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Sourcedb.igg.cas.cnInternational Geology Review, Vol. 49, 2007, p. 374–388.
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Review of the Lithium Isotope System as a Geochemical Tracer
YAN-JIE TANG,1 HONG-FU ZHANG, AND JI-FENG YING State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China Lithium isotopes have many advantageous characteristics as geochemical tracers for a wide range of geological processes, covering fluid/melt-mineral interactions from the Earth's surface tothe mantle. The latest developments in Li isotope geochemistry, in terms of measuring techniques,major geochemical reservoirs, the contrasting behaviors during near-surface weathering, and crust/mantle cycling of materials in subduction zones, as well as the signature of convective mantle, andcontrolling factors in Li isotopic fractionation are briefly reviewed in this article. With better quan-tification of partition coefficients (mineral-melt/fluid) and fractionation factors (such as temperature anddiffusion coefficients), the Li isotope system will become a powerful geochemical tracer in future studies. for most mantle minerals (Ryan and Langmuir,1987; Brenan et al., 1998a, 1998b; Seitz and Wood- LITHIUM ISOTOPE GEOCHEMISTRY is a burgeoning land, 2000; Seitz et al., 2004). The large mass differ- research field, and has developed rapidly in recent ences ( 15%) between its two stable isotopes, 6Li years with the advance of mass spectrometric tech- and 7Li (their abundances are 7.5% and 92.5%, niques that have overcome intrinsic difficulties in respectively) produce large isotopic fractionation in isotopic measurement of this light element. It was terrestrial systems, from –20 to +40‰ (Hoefs and first used effectively following the development of a Sywall, 1997; Rudnick et al., 2004), up to 80‰ varia- borate technique for measurement by thermal ion- tion of 7Li/6Li (Rudnick and Nakamura, 2004). Li ization mass spectrometry (TIMS) (Chan, 1987), and isotopes appear to be strongly fractionated in low- further enhanced by the advent of multiple-collector temperature systems and highly variable in altered inductively coupled plasma mass spectrometry oceanic crust, hydrated mantle rocks, and eclogitic (MC-ICPMS) (Tomascak et al., 1999a). There is a slabs (Chan and Edmond, 1988; Chan et al., 1994; growing interest in Li isotope geochemistry as the Decitre et al., 2002; Pistiner and Henderson, 2003; nature of its potential to reveal important information Zack et al., 2003; Moriguti et al., 2004; Rudnick and covering a wide range of geochemical processes, such Nakamura, 2004). With more people becoming inter- as continental weathering (Huh et al., 1998, 2001), ested in Li isotope geochemistry, it is important to alteration of the oceanic crust (Chan et al., 1994, have a comprehensive review of the current research Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 2002a; Seyfried et al., 1998), and crust-mantle cycling state in this area. The purpose of this paper is to of materials (Moriguti and Nakamura, 1998a; Tomas- briefly summarize recent developments in Li isotope cak et al., 1999b, 2002; Zack et al., 2003; Elliott et al., geochemistry, and to better understand its potential 2004; Woodland et al., 2004).
to serve as a geochemical tracer in crust/mantle Lithium has many favorable characteristics as a recycling, mantle metasomatism, and near-surface potential geochemical tracer. For example, the radius of Li+ is similar to that of Mg2+, so it cansubstitute for magnesium in olivine, enstatite, anddiopside (Seitz and Woodland, 2000). This behavior Measurements and Compositions
contrasts with that of the large alkali ions (K, Rb, of Li Isotopes
and Cs) and means that Li behaves like a moder-ately incompatible element during partial melting of Measurement of Li isotopic ratios mantle rocks. Li can be highly soluble in H O-rich Like other stable isotope systems, variations in fluids, but has D values between 0.1 and 1.0 Li isotopes are reported in terms of parts per mildeviations from an international standard and expressed in a delta notation: LITHIUM ISOTOPE SYSTEM δ7Li (‰) = [(7Li/6Li) – 1] × 1000. ICP-MS is close to 0.5‰ (Tomascak, 2004). In con- trast with TIMS, ICP-MS produces a large fraction- The standard is conventionally the U.S. National ation and required correction using bracketing Institute of Standards and Technology (NIST) high- purified Li CO reference material NIST L-SVEC, Other techniques for the measurement of Li which has 7Li/6Li = 12.02 ± 0.03 (Flesch et al., isotopes include secondary ionization mass spec- 1973). In early literature, both δ7Li and δ6Li have trometry (Chaussidon and Robert, 1998) and laser- been used, but increasingly the δ7Li has become excited atomic fluorescence spectroscopy (Smith et standard. In this case, positive values of δ7Li reflect al., 1998). Although these methods have relatively heavier isotopic ratios, in keeping with other stable low precision (1σ standard deviations 3‰), their isotope systems. Measurements reported as δ6Li can in situ spatial analytical ability is unique, and have be readily converted to δ7Li, yielding a value that been used to study minerals from mantle xenoliths generally closely approximates the corresponding (Bell et al., 2004; Hervig et al., 2004; Ottolini et al., 2004; Woodland et al., 2004), pyroxenes from Mass spectrometric techniques for Li isotopic meterorites (Beck et al., 2004; Barrat et al., 2005), measurement in geological materials have been synthetic minerals from experimental studies (Rich- well reported by Chan (2004). The first precise ter et al., 2003; Lundstrom et al., 2005) and glass measurements of Li isotopic ratios were performed inclusions in basalts (Kobayashi et al., 2004).
using TIMS (Chan, 1987; Moriguti and Nakamura,1998b; James and Palmer, 2000a). However, the Li isotopic compositions of major reservoirs TIMS technique is time consuming and suffers from A number of studies, starting with the pioneering a complicated ion exchange procedure to obtain work of Chan et al. (1987), have established that pure Li solutions (Moriguti and Nakamura, 1998b).
large fractionations of Li isotopes occur in the rock- Although Li is efficiently ionized by solid source H O system. The rapidly accumulating data support mass spectrometers, highly reproducible isotopic some general constraints on the Li isotopic charac- ratios are more difficult to obtain by this technique ter of the major geochemical reservoirs, as summa- than for the gas-source machines used to measure rized in Tomascak (2004), which thus far is the most oxygen isotopes. The reproducibility of Li isotope comprehensive review of Li isotope geochemistry, measurements has thus lagged behind oxygen iso-tope measurements, typically 0.5–1‰ compared including data from both published literature and to 0.05‰ (2σ standard deviations). This has ham- meeting abstracts. The ranges of Li isotopic ratios in pered application of Li isotopes to studying mantle major reservoirs are compiled in Figure 1.
processes, where variations in Li isotopic ratios are The mantle, as represented by fresh basalt (i.e., only a few per mil, but it has been sufficiently pre- MORB, and Hawaiian basalt, the later being repre- cise for documenting the larger isotopic variations sentative of OIB), appears to have average δ7Li val- in the near-surface environment. Despite an unsta- ues of about +3 to +5‰ (Fig. 1). The upper mantle ble instrumental fractionation, TIMS can yield very is inferred to have a δ7Li equal to that of average Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 stable isotopic data during a run, and requires no MORB, +4 ± 2‰ (Chan et al., 1992, 2002b; external standard with a proper loading form. As a Tomascak and Langmuir, 1999), because Li isotopes result, a large fraction of high quality data has been do not fractionate significantly at the high tempera- generated by TIMS (e.g., Huh et al., 1998; Moriguti tures of mantle melting, >1000°C (Tomascak et al., and Nakamura, 1998a, 1998b; James et al., 1999; 1999b). However, the mantle is not homogeneous. It James and Palmer, 2000a; Huh et al., 2001; Chan, has been shown by Nishio et al. (2004) that some mantle-derived peridotites have extremely light δ7Li The advent of multiple-collector ICPMS offers values down to –17‰. In situ Li isotopic analysis of the opportunity for small concentrations of Li to be glass inclusions in olivine phenocrysts from Hawai- analyzed to high precision (Tomascak et al., 1999a; ian lavas also shows extremely large δ7Li ranges, Nishio and Nakai, 2002; Bouman et al., 2004; from –11 to +5‰ (Beck et al., 2004). Two oceanic Magna et al., 2004), which is highly significant for island basalts reported by Ryan and Kyle (2004) documenting Li isotopic variations in the mantle.
have heavy δ7Li values (8‰ and 10‰). These The precisions of TIMS and ICP-MS for natural observations indicate the high heterogeneity of the samples are comparable and the 2σ uncertainty of TANG ET AL. FIG. 1. Li isotopic composition of various reservoirs. Data sources: seawater (Chan and Edmond, 1988; You and Chan, 1996; Moriguti and Nakamura, 1998b; Tomascak et al., 1999b; James and Palmer, 2000a; Rudnick et al., 2004);river water (Huh et al., 1998, 2001); high-temperature vent fluids (Chan et al., 1993; Foustoukos et al., 2004; Kisakureket al., 2004); arc lavas (Moriguti and Nakamura, 1998a; Tomascak et al., 2000, 2002; Chan et al., 2002b); oceanic islandbasalts (Tomascak et al., 1999b; Chan and Fery, 2003); fresh MORB (Chan et al., 1992, 2002b; Moriguti and Nakamura,1998a; Tomascak and Langmuir, 1999); altered MORB (Chan et al., 1992, 2002a); marine sediments (Chan et al., 1994;Zhang et al., 1998; James et al., 1999; Chan and Kastner, 2000; Bouman et al., 2004); loess, shales, and upper continen-tal crust (Teng et al., 2004); eclogite data (Zack et al., 2003); peridotite and pyroxenite xenoliths (Tang et al., unpubl.
data; Tomascak, 2004); other reservoirs (Tomascak, 2004). Despite a range beyond cited analytical uncer- partitioned into surface waters, leaving lighter Li in tainties (δ7Li +29.3 to 33.3‰) (Rudnick et al., the weathered residua, which is supported by the 2004, and references therein), seawater has a rela- isotopically heavy compositions of groundwater tively homogenous Li isotopic composition, consis- (Hogan and Blum, 2003), the isotopic fractionation tent with the relatively long residence time of Li in between dissolved and suspended loads of rivers the oceans (Li, 1982), and is isotopically quite (Huh et al., 2001) and experimental data (Pistiner heavy, with an average of δ7Li +32‰ (Chan and and Henderson, 2003). Orogenic eclogites (in the Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 Edmond, 1988; You and Chan, 1996; Tomascak et deep crust) have δ7Li values lighter than the upper al., 1999a; James and Palmer, 2000a). Similarly, crust, which reflect loss of heavy Li during meta- marine sediments have a wide range of δ7Li (–1.6 to morphic dehydration of subducting slab (Zack et al., +25‰) (Chan et al., 1994; Zhang et al., 1998; James 2003). Rivers show a large range in δ7Li, encom- et al., 1999; Chan and Kastner, 2000; Bouman et al., passing mantle and seawater values. The important 2004), whereas most altered basalts and marine sed- processes affecting river-dissolved Li isotopic com- iments have δ7Li values that are generally interme- positions are fractionation between solution and diate between that of the mantle and seawater, due secondary minerals, and thus the intensity of weath- to the uptake of heavier seawater Li. ering (Huh et al., 1998).
Compared to the mantle, the upper continental crust has an even lighter δ7Li of 0 ± 2‰ (Teng et al., Li Isotopes as Geochemical Tracers
2004), obtained by dissolved and suspended riverloads (Huh et al., 2001), shales, loess, and granites.
Li isotopes are significantly fractionated near the This isotopically light composition is inferred to Earth's surface (Huh et al., 2001, 2004; Pistiner and reflect the influence of weathering, with heavy Li Henderson, 2003; Kisakürek et al., 2004; Rudnick LITHIUM ISOTOPE SYSTEM et al., 2004). The legacy of this process provides a The Li isotopic composition of river water is not robust tracer of surface material that is returned highly sensitive to that of the bedrock, in contrast to (recycled) to the mantle (Weaver et al., 1986; radiogenic isotopic ratios used to monitor chemical Brenan et al., 1998b; Moriguti and Nakamura, weathering (Huh et al., 1998). There is thus promise 1998a; Zack et al., 2003; Bouman et al., 2004).
in using the evolution of the Li isotope ratio of Thus, a comprehensive understanding of Li isotope seawater to assess past changes in the intensity of fractionation during weathering is necessary for continental weathering (Raymo et al., 1988). In using Li isotopes to trace chemical cycles and igne- other words, it might be anticipated that a change to ous processes. In this section, the behaviors of Li lighter Li isotopic compositions in ancient oceans isotopes in the near-surface environment are out- would reflect lower weathering rates in the past.
lined in order to understand controls on Li isotope Whereas the general process of Li isotopic fraction- fractionation, and to present geochemical applica- ation at the surface is clear, a more detailed under- tions addressing current geologic problems.
standing of their behavior during weathering,transport, and incorporation into the geological Tracing weathering processes record is required before Li isotopic ratios can be Li has several advantages as an isotope tracer of used as weathering proxy (Henderson, 2002). An weathering. Being a fluid-mobile trace element, Li exciting prospect for the future is attracting an tends to be widely distributed in the Earth's crust; Li increasing number of groups to investigate this is present only in the +1 valence state, so its isotopic potential (Hoefs and Sywall, 1997; Kosler et al., composition is not influenced by redox reaction. 2001; Pistiner and Henderson, 2003; Marriott et al.,2004a). For evaluating their use as a paleo-environ- During weathering, the lighter isotope 6Li is pref- mental indicator, Marriott et al. (2004b) measured erentially retained in the solid phase whilst 7Li goes the δ7Li and Li/Ca ratios in calcite and aragonite as into solution (Huh et al., 1998, 2001, 2004; Pistiner a function of salinity, and concluded that biological and Henderson, 2003; Kisakürek et al., 2004)—i.e., controls did not influence the incorporation of Li heavy Li is leached from rocks due to weathering at into biogenic carbonates. Thus δ7Li in carbonates the Earth's surface. Thus river waters have heavy Li might provide a faithful record of Li isotopic compo- isotopic compositions compared to the original bed- sition of seawater through time (provided post-depo- rock and associated suspended load (Huh et al., sitional processes have not influenced δ7Li).
2001). Moreover, fractionation seems to be only Partitioning of Li between water and clays can slight in stable and warm climates where weathering produce major isotopic fractionation in the marine is slow, but it may become large in tectonically system (Chan et al., 1992; Seyfried et al., 1998; active regions where weathering is rapid (Huh et al., Zhang et al., 1998). Unlike radiogenic isotope 2001, 2004). Fractionation of Li isotopes can also systems (e.g., 87Sr/86Sr), where seawater represents occur during sorption onto mineral surfaces (James an intermediate mix of hydrothermal input (mantle- and Palmer, 2000b; Pistiner and Henderson, 2003).
dominated signature) and river water (continental Different minerals have different 7Li/6Li ratios, so signature), seawater has an Li isotope ratio heavier Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 incongruent weathering can produce significant than its inputs (Fig. 2). This paradox can be isotope shifts (Pistiner and Henderson, 2003; Huh explained by the low-temperature alteration of the oceanic crust to hydrous minerals (such as smec- The behavior of Li isotopes during continental tite). These alteration products are important sinks weathering demonstrates that processes affect the for oceanic Li and also preferentially uptake 6Li isotopic composition of regoliths. For example, from seawater during this process (Chan et al., Kisakürek et al. (2004) showed that a thick laterite 1992, 2002a). Enhanced removal of 6Li from the developed on the basaltic flows contains a oceans results in a heavy seawater composition of significant aeolian input of isotopically light Li, δ7Li +32‰ (James and Palmer, 2000a), heavier presumably derived from the weathering of crystal- than its average input. Although the Li gained by line basement. Rudnick et al. (2004) reported the alteration materials is lighter than mean river that extreme isotopic fractionation (δ7Li down to water, δ7Li < +23‰ (Huh et al., 1998), it is never- –20‰) during the development of saprolite is theless heavier than fresh oceanic basalts, δ7Li accompanied by input of heavier Li from ground +3‰ (Chan et al., 1992; Moriguti and Nakamura, TANG ET AL. FIG. 2. Schematic illustration of Li isotope systematics in the hydrological cycle, modified from Elliott et al. (2004).
Data sources are the same as those identified in Figure 1.
As summarized in Elliott et al. (2004), weather- and magmatic crystallization (Richter et al., 2003).
ing of continental rocks results in the heavy Li (3) Large mass differences between its two isotopes isotopic compositions of river water that feeds the lead to large isotopic fractionations in terrestrial oceans. Low-temperature alteration of oceanic crust systems (Hoefs and Sywall, 1997; Rudnick and then makes seawater heavier than river water (Fig.
Nakamura, 2004), which develop during low-tem- 2). The process of increasing the δ7Li of altered oce- perature alteration associated with water at the anic crust, as part of the low-temperature Li cycle, Earth's surface. In contrast, Li isotopic fractionation showing a distinctive signature to the oceanic plate is extremely small when the temperature exceeds that can be used to trace it. On the other hand, sea- 350°C (Chan et al., 1994; Tomascak et al., 1999b), water Li concentrations and δ7Li show potential as and thus is insignificant in magmatic processes, proxies for global silicate weathering processes, which suggests that Li isotopic composition in inasmuch as rivers presently contribute 50% of the mantle-derived magmas may directly indicate the Li input to the oceans (Kisakürek et al., 2005).
composition of source materials except for latermodification. Hence, the Li isotopic variations in Tracing crust/mantle recycling igneous rocks, especially those derived from the In terms of solid Earth geochemistry, one of the mantle, may indicate the involvement of surface great expectations for the fledging field of Li isotope material subducted into the source regions, preserv- geochemistry is to elucidate the complicated pro- ing the variable Li isotopic compositions imprinted Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 cess of recycling of oceanic crust in subduction by water/rock interactions under near-surface zones. Lithium isotopes have great potential to trace crust/mantle recycling in virtue of unique physico- In contrast to earlier work on the Izu arc chemical characteristics: (1) Li can be concentrated (Moriguti and Nakamura, 1998a), Moriguti et al.
in crust materials compared to mantle materials (2004) reported that d7Li did not change systemati- reflecting its moderate incompatibility during par- cally across the Northeast Japan arc, where all lavas tial melting and fractional crystallization (Ryan and had MORB-like δ7Li values. They attributed the dif- Langmuir, 1987; Brenan et al., 1998a). (2) It is ferences in Li systematics between the Northeast expected that Li is strongly enriched by seawater Japan arc and the Izu arc to differing extents of Li weathering and alteration, and then released in the isotopic fractionation due to differing physical char- subduction zone due to its strong mobility in fluid- acteristics in the subduction zone, such as related processes, such as weathering and alteration the thermal structure of the subducting slab and on the sea floor, during metamorphism in the sub- subduction angle. In a similar study, Leeman et al.
duction zone (Seyfried et al., 1984; You et al., 1996; (2004) also found MORB-like δ7Li across the Brenan et al., 1998b), and attending partial fusion Cascades arc in southern Washington. They attrib- LITHIUM ISOTOPE SYSTEM FIG. 3. Schematic illustration of Li isotope systematics in a subduction-zone setting, modified from Zack et al.
(2003). Heavy Li in fluids from the décollement zone and serpentinite diapers reflect Li isotopic fractionation during thedehydration of altered oceanic crust. The high δ7Li of fluids escaping the slab at low temperatures likely enrich theforearc mantle wedge and hydrate the adjacent mantle in 7Li, which may be the sources of arc lavas. Thus, high δ7Livalues in arc lavas (up to +12‰) might be explained by incorporation of the forearc mantle. Data sources are the sameas those identified in Figure 1.
uted the lack of obvious slab contribution to the high source of the lavas, which were likely introduced temperatures, causing efficient dehydration in the from recycled components. forearc. Evaluating the contribution of slab compo- Up to now, the use of Li isotopes in studying nents to subduction-zone lavas indicates that not island-arc processes is a very new tool in geochem- only chemical characteristics of the subducted slab istry. Most studies have focused on the volcanic out- but also the physical, as well as chemical character- put of Li at subduction zones (Moriguti and istics of the subduction zone could affect the Li Nakamura, 1998a; Chan et al., 2002b; Tomascak et isotopic compositions of the slabs, producing heter- al., 2002), and several articles on the slab input of ogeneity of the mantle. Li—i.e., marine sediments and the altered down- Assuming that isotopically fractionated Li can going oceanic crust (Bouman et al., 2004; Elliott et Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 traverse the subduction zone, one might see vari- able δ7Li in intraplate magmas that are suggested to Altered oceanic crust has a heavy Li isotopic sample ancient, recycled oceanic lithosphere. Ryan composition. Heterogeneous distribution of sub- and Kyle (2004) found intraplate basalts with ducted, altered oceanic crust in the mantle will MORB-like δ7Li in Antarctica, but δ7Li changed result in the heterogeneity of Li isotopic ratios. The during differentiation. Because Li does not frac- accumulating Li isotope dataset of mantle-derived tionate during crystal fractionation (Tomascak et materials indicates a significant range in Li isotopic al., 1999b), they suggested that the changing isoto- ratios, which provides powerful evidence for the pic composition reflects assimilation of lithospheric widespread distribution of recycled material in the Li (Ryan and Kyle, 2004). In contrast to the rather convecting mantle. However, the passage of Li from constant δ7Li values seen in the whole-rock data, surface to depth is not simple and is strongly affected widely varying δ7Li (–10 to +8‰) in glass inclu- by the subducting processes (Fig. 3). Water is suc- sions in olivines from Hawaiian lavas was reported cessively released during subduction, first by poros- (Kobayashi et al., 2004). The large isotopic varia- ity reduction during compaction and then through a tions were attributed to heterogeneities in the series of prograde dehydration reactions as the TANG ET AL. descending slab becomes hotter (Kerrick and Con- subducted deeply and may form a distinct mantle nolly, 2001a, 2001b). As in the near-surface environ- reservoir that could be sampled by plume-related ment, the partitioning of Li between residual solid and liquid will result in isotopic fractionation (Chan The signatures of Li isotopic compositions in the and Kastner, 2000). Although isotopic fractionation subduction zone can be used to trace mantle con- diminishes with increasing temperature, there is still vection. Deep-subducted altered oceanic crust significant inferred (Chan et al., 1993; Huh et al., becomes isotopically light, while the mantle adja- 1998) and measured (Seyfried et al., 1998) Li isoto- cent to the slab becomes heavy (Fig. 3). Conse- pic fractionation between minerals and water during quently, there are two influxes to the mantle because hydrothermal circulation at 350°C. Fluids, which of the subduction, which have isotopic signatures are expelled during subduction and so-called décol- different from the mantle itself. Inasmuch as Li lement, are isotopically heavier than the associated isotopes are stable, these signatures of Li isotopic sediments (Chan and Kastner, 2000). Analyses of compositions can be associated with low-tempera- serpentinites show a wide range of δ7Li (Ryan et al., ture processes near the surface of the Earth. As a 2001), which clearly indicates that heavy subducted result, Li isotopes can provide an important Li shifts to the oceans and shallow mantle wedge, approach to trace geochemical processes in the sub- but the amount of Li shifted from the slab in the duction zone and corresponding mantle convection. early dehydration processes is estimated to be a Recent research showed that old, altered oceanic minor fraction of the total Li subducted (Chan and crust is isotopically lighter than its younger counter- Kastner, 2000). The forearc mantle wedge may form part, which may reflect exchange with overlying a sink for the high δ7Li released from the subducted sediments, or possibly a change in δ7Li of sea water oceanic crust (Tomascak et al., 2002; Zack et al., over time (Bouman et al., 2004). Chan et al. (2002a) 2003), which may be the source for serpentinites presented Li concentrations and isotopic profiles with high δ7Li. through the upper 2 km of the oceanic crust (Fig. 4); A few arc lavas have Li isotopic ratios heavier those profiles record alteration conditions and the Li than MORB (Tomascak et al., 2002) (Fig. 1), which exchange processes as a function of depth. In the implies that little of the budget of heavy Li in the upper part of the section, interaction with seawater subduction zone is lost to the surface. This can be at low temperature leads to enrichment of Li and related to the relative affinity of Li for mantle heavier isotopic composition due to incorporation of phases. Due to its similar ionic radius to Mg, Li can Li into secondary phases. The rocks from the lower occupy the abundant Mg lattice sites available in portion fall in the domain of unaltered MORB due to the mantle, although experiments show that Li is restricted fluid flow and rock-dominated conditions.
moderately incompatible in melt/fluid (Brenan et The transition zone is enriched in Li but has a light al., 1998a, 1998b). Thus heavy Li, initially carried isotopic composition resulting from precipitation from the slab in aqueous fluids, isotopically re- from mixtures of seawater and Li-rich hydrothermal equilibrates with the mantle (Tomascak et al., fluids. Sheeted dikes exhibit decreasing Li concen- 2002). The heavy Li isotopic signature from the slab trations with depth. Interaction of seawater-derived Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 is effectively transferred to the cold portion of man- hydrothermal fluids at high temperatures and low tle lying above the slab (Fig. 3).
water/rock ratios results in depletion of Li and light The extremely light Li isotopic ratios in Alpine isotopic compositions in greenschist- and sub- eclogites provide evidence for the loss of heavy Li amphibolite–facies rocks. The range of δ7Li may be from the subducted oceanic crust during dehydra- explained by varying fluid compositions, tempera- tion (Fig. 3). Zack et al. (2003) argued that the low ture-dependent isotopic fractionations, and water/ δ7Li values were likely produced by isotopic frac- rock ratios. Crustal rocks of different alteration tionation through Rayleigh distillation during dehy- zones thus exhibit distinct characteristics in Li dration of clays or chlorite at early stages of abundances and isotopic compositions, making Li a metamorphism. The Alpine eclogites thus represent sensitive indicator of the conditions and history of the light complement of the heavy isotopic signature that left the slab to reside in the overlying mantle.
These suggestions are consistent with isotopically Signature of the convective mantle heavy Li being released into the forearc mantle Understanding material cycles in the Earth wedge, whereas an isotopically light component is provide a better opportunity to study the nature and LITHIUM ISOTOPE SYSTEM FIG. 4. Variation of Li concentration (A) and isotopic ratios (B) with depth for whole rocks and smectites from DSDP/ ODP Sites 504B and 896A, shown with lithologic sections from Chan et al. (2002a). Also shown are concentration datafor the transition zone from Mengel and Hoefs (1990) and at 1570 mbsf from Zuleger et al. (1996).
origin of the chemical heterogeneity of the mantle (Seitz et al., 2004). The degree of intramineral frac- shown in Figure 5. Materials at the Earth's surface ) also negatively correlates with undergo various physicochemical processes, such as equilibration temperature, suggesting that Li water/rock interaction underneath the ocean floor isotope fractionation may occur at high magmatic and subsequent dehydration/partial melting during temperatures (950–1150° C). Another study infers subduction (Moriguti and Nakamura, 1998a; that an anhydrous EM1-like metasomatic agent may Seyfried et al., 1998; Chan et al., 2002a; Zack et al., have an extremely low δ7Li value, whereas a 2003). Water-soluble elements (such as Li) during hydrous EM2-like agent may have a positive d7Li dehydration processes are preferentially released to (Nishio et al., 2004) (Fig. 6). Due to the loss of metasomatize the overlying mantle wedge, which is heavier Li from the subducted slab during low-tem- the source region for island-arc magma (Ishikawa perature dehydration, the δ7Li value of subducted, and Nakamura, 1994; Ryan et al., 1995). The slab is highly altered MORB would be extremely low com- finally introduced into the mantle and may form a pared to that of fresh MORB. Thus, the metasomatic chemical anomaly, which could eventually become agent with extremely low δ7Li is derived from sub- the source region for mantle plumes expressed at the ducted, highly altered basalt (Fig. 6). The enrich- Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 Earth's surface as ocean island basalts (Hofmann ment of isotopically light Li may be a feature of the and White, 1982). Unlike radiogenic isotopic ratios, EM1 mantle reservoir. Because of apparent sensitiv- the Li isotopic ratio is not affected by time or parent/ ity of the Li isotopic composition to the alteration daughter fractionation. Therefore, it is expected that profile of subducted MORB, it may provide comple- Li isotopic compositions provide complementary mentary information to Sr, Nd, and Pb isotopic com- information to familiar radiogenic isotopes.
positions regarding the mantle source.
The Li isotopic signatures of minerals in mantle- MORB and OIB represent primary samples of derived xenoliths have provided direct information the mantle (Hofmann, 1997). MORB are derived on the evolution of lithosphere. Values of δ7Li vary from shallow (dominantly less than 60 km) decom- systematically between minerals; olivine has the pression melting of the mantle, whereas OIB can be heaviest δ7Li (+1.4‰ to +4.5‰), followed by ortho- generated by upwelling, mostly related to mantle pyroxene (–1.0‰ to +3.9‰), and clinopyroxene has plumes, which have originated from a deeper mantle the lightest (–2.4‰ to +3.0‰), whereas whole-rock than that of MORB. Generally, a deep mantle layer δ7Li, from +1‰ to +4‰, correlates negatively with is fed by subducted slabs and ultimately forms the the degree of metasomatism of mantle xenoliths source for OIB (Tackley, 2000). Based on the behavior TANG ET AL. FIG. 5. Volumes of δ7Li for clinopyroxenes from different world localities compared with published MORB and Hawaiian data. Data sources: Hawaiian lavas (Tomascak et al., 1999b; James and Palmer, 2000a; Chan and Fery, 2003;Pistiner and Henderson, 2003); N-MORB (the published MORB data are N-MORB–depleted compositions) (Chan et al.,1992; Moriguti and Nakamura, 1998a); clinopyroxenes in world xenoliths (Seitz et al., 2004); clinopyroxenes from theNorth China craton (Tang et al., 2006; unpubl. data); other data (Nishio et al., 2004).
of Li in the subduction zone, the lower mantle would It is obvious that further work is required to build possibly evolve toward a lighter Li isotopic composi- on the above observations. From the primary under- tion than the upper mantle. Thus OIB should theo- standing of the mantle based on the current data, the retically have a lighter Li isotope ratio than MORB.
sources of OIB and enriched MORB (heavy in δ7Li) Therefore, the spread of δ7Li in MORB data (Fig. 5) should have a common origin in the down-dragged requires recycled materials, with lighter Li isotopic mantle adjacent to the subducting slab (Fig. 3). Flu- compositions, to be mixed with the upper mantle.
ids released from the downgoing plate will be heavy, This has long been proposed as a logical corollary of but variable in composition, depending on the prior plate tectonics (Allégre and Turcotte, 1986), and has history of dehydration. This process could account been recently corroborated by the subtle variations for the large variability in δ7Li not clearly related to in oxygen isotopic ratios (Eiler et al., 2000). Based other chemical tracers (Tomascak et al., 2002).
on the behavior of Li isotopes in the subduction Material from the slab has to pass into a hot portion zone, the idea of recycled material is strongly sup- of the mantle wedge before it triggers large-scale ported as well.
melting. The colder mantle layer overlying the slab Li isotope analysis of OIB from Hawaiian lavas will acquire a recycled signature, such as heavy Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 (Tomascak et al., 1999b; James and Palmer, 2000a; δ7Li, from the passage of slab-derived fluids through Chan and Fery, 2003; Pistiner and Henderson, it (Fig. 3). Similarly, the mantle layer may also 2003) has demonstrated that they are slightly acquire a sedimentary signature. This overlying heavier than N-MORB, although there is large over- mantle will be viscously coupled to the slab, and so lap between the δ7Li values (Fig. 5). New data for carried down beyond the subduction zone. As the clinopyroxenes from xenoliths (Nishio et al., 2004; slab descends and warms, the overlying enriched Seitz et al., 2004) provide an important supplement layer will cease to be viscously coupled. Depending to the dataset of mantle-derived lavas. Despite some on the thermal and dynamic history of the plate, the samples possessing δ7Li values that overlap those of heavy δ7Li layer of mantle may be carried to vari- OIB, the clinopyroxenes from East Russia and able depths to contribute to both MORB and OIB Southwest Japan display δ7Li as low as –17‰.
sources. This scenario of recycling is highly specu- These extremely light Li isotopic ratios implicate lative. What is clear, however, from our current the involvement of subducted oceanic crust in the understanding of the behavior of Li isotopes, is that subcontinental mantle lithosphere represented by there is ample evidence for the role of recycled these xenoliths (Nishio et al., 2004).
material in some OIB and MORB sources.
LITHIUM ISOTOPE SYSTEM FIG. 6. Variation of Li concentrations vs. Li isotopic ratios. The black arrow shows the trend for alteration of oceanic crust, and the grey arrow depicts the dehydration trend of subducted oceanic crust. Data sources: fresh MORB (Morigutiand Nakamura, 1998a; Chan et al., 2002a, 2002b); altered MORB (Chan et al., 1992); dehydrated slab (i.e., eclogite)(Zack et al., 2003); arc lavas (Moriguti and Nakamura, 1998a; Tomascak et al., 2000, 2002; Chan et al., 2002a, 2002b);chondrite range (McDonough et al., 2003; Tomascak, 2004); minerals in peridotitic xenoliths from Hannuoba, NorthChina craton (Tang et al., 2006); clinopyroxenes in hydrous and anhydrous xenoliths (Nishio et al., 2004).
Controlling Factors in
seawater and oceanic crust, slab and fluid, and Li Isotopic Fractionation
intramineral isotopic fractionations between mantleminerals probably generated by diffusion during Temperature may be a key factor of Li isotopic mantle melting due to different partition coefficients fractionation (Seyfried et al., 1998; Tomascak et al., of Li in crystal-melt/fluid systems (Brenan et al., 1999b; Coogan et al., 2005). According to experi- 1998a). As a result, effective diffusion coefficients ments involving Li isotopic fractionation between of Li in water, minerals, or rocks may constrain the clinopyroxene and Cl- and OH-bearing aqueous isotopic fractionation of Li associated with diffusion Downloaded By: [Chinese Academy of Sciences] At: 07:21 20 December 2010 fluids between 500 and 900°C at 2.0 GPa (Wunder (Barrat et al., 2005; Richter et al., 2006; Teng et al., et al., 2005), 7Li is always preferentially partitioned into the fluid, and the isotopic fractionation of Li is Teng et al. (2006) found Li isotopic fractionation temperature-dependent and approximated by the accompanying Li diffusion from an Li-rich pegma- = –4.61 × (1000/T [K]) + 2.48 tite into country rocks. They attributed extreme frac- (R2 = 0.86). This equation may be roughly applica- tionations ( 30‰) to the large differences in ble for quantification of fluid-rock interaction where diffusion coefficients between 6Li and 7Li, and Li Li in minerals resides in octahedral coordination concentrations between the pegmatite and country (such as, pyroxene, amphibole, mica, and chlorite) rocks. Experimental results of kinetic isotopic frac- (Wunder et al., 2005). tionation during diffusion of ionic species in water As discussed above, heterogeneity of lithium (Richter et al., 2006) show that isotopic fraction- isotopes during crust-mantle cycling is mainly pro- ation in water is much smaller than that in molten duced by hydration-dehydration processes, which silicate liquids (Richter et al., 2003). They consid- may be related to different mechanisms of isotopic ered that the distinction between water and silicate fractionation, such as isotopic fractionation between liquids is that water surrounds dissolved ions with TANG ET AL. hydration shells, which likely play an important but active isotopes such as Sr, Nd, Pb, Os, and Hf in still poorly understood role in limiting the isotopic fractionation associated with diffusion (Richter etal., 2006).
Recently, we obtained Li isotopic compositions of minerals in a suite of spinel peridotitic xenoliths Financial support is acknowledged from the from the Hannuoba Cenozoic basalts, North China National Science Foundation of China (grants craton. The data (Fig. 6) show that the olivine sepa- 40534022, 40225009, and 40503004).
rates have positive δ7Li values (+3.3 to +6.4‰). Incontrast, the pyroxenes have low and negative δ7Li (–3.3 to –8.2‰ for clinopyroxene, and –4.0 to –6.7‰ for orthopyroxene), which require a metaso-matic agent with low δ7Li values. Considering that Allegre, C. J., and Turcotte, D. L., 1986, Implications of a the highly altered oceanic crust preserves δ7Li as 2-component marble-cake mantle: Nature, v. 323, p.
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