronouncedin the SLC than in the Pali and Kaau peridotites.The di¡erences in both trace and major elementsand Nd, Sr and Hf isotope data between the SLCand Pali and Kaau peridotites (Figs. 2^4) point toa heterogeneous lithospheric mantle beneathOahu. Although more isotope data on the xeno-liths from the other vents are needed before wecan de¢ne their exact di¡erences, we suggest thatthese di¡erences probably result from the hetero-geneous distribution of melt and/or melt channelsin the lithosphere. If the network of melt channelsassociated with the SLC vent was more wide-spread than the equivalent network beneath Paliand Kaau, then this would allow for larger ex-tents of metasomatism to take place in the SLClithosphere than in Pali and Kaau. A more exten-sive and possibly deeper melt network beneathSLC can also explain the abundance of deeper,garnet-bearing xenoliths in SLC and their absencein the Pali and Kaau vents [3].The near total absence of Koolau-type isotopic

signatures in the Oahu mantle xenolith populationremains a striking observation (only two perido-tites with such isotope compositions have beenreported and these have been interpreted as crus-tal cumulates [7,16]). Even if the main shield-stageKoolau lavas are not as isotopically extreme asthe later subaerial Maakapuu £ow [43,44], thegeneral absence of peridotites with 143Nd/144Nd6 0.5130 and 87Sr/86Srs 0.7035 suggeststhat the Koolau lavas did not signi¢cantly inter-

act with the lithosphere. This makes the MORB-like Nd^Hf isotopic composition of the Pali sam-ple even more signi¢cant, since the Pali vent islocated within the Koolau caldera and potentiallycloser to the Koolau melt network. This, com-bined with the recent report of Koolau-type87Sr/86Sr and 143Nd/144Nd ratios in plagioclaseseparates from the more shallow plagioclase peri-dotites from Pali [14,45], suggests changes in the‘plumbing’ of the volcano with depth. The deeperKoolau magma conduits must have been wellshielded from the surrounding lithosphere at thespinel lherzolite ¢eld (30^50 km [3]), while theywhere able to impregnate the more shallow anddelaminated lithosphere and precipitate plagio-clase [45].The MMI model presented here can explain the

large Hf/Nd isotope decoupling in the SLCPs, butthe lack of such compositions in ocean island ba-salts (OIBs) suggests that such MMI melts arevolumetrically minor relative to the eruptinglavas. However, the limited Nd^Hf isotope dataon the HV lavas [30] de¢ne a slope of 5.8 in Nd^Hf isotope space (Fig. 4), signi¢cantly steeperthan the slope of the terrestrial array (1.36 [46])and the slope of Hawaiian lavas (1.0 [47]). Thesteep slope and the relatively high 176Hf/177Hffor a given 143Nd/144Nd in the HV can be ex-plained by the contribution of some MMI meltswith high 176Hf/177Hf to the erupting HV. It hasalso been suggested that the shallow slope de¢nedby some Samoan basalts in Nd^Hf space [48] canbe explained by some contribution of melts thatinteracted with ancient recycled mantle litho-sphere. Further data are needed to determinewhether the HV Hf isotope compositions andthe variable slopes de¢ned in Nd^Hf space bydi¡erent OIBs correlate with indices of MMI(e.g. Hf, Zr depletions) and what the contributionof the depleted lithosphere (recent or ancient-re-cycled) is in plume-related volcanism.

Acknowledgements

This work was supported by NSF Grants OCE9977642 to G.S. and OCE-0241681 to G.S. andV.J.M.S. We thank Andreas Stracke for com-



ments on an earlier version of the manuscript andMihai Ducea and John Lassiter for their reviewswhich improved the ¢nal manuscript. We thankTom Beasley for his assistance with the electronprobe analyses at FCAEM, and Shantanu Keshavfor assistance and discussions.[KF]

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Hf–Nd isotope decoupling in the oceanic lithosphere: constraints from spinel peridotites from...

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