the terrestrial li isotope cycle: light-weight constraints on mantle convection

Frontiers

The terrestrial Li isotope cycle: light-weight constraints onmantle convection

Tim Elliott a;�, Alistair Je¡coate a, Claudia Bouman b;1

a Department of Earth Sciences, Wills Memorial Building, Queens Road, University of Bristol, Bristol BS8 1RJ, UKb Faculteit der Aard- en Levenswetenschappen, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

Received 15 September 2003; received in revised form 29 January 2004; accepted 1 February 2004

Abstract

The two stable isotopes of Li are significantly fractionated by exchange reactions with clays near the Earth’ssurface. The isotopic legacy of this process provides a robust tracer of surface material that is returned (recycled) tothe mantle. Altered oceanic crust has a heavy Li isotopic composition (high 7Li/6Li). Heterogeneous distribution ofsubducted, altered oceanic crust in the mantle will result in variations in Li isotope ratios. A rapidly accumulatingdataset of Li isotope analyses on mantle-derived materials indeed indicates a significant range in Li isotope ratios.This observation provides powerful evidence for the widespread distribution of recycled material in the convectingmantle. There is substantial overlap in Li isotopic compositions of ocean island basalts (OIB) and mid-ocean ridgebasalts (MORB). Some OIB, however, have slightly heavier Li compositions than typical, depleted MORB. At facevalue this suggests a larger contribution of recycled oceanic crust in the sources of some OIB than in the uppermantle. Yet recent evidence implies that heavy Li is lost from the slab at subduction zones and the recycled residualcrust is left with an isotopically light signature. Extremely light Li isotope ratios are observed in some continentalmantle xenoliths but not in OIB proposed to contain large proportions of recycled crust. Studies of subduction zonelavas imply the heavy Li isotope signature of altered oceanic crust is transferred to the mantle above the subductingplate. Thus cycling of sub-arc mantle may be an important process in forming chemical heterogeneities sampled byOIB. Inferences from initial Li isotope results are thus notably different from those from the longer established butmore equivocal radiogenic isotope tracers. Li isotopes promise to provide significant new constraints on thedistribution of recycled material in the mantle and its implications for mantle convection.; 2004 Elsevier B.V. All rights reserved.

Keywords: Li isotopes; mantle evolution; recycling

0012-821X / 04 / $ ^ see front matter ; 2004 Elsevier B.V. All rights reserved.doi:10.1016/S0012-821X(04)00096-2

* Corresponding author. Tel. : +44-117-954-5426; Fax: +44-117-925-3385.E-mail addresses: [email protected] (T. Elliott), a.je¡[email protected] (A. Je¡coate), [email protected]

(C. Bouman).

1 Present address: Finnigan Advanced MassSpectrometry, Thermo Electron Corperation, Barkhausenstr. 2, 28197 Bremen,Germany.

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1. Introduction

Mantle convection stirs the Earth’s interior.Understanding this ¢rst-order planetary processhas been improved by increasingly sophisticated£uid dynamic models [1] and better resolved seis-mic images of the mantle’s velocity structure [2].It is also necessary to develop in tandem geo-chemical tracers of convection. The importanceof geochemistry is that it provides a time-inte-grated signal of the e¡ects of convection andcan trace the fate of material with distinctive com-positional signatures. In partnership with thecomplementary present-day picture from seismol-ogy, geochemistry provides the observables forconvection model testing [3,4].The formation and subduction of oceanic plates

is an inherent part of mantle convection. The re-turn to the mantle of oceanic crust (and its de-pleted residue), variably in£uenced by its near-surface residence, is frequently termed ‘recycling’.Determining how recycled material is distributedin the present-day mantle informs on the style ofmantle convection. In seeking this elusive goal, anumber of isotope systems have been employed astracers of recycled plates. Radiogenic isotope sys-tems are most commonly used (e.g. [3,4]), but canbe strongly in£uenced by processes other thanoceanic crustal formation, alteration and subduc-tion. Although many popular interpretations ofradiogenic isotope variations implicitly invoke re-cycled material, the evidence is equivocal [5].Stable isotope tracers are highly desirable to

ground-truth inferences from radiogenic isotopes.Stable isotope fractionation is dominantly the re-sult of low-temperature processes near the Earth’ssurface. Thus signi¢cant variations in the stableisotope ratios of mantle-derived samples unam-biguously re£ect variable contributions of re-cycled material. This logic has prompted a carefulre-examination of the oxygen isotope signaturesof mantle-derived basalts using high-precision la-ser £uorination techniques (see [6], for recentsummary). Such oxygen isotope studies have iden-ti¢ed some lavas that clearly contain recycled ma-terial in their source [7]. Yet most mantle-derivedmelts show a rather small range in oxygen isotopecompositions. Whilst this may re£ect a lack of

recycled input, it may also re£ect a lack of sensi-tivity of oxygen isotopes to some recycling pro-cesses.Here we highlight the potential of the Li iso-

tope system in tracking the fate of subducted ma-terial and its distribution by mantle convection.We initially summarise the behaviour of Li iso-topes in the near-surface environment to under-stand the controls on Li isotope fractionation. Wethen consider the e¡ects of subduction zone pro-cesses on the Li isotope systematics of the alteredoceanic crust. Finally we assess the burgeoning,but still modest, Li isotope data on mantle-de-rived samples. Li isotope geochemistry is a juve-nile research area, dominated by work in the lastdecade. Systematic study of the Li isotope ratiosof mantle-derived rocks is even more immature.Thus we can only present a tantalising aperitif ofwork that may follow and note that our interpre-tations are still at the frontier stage.

2. Preliminaries

Lithium has two stable isotopes, 6 and 7, whichhave the biggest relative mass di¡erence of anyisotope pair aside from hydrogen^deuterium.The potential for mass-dependent fractionationis thus obvious. Unlike the better established lightstable isotope systems (e.g. C, O, S), Li is a tracecation and does not form an integral part ofhydrological, atmospheric or biological cycles.Hence measurements of Li isotopes are likely toprovide rather di¡erent information than is avail-able from the more commonly used isotope sys-tems.As with other stable isotope measurements, Li

isotope ratios are typically expressed in a deltanotation:

N7Li ¼ ½ð7Li=6LisampleÞ=ð7Li=6ListandardÞ31�U1000

where the standard is conventionally the NationalInstitute of Standards highly puri¢ed Li2CO3

reference material NIST L-SVEC [8] which has7Li/6LiW12.15 [9]. As has occurred in the earlystages of development of other isotope systems,some confusion has arisen over the form in whichdata are reported. Both N

7Li and N6Li have been

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T. Elliott et al. / Earth and Planetary Science Letters 220 (2004) 231^245232

used in the literature, but increasingly the N7Li

notation has become standard (e.g. [10]). In theN7Li formulation, more positive values are alsoisotopically heavier, in keeping with other stableisotope systems. Measurements reported as N

6Lican be readily converted to N

7Li. Simply changingthe sign of N6Li measurements yields a value thatgenerally closely approximates the correspondingN7Li.Measurements of Li isotope ratios have tradi-

tionally been made using thermal ionisation massspectrometry [11^13]. Although Li is e⁄cientlyionised by solid source mass spectrometers, highlyreproducible isotope ratios are more di⁄cult toobtain by this technique than for the gas-sourcemachines used to measure oxygen isotopes. Thereproducibility of Li isotope measurements hasthus lagged behind oxygen isotope measurements,typically V1x compared to V0.05x (all er-rors in this contribution are cited as 2c standarddeviations). This has hampered application of Liisotopes to studying mantle processes, where var-iations in Li isotope ratios are only a few per mil(see below), but it has been su⁄ciently precisefor documenting the larger isotopic variations inthe near-surface environment. Thermal ionisationmeasurements are also time-consuming and so theLi isotope database has remained relatively lim-ited. The advent of plasma ionisation multi-collector mass spectrometry has allowed a con-siderably faster throughput of smaller samples[14,15]. New-generation plasma ionisation multi-collector mass spectrometers have also enabled amajor improvement in reproducibility, typicallyR 0.2x [16], much closer to oxygen isotope mea-surements and better than the best thermal ionisa-tion data ( R 0.5x) [10]. This advance is highlysigni¢cant for documenting Li isotopic variationsin the mantle.

3. Behaviour of Li in the hydrological cycle

The fractionation of Li isotopes is dominatedby partitioning of Li between clays and water [17^21]. It is important to review brie£y the funda-mentals of these low-temperature processes in or-der to understand the potential of Li isotopes as a

tracer of subducted material. This pioneeringwork of Chan and co-workers in the last decadehas laid the essential foundations for exploiting Liisotopes as high-temperature tracers [17,22,23].Readers interested in more detail on this and oth-er aspects of Li geochemistry are referred to arecent and comprehensive review by Tomascak[24].Weathering of the continents both releases Li

and isotopically fractionates it [23]. Minerals arecharacteristically enriched in 6Li relative to co-ex-isting aqueous £uid [21,25]. Thus river watershave heavy Li isotope compositions compared tothe original bed-rock and associated suspendedload [20]. The Li isotopic composition of riverwater is not highly sensitive to the Li isotopecomposition of the bed-rock of the catchmentarea, in contrast to some radiogenic isotope ratiosused to monitor chemical weathering [23]. Thereis thus promise in using the evolution of the Liisotope ratio of seawater to assess past changes inthe intensity of continental weathering, with itsattendant implications for atmospheric CO2 andlong-term climate change (e.g. [26]). In a simplis-tic scenario it might be anticipated that a changeto lighter Li isotope compositions in ancientoceans would re£ect lower weathering rates inthe past. Whilst the general process of Li isotopicfractionation at the surface is clear, a more de-tailed understanding of the behaviour of Li iso-topes during weathering, transport and incorpo-ration into the geological record is required beforeLi isotope ratios can be used as weathering proxy(for recent review on chemical proxies of pastconditions see [27]). However, an increasing num-ber of groups are investigating this potential andthis is an exciting prospect for the future [21,28^32].In the marine system too, partitioning of Li

between water and clays results in major isotopicfractionation [17,18,25]. This results in an initiallypuzzling observation. Unlike radiogenic isotopesystems, such as 87Sr/86Sr, where seawater repre-sents a intermediate mix of a hydrothermal input(unradiogenic, mantle-dominated signature) andriver water (radiogenic, continental signature),seawater has a Li isotope ratio heavier than itsinputs (Fig. 1). This paradox is explained by the

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T. Elliott et al. / Earth and Planetary Science Letters 220 (2004) 231^245 233

Fig. 1. (A) Cartoon of the main £uxes that control Li isotope mass balance in the oceans. Flux estimates taken from [23]. (B)Analyses that constrain the ocean Li isotope mass balance. Individual analyses are shown as open symbols with weighted meansindicated by larger symbols. Typical uncertainties are R 1x. Data sources: rivers [23], high-temperature vents [22], seawatercompilation from [13], fresh MORB [17,34]. The oceanic plate is divided into three stratigraphic components to illustrate theupper (top 500 m), high-N7Li, low-temperature altered basalts [17,33]; lower (s 500 m) basalts, some of which represent the lightresidue of high-temperature hydrothermal £uids (note these are also depleted in Li and only weakly a¡ect the weighted mean ofthis section) [33]; serpentinised upper mantle [74]. The low-temperature altered basalts have strongly elevated Li concentrationsand dominate the Li isotope budget of the altered crust.

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low-temperature alteration of the oceanic crust tohydrous minerals (such as smectite and phillip-site). These alteration products are importantsinks for oceanic Li and also preferentially re-move 6Li from seawater during this process[17,33]. Enhanced removal of 6Li from the oceansresults in a complementary, heavy seawater com-position of N

7LiW32 (e.g. compilation in [13]),heavier than its average input. Although the Ligained by the alteration material is lighter thanmean river water, N7Li6 23x [23], it is neverthe-less heavier than fresh oceanic basalts, N

7LiW3[17,34].In summary, weathering of silicates on the con-

tinents produces river water with isotopicallyheavy Li which feeds the oceans. Low-tempera-ture alteration of oceanic crust serves to makeseawater even heavier than river water. Eventhough altered oceanic crust is isotopically lightcompared to mean river water, it is notably heav-ier than mantle values (Fig. 1B).The process of increasing the N

7Li of alteredoceanic crust as part of the low-temperature Licycle thus imparts a distinctive signature to theoceanic plate that can be used to trace it. We ¢rst,however, assess if the magnitude of this heavy,recycled £ux is su⁄cient to a¡ect the Li isotopiccomposition of the mantle as a whole. The £uxmight seem most straightforwardly calculated byusing an estimate for average altered oceanic crustand present-day rates of plate subduction. Al-though it is clear that altered oceanic crust isheavy [17,33], Li isotope analyses of altered oce-anic crust are only available from a few localitiesworldwide. Only three samples have been ana-lysed that are older than 10 Myr. Constructing arepresentative value for average altered oceaniccrust that has experienced a full range of altera-tion processes is therefore di⁄cult. Thus we preferto estimate the net Li isotope £ux to the oceaniccrust, and ultimately the subduction zone, indi-rectly by examining the Li isotope mass balancein the oceans. Using the commonly made assump-tion of a steady-state seawater composition, theLi isotope £ux to the altered oceanic crust mustbalance the input £ux from rivers and hydrother-mal vents. Since the hydrothermal £ux is itselfdominantly derived from the oceanic crust itself,

the net £ux to the oceans equals the riverine £uxof V6U1010g/yr with N

7Li = 23x [23].The rami¢cations of returning this £ux of heavy

Li to the mantle via subduction of the alteredoceanic crust are explored in Fig. 2. It is evidentthat the magnitude of the recycled £ux is justsu⁄cient to in£uence the composition of the man-tle as a whole. Moreover, if recycled material isnot e⁄ciently mixed throughout the whole mantleit can signi¢cantly change (V1x) the Li isotopecomposition of a more restricted portion of themantle, e.g. the upper mantle (Fig. 2). The relativeLi isotopic compositions of di¡erent mantle reser-voirs thus have clear implications for assessing thestyle of mantle convection.

Fig. 2. Li isotopic evolution of crustal and mantle reservoirsin response to weathering and subduction of Li. The simplis-tic model illustrates the cumulate e¡ect of removing the riv-erine Li £ux (5.6U1010 g/yr, N7Li = 23x [23]) from the con-tinental crust and adding it to the mantle via subduction.The graph shows the e¡ects of two scenarios: adding thesubducted £ux to the whole mantle (4U1027 g) and justupper mantle (1.33U1027 g). Both reservoirs are initially as-sumed to have [Li] = 1 Wg/g and N

7Li = 3x. The continentalcrust (2U1025 g) is assumed to have an initial N

7Li = 3x(since it is ultimately derived from the mantle) and [Li] = 22Wg/g [35]. Each reservoir is assumed to be homogeneous andtherefore changes are averaged over the whole reservoir. Themodel is run in 1U108 yr time steps for 1.5U109 yr. Afterthis time, the composition of the model continental crustcomposition is at the lower end of continental compositesanalysed by [35] and so the model is run no further. Thecomposition for current continental crust is from [35], illus-trating a preferred average for whole crust and 1 standarddeviation of all upper crustal samples measured.

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The increase in N7Li of the mantle sketched in

Fig. 2 ultimately results from weathering of thecontinents, followed by cycling heavy Li throughthe oceans and altered oceanic crust into the man-tle. Continental crust is originally derived fromthe mantle and these two reservoirs should ini-tially have had the same Li isotope ratio. Contin-ued addition of heavy Li into the mantle by sub-duction must have a complementary e¡ect on theLi isotope ratio of the continents. The Li isotopecomposition of the continental crust thus providesan important constraint on how heavy the mantlecan have become through time. Valuable recentstudies report a light Li isotopic composition(N7LiW0) for both the upper [35,36] and lower[37] continental crust with a mean [Li] = 22 ppm[35]. The light Li isotope ratio of the present-daycontinental crust can be produced in only 1 Gyrof recycling at current rates. This suggests that therecycled Li £ux calculated from present-day oce-anic mass balance may be anomalously high, pos-sibly as a result of enhanced current weatheringrates from Himalayan uplift (e.g. [26]) or simplydue to inaccuracies in the approach. Regardless ofthe rate at which the continents have becomelight, however, the Li isotope composition of thecontinental crust limits the change in the N

7Li ofthe whole mantle to be V0.3x.Before considering how subduction modi¢es the

Li alteration £ux into the mantle it is worthbrie£y contrasting the Li isotope signature of al-tered oceanic crust with longer established mea-surements of oxygen isotopes. Oxygen is nearlyequally abundant in most silicate rocks, fromclays to mantle minerals, whereas altered upperoceanic crust has a Li concentration some 10times greater than the mantle. Recycling of al-tered oceanic crust to the mantle will thus havea greater leverage on the Li isotope system. More-over, heavy oxygen isotope ratios in the upperoceanic crust are compensated by light oxygenisotope ratios in the lower oceanic crust [38].Thus the oxygen isotopic composition of the com-plete sequence of altered oceanic crust is the sameas pristine mantle. Nevertheless, oxygen isotopeshave provided striking evidence for the presenceof speci¢c portions of recycled crust in someocean island basalt (OIB) sources, notably sedi-

mentary material [39] and lower portions of thealtered oceanic crust [40]. Yet oxygen isotopes areinsensitive to the wholesale addition of alteredoceanic crust to a mantle source. Interpretationof the similarity of most peridotite, mid-oceanridge basalt (MORB) and OIB oxygen isotoperatios [7] is therefore ambiguous.

4. The subduction zone ¢lter

Plate tectonics inevitably mixes altered oceaniccrust, with its budget of heavy Li, back into themantle. However, the passage of Li from the sur-face to depth is not simple and is strongly a¡ectedby the processes occurring during subduction(Fig. 3A). Water is successively lost during sub-duction, ¢rst through porosity reduction duringcompaction and then through a series of pro-grade dehydration reactions as the descendingslab becomes hotter (e.g. [41,42]). As in thenear-surface environment, the partitioning of Libetween residual solid and liquid will result inisotopic fractionation. Although isotopic fraction-ation diminishes with increasing temperature,there is still signi¢cant inferred [22,33] and mea-sured [25] Li isotopic fractionation between min-eral phases and water during high-temperaturehydrothermal circulation at V350‡C. Thus thepotential for isotopic fractionation extends wellinto the subduction zone.Fluid is expelled out of the subduction zone as

a result of compaction of the sedimentary pile.These £uids that return to the ocean along theso-called de¤collement are isotopically heavierthan the associated sediments [19]. Further insightinto the shallow subduction zone is provided byserpentine diapirs which sample the highly hy-drated fore-arc. Analyses of Mariana serpentinitesshow a wide range of N7Li from light (30.5x) toheavy values (+10x) [43], although ¢ner-grainedfractions show dominantly heavier values (N7Li5^9x) [44]. Thus some of the heavy subductedLi is clearly lost to the oceans and shallow wedge,but the amount of Li lost from the slab in theseearly dehydration processes is estimated to be aminor fraction of the total Li subducted [19,45].In contrast, the budget of B is signi¢cantly de-

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Fig. 3. (A) Cartoon of the Li isotope behaviour in the subduction zone, illustrating processes discussed in the text. Sketch indi-cates transfer of £uids with heavy Li to hydrate the adjacent mantle (hatched). The hydrated mantle is viscously coupled anddown-dragged with the complementary, isotopically light, dehydrated oceanic crust. Fluids that traverse into the hotter interiormantle wedge and induce melting have already isotopically equilibrated their Li with the preceding mantle. Thus arc lavas haveN7Li comparable to the pristine mantle wedge. (B) Li isotopic composition of inputs and outputs from subduction zone. Individu-al data are shown with weighted means represented as larger symbols. Note that despite the heavy mean compositions of bothincoming sediments and altered ocean crust, the vast majority of arc lavas have N

7Li within error of MORB (uncertainties formost arc measurements R 1x). Samples sources: eclogites [52], altered oceanic crust (see Fig. 1), oceanic sediments[18,19,34,49,75], arc lavas [34,48,49,76]. The older Panama samples reported in [76] are not plotted due to the possible e¡ects of10 Myr of weathering in a tropical environment, which can increase N

7Li of samples [10].

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pleted by fore-arc dehydration [45,46] which com-promises its potential as a tracer of recycled ma-terial.A major mass £ux at subduction zones is asso-

ciated with magmatism at the volcanic front. Sub-duction zone magmas are characterised by somedistinctive chemical signatures. Notably arc lavashave elevated concentrations of elements empiri-cally observed to be mobile in aqueous £uids(such as Li) relative to elements that form highlycharged ions and have low solubilities in aqueous£uids (e.g. Y) but otherwise behave similarly dur-ing magmatic processes (for a recent review see[47]). Thus arc lavas have high Li/Y ratios (e.g.[48]). Analogous ratios, such as Ba/Th, have longbeen used to implicate the addition of aqueous-rich £uids from the down-going slab to the sourceof arc lavas (in the mantle wedge). The elevatedLi/Y of arc lavas would hence be anticipated to beassociated with heavy Li, £uxed from the sub-ducting slab. It is surprising that very few arclavas have Li isotope ratios signi¢cantly heavierthan MORB [48] (see Fig. 3B). Two studies of arclavas show variations of Li isotope ratios withsome key trace element ratios, suggesting a trendto heavier Li ratios with larger slab-derived inputs[34,49]. Such systematic variations are not ob-served, however, in comprehensive studies of anumber of other arcs ([48], Bouman, unpublisheddata).That few arc lavas have heavy Li isotope ratios

implies that little of the budget of heavy Li thatenters the subduction zone is lost to the surface asa result of arc magmatism. Yet, as discussedabove, several ‘£uid-mobile’ tracers of the slabare evident in arc lavas. The lack of an associatedheavy Li isotopic signature can be related to therelative a⁄nity of Li for mantle phases. Li has anionic radius comparable to Mg and can occupythe abundant Mg lattice sites available in themantle. Although apparently preferentially trans-ported by £uids, high-temperature partitioning ex-periments show that Li is only moderately incom-patible in either melt or £uid relative to mantlephases [50,51]. Thus it is likely that heavy Li ini-tially carried from the slab in aqueous £uids iso-topically re-equilibrates with the mantle [48]. Theheavy Li isotopic signature from the slab is e¡ec-

tively transferred to the cold portion of mantlelying above the slab (Fig. 3A).Striking evidence for the loss of heavy Li from

the subducted oceanic crust itself is found in theextremely light Li isotopic ratios (N7Li as low as311x) of Alpine eclogites [52] (see Fig. 3B).These high-pressure metamorphic rocks are be-lieved to be fragments of deeply subducted ocean-ic crust that have been rapidly exhumed by thrust-ing during the Alpine orogeny. Zack et al. [52]argue that the loss of Li during pro-grade meta-morphism during subduction is associated withsigni¢cant isotopic fractionation. As in the surfaceenvironment, Li in the £uid phase is enriched inthe heavy isotope leaving the dehydrated slab iso-topically light. If accumulated e¡ects of dehydra-tion are modelled as a Rayleigh distillation pro-cess, the ¢nal ‘anhydrous’ residue at the end ofsubduction zone processing can have extremelylight Li isotope ratios. This mechanism can beinvoked to account for the very light Li isotoperatios observed in the eclogites (see Fig. 3B).Comparable isotopic fractionation during subduc-tion has been noted in the boron isotope system[53^55]. The Alpine eclogites thus represent thelight complement of the heavy isotopic signaturethat has left the slab to reside in the overlying,viscously coupled mantle.The e¡ects of subduction thus provide addi-

tional interest in the use of Li isotopes as tracersof mantle convection. The deep recycled, dehy-drated altered oceanic crust becomes isotopicallylight. Meanwhile the cool mantle adjacent to theslab becomes heavy. Thus we have two inputs tothe mantle as a result of subduction that are iso-topically distinct from the mantle itself. More-over, since these are stable isotope variations,these signatures can be unambiguously linked tolow-temperature processes near the surface of theEarth. Li isotopes therefore provide a means totrace the mixing of subducting slab and adjacentmantle wedge back into the convecting mantle. Itis of great interest to see how Li isotope signa-tures of recycling compare to radiogenic isotopemeasurements. Although the latter have long beenused to infer the distribution of recycled material,their variations can be interpreted in other waysand are much more equivocal.

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5. Mantle signatures

MORB and OIB represent our primary sampleof the mantle (for a review see [56]). The meltingprocess averages melts from a large volume ofsource rock and so studying melts is an e⁄cientapproach to sample the mantle. MORB are theproduct of shallow (dominantly less than 60 km)decompression melting in response to platespreading. MORB thus e¡ectively sample the am-bient shallow mantle. OIB are thought to be de-rived from melting of mantle that is hotter, orwith a low solidus, such that it commences melt-ing deeper than MORB mantle. Thus OIB can begenerated by upwelling even beneath thick ocean-ic lithosphere. The upwellings required to produceOIB are frequently attributed to mantle plumes.Since plumes need to be sourced at a major ther-mal boundary, most appealingly the core^mantleboundary, they are generally thought to sampledeeper mantle than the source of MORB.OIB are typically more ‘enriched’ than MORB,

with higher incompatible element contents and iso-tope ratios that re£ect long-term elevation of moreincompatible relative to less incompatible elements(e.g. high 87Sr/86Sr). Maintaining this contrast ingeochemical signatures between MORB and OIBsources is an important constraint on mantle con-vection models [3,4,57,58]. The nature of the ‘en-richment’ in di¡erent OIB is not uniform andseveral end-member compositions have been iden-ti¢ed. The role of recycled material has long beeninvoked to account for several of these end-mem-ber compositions. In a popular general scenario,manifest in a variety of di¡erent speci¢c forms(see [59]), a deep mantle layer is fed by subductedplates and ultimately forms the source for OIB.From the behaviour of Li in the subduction zonediscussed above, this lower mantle layer wouldlikely evolve with a lighter Li isotope ratio thanthe upper mantle. Thus OIB might be expected tohave a lighter Li isotope ratio than MORB. Thisshould be particularly marked for the so-calledHIMU OIB which are argued to have a largeportion of recycled ma¢c crust in their sources(see [56]).Before examining the Li isotope data for oce-

anic basalts, several important caveats need to be

addressed. Whilst variations in stable isotope ra-tios in mantle-derived rocks represent the leastambiguous signal of the (variable) presence ofmaterial that has been a¡ected by processes closeto the Earth’s surface, it is vital to assess if this isfrom deep recycled material or simply shallow-level contamination during magmagenesis [60,61]or even recent weathering [10]. Conclusively rul-ing out a role of crustal assimilation is di⁄cult,but the potential e¡ects can be minimised by ana-lysing relatively ‘primitive’ samples. In the case ofLi, it is possible to measure the isotope ratio ofthe early crystallising phase olivine, which shouldbe least a¡ected by contamination processes. Theability to analyse olivines is also an asset in study-ing older samples where identi¢cation of possiblealteration in the whole rock is di⁄cult [10]. Fresholivines can be readily picked from many OIB inthe same way that fresh glass can be preparedfrom MORB to produce reliable sample material.Although fractionation of stable isotopes is di-minished at high temperatures, it is important toquantify the possible isotope fractionation in-duced by magmatic processes. Neither variablemelting [62] nor basaltic di¡erentiation processes[63], however, appear able to generate signi¢cantvariations in Li isotope ratios.Li isotope data remain sparse for samples from

our planet’s interior. In the case of MORB, thereare only six measurements of fresh samples in theliterature and these range in N

7Li from 1.5 to4.7x [17,34]. Five of these six measurementsrange from 3.2 to 4.7x, which is within errorof the techniques used in these analyses. A recenthigh-precision study ( R 0.3x) of hand-picked,fresh glasses from the East Paci¢c Rise (EPR)yields a range of N

7Li of 3.1^5.2x [64]. Signi¢-cantly, the Li isotope compositions in this studycorrelate with radiogenic isotope and highly in-compatible element ratios. Covariation of Li iso-topes with 143Nd/144Nd, for example, makes con-tamination an unlikely explanation for the rangein Li isotopes. The EPR dataset supports otherunpublished studies that argue for a ‘depleted’,N-MORB N

7Li of 3^4x [17,65] and signi¢cantLi isotope variations within MORB globally [66].The spread of N7Li in MORB data requires re-

cycled material to be mixed variably back into the

EPSL 7032 17-3-04


upper mantle. This has long been proposed as alogical conceptual corollary of plate tectonics [67].Yet only very recently has the ¢rst unequivocalevidence of this process been presented, in the

form of subtle variations in oxygen isotope ratios[68]. The Li isotope data provide strong addition-al support to this idea.Li isotope analyses of OIB are also scarce. An

Fig. 4. (A) Li isotopic compositions of all published MORB [17,34] and Hawaiian lavas [10,13,21,63]. The published MORB sam-ples are all N-MORB (depleted compositions) and can be compared with other unpublished N-MORB averages [64,65]. The Ha-waiian samples have uncertainties of 0.5x, except for Kilauea where measurements have errors of V1x. MORB samples like-wise have errors of V1x apart from the analyses of [64] which have a reproducibility of 6 0.3x. Some of the Hawaiiansamples reported in [10] are altered and we have only plotted samples that have K/Rb ratios that fall within the range of 380^630, cited as representative of pristine samples [10]. The samples from Mauna Kea have been sub-divided into types 1^3 that dis-play distinct chemical signatures and variable contributions from di¡erent source components (see [10]). (B) N

7Li of clinopyrox-enes from several mantle nodule localities [71] compared to MORB and Hawaiian data reported in A.

EPSL 7032 17-3-04


important OIB datum is that of the internationalstandard BHVO, a recent eruption from Hawaii.It is well established that this basalt has N

7Lis4.7x [10,13,21] and so is heavier than N-MORB.This lonely data point has been considerably aug-mented by further analyses from Hawaii [10,63](see Fig. 4A). The latter study is at higher preci-sion ( R 0.5x compared to typical R 1x of pre-vious studies) and also includes analyses of theolder Koolau lavas. The Koolau lavas are re-markable for an enriched, heavy oxygen isotopecomponent in their source, clearly implicated assedimentary in origin [39]. Additionally samplesfrom Mauna Kea can be sub-divided into groupsthat re£ect di¡erent proportions of distinct com-ponents in their sources. For example, the type 1lavas have high SiO2 and have been argued tocontain recycled, ma¢c oceanic crust whereastype 3 lavas, with the highest 3He/4He, containthe largest proportion of ‘undegassed’ mantle.There is considerable overlap between the N

7Liof MORB and Hawaiian lavas (Fig. 4A). Never-theless, the Hawaiian lavas are on average heavierthan N-MORB. Despite the notable geochemicalcontrasts between the di¡erent Hawaiian compo-sitions (Koolau and various Mauna Kea types)their di¡erences in N

7Li are subtle. It is worthnoting, however, that the Mauna Kea lavas in-voked to contain the largest component of re-cycled ma¢c, oceanic crust (type 1) have heavierN7Li than the other lava types. This is in keepingwith unpublished data [69,70] that documentheavy Li isotope ratios (5^7x) in end-member‘HIMU’ lavas, which are traditionally thoughtto contain large amounts of recycled ma¢c ocean-ic crust [56].Recent analyses of clinopyroxenes from mantle

nodules [71] provide an exciting counter-poise tothe data from mantle-derived lavas (Fig. 4B).Whilst some samples have N

7Li that overlap OIBvalues, the clinopyroxenes from Far East Russiaand southwestern Japan display N

7Li as low as317x. These extremely light Li isotope ratiosimplicate the involvement of material from sub-duction-processed oceanic crust in the sub-conti-nental mantle lithosphere represented by thesenodules [71]. The samples also clearly documentthe possibility for large mantle N

7Li variations. In

a similar vein, Kobayashi et al. [72] report ion-probe data from Hawaiian olivine-hosted melt in-clusions with N

7Li as low as 310x. Nevertheless,this isotopically light component does not appearto represent a signi¢cant proportion of the sourcesince it is not evident in the bulk lava composi-tions (Fig. 4A).

6. Implications

The observations above lead to some valuableinferences. For OIB, like MORB, a range of N7Lipoints to variable amounts of recycled material intheir sources. Very light Li isotope ratios fromsubduction-processed slab is dramatically evidentin some mantle nodules and melt inclusions, butnot in OIB lava compositions. Indeed, samplesproposed to contain a large proportion of ancientbasaltic slab are generally heavier than N-MORB.Likewise there is no evidence of lighter Li in sam-ples frequently linked to a lower mantle reservoir.High 3He/4He samples from Hawaii (type 3Mauna Kea) have marginally higher N

7Li thanN-MORB (Fig. 4A). Thus there is no indicationof the drastic division in Li isotope signature be-tween OIB and MORB that should result fromsubduction zone processing and preferential accu-mulation of subducted slabs in the lower mantle(e.g. [73]).Clearly much more work is required to build on

these initial observations. From the tantalisingglimpse into the mantle provided by the current,we suggest that the trend to heavy N

7Li in OIB(and enriched MORB) sources has a common ori-gin in the down-dragged mantle adjacent to thesubducting slab (Fig. 3A). Fluids released fromthe down-going plate will be heavy, but variablein composition, depending on the prior history ofdehydration. This process could account for thelarge variability in N

7Li not clearly related to oth-er chemical tracers. Material from the slab has topass into a hot portion of the mantle wedge be-fore it triggers large-scale melting. The coldermantle layer overlying the slab will acquire a re-cycled signature, such as heavy N

7Li, from thepassage of slab-derived £uids through it (Fig.3A). The mantle layer may also similarly acquire

EPSL 7032 17-3-04


a sedimentary signature. This overlying mantlewill be viscously coupled to the slab and so car-ried down beyond the subduction zone. As theslab descends and warms, the overlying enrichedlayer will cease to be viscously coupled. Depend-ing on the thermal and dynamic history of theplate, the heavy N

7Li layer of mantle may be car-ried to variable depths to contribute to bothMORB and OIB sources.This scenario of recycling is still highly specu-

lative. What is clear, however, from our currentunderstanding of the behaviour of Li isotopes isthat although there is ample evidence for the roleof recycled material in some OIB and MORBsources, this does not appear to be in the formof subduction zone-processed ma¢c oceanic crust.Interestingly these sentiments are in keeping withother recent discussions on the geochemistry ofOIB [5].

Acknowledgements

The manuscript was considerably improved bythe reviews from Peter van Keken, Paul Tomas-cak and an anonymous reviewer. Comments fromChris Hawkesworth and George Hel¡rich alsohelped point things in the right direction. Theeditorial encouragement and patience of AlexHalliday is gratefully acknowledged. Our develop-ments in Li isotope geochemistry have been gen-erously supported by the Leverhulme Trust, NWOand NERC.[AH]

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EPSL 7032 17-3-04


Tim Elliott graduatedfrom Cambridge in1987 to undertake aPhD at the OpenUniversity in MiltonKeynes, ¢nishing in1991. Having had his¢ll of the delights ofnew towns he left the

country to spend 2 years at Lamont-Doherty GeologicalObservatory, New York, USA and 6 years at the Faculteitder Aardwetenschappen, Vrije Universiteit, Amsterdam, TheNetherlands. He became a lecturer at the Department of EarthSciences, University of Bristol in 1999.Alistair Je¡coate graduated from Royal Holloway College,University of London in 2001. He joined the nascent isotopelab at the Department of Earth Sciences in Bristol where he iscurrently ¢nishing his PhD.

Claudia Bouman graduated from the Uni-versiteit van Utrecht, The Netherlands in1998. She has just submitted her doctoralthesis from the Vrije Universiteit, Amster-dam and currently works as an applicationspecialist for Thermo-Finnigan, Bremen.

EPSL 7032 17-3-04


the terrestrial li isotope cycle: light-weight constraints on mantle convection

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