Li Partitioning Research Articles

Controls on Li partitioning and isotopic fractionation in inorganic calcite

The δ7Li of marine carbonates has been interpreted as an archive of the evolution of seawater δ7Li, and therefore continental weathering, through geological time. However, little is known about the incorporation of Li into calcium carbonate minerals and, consequently, the controls on Li partitioning (DLi) and isotopic fractionation (Δ7Lisolid-fluid) associated with Li incorporation. Crucially, we lack a fundamental understanding of how Li partitioning and Δ7Lisolid-fluid change in response to the chemical and physical conditions of crystal formation. Here, we present DLi and Δ7Lisolid-fluid data from a series of inorganic calcite precipitation experiments where temperature, and solution pH and dissolved inorganic carbon (DIC) were independently varied. We find DLi values in the range 0.8–1.5 × 10−3, which show no relationship with temperature, a strong positive correlation with pH, and a weak positive correlation with DIC. At face value, these patterns are inconsistent with the results of previous precipitation studies. However, the correlations with pH and DIC are consistent with a strong precipitation rate control on DLi that aligns well with previous data, with a likely secondary influence from the incorporation of Li-HCO30 ion pairs from solution. We find Δ7Lisolid-fluid values in the range −6 to −2 ‰, which show no relationship with temperature or pH, and a weak positive correlation with DIC and crystal precipitation rate. These results do not agree with previously published data. Considered alongside previously published data, we observe no consistent relationship between Δ7Lisolid-fluid and any reported physical or chemical experimental parameter, highlighting the need for substantial further work to determine whether systematic controls on Li isotopic fractionation exist in carbonate minerals, and whether they may be environmentally significant.

The effect of solution chemistry on elemental and isotopic fractionation of lithium during inorganic precipitation of calcite

The elemental and isotopic fractionation of lithium between inorganic calcite and aqueous solutions has been measured as a function of solution chemistry in mixed CaCl2-NaHCO3 aqueous solutions spanning a range of pH from ~6 to ~9.5 at room temperature. Vaterite added to the solution recrystallized to calcite with limited change in solution chemistry. With this experimental design solution pH and Na/Ca strongly correlate. The ratio of the concentration of Li in the calcite to that of the solution (Kd,Li = [Li]calcite/[Li]solution), increased by about an order of magnitude with increasing pH. The Li exchange coefficient (Kd,Li/Ca=([Li]calcite/[Ca]calcite)/([Li]solution/[Ca]solution) decreased by about four orders of magnitude, from ~10−2 to ~10−6 with increasing pH due to the large decrease in solution Ca content with increasing experiment pH. The Li isotope fractionation also changed systematically as the pH of the solution increased from 6.5 to 9.5 with the calcite ranging from about −6‰ to +2‰ relative to the solution it grew from. No change in either the elemental or isotopic partitioning was observed with either a change in the Li concentration of the solution or with a change in the experimental duration (5–400 days). To better understand the role of diffusive Li isotope fractionation a series of experiments were performed to determine the relative diffusivity of 6Li and 7Li in solutions with different pH. The results suggest little or no difference in relative diffusivity (D6Li ~ 0.998D7Li) across the range of solution chemistry considered (pH 4 to ~7.5). The change in Li partitioning between calcite and solution with changing solution chemistry is consistent with Li incorporation into calcite as LiHCO3 and hence a dependence on solution H+/Ca2+. If this interpretation is correct, it means that the concentration of Li in inorganic calcite precipitated from seawater at fixed Ca and Li concentrations will increase with decreasing pH. The change in the Li isotope fractionation factor with changing solution chemistry may either reflect a kinetic isotope fractionation associated with changing calcite growth rates or a change in the equilibrium fractionation factor due to changing Li speciation in the solution. These results, while not directly applicable to pristine biogenic calcite, have important implications for interpreting the Li content of calcite that has undergone any diagenetic modification (e.g., all bulk carbonate Li isotope data). Diagenesis under variable conditions provides a plausible alternative explanation for Li isotope excursions in carbonate sediment sequences that have been interpreted as indicating changing seawater δ7Li that deserves further investigation.

Li Partitioning Into Coccoliths of Emiliania huxleyi: Evaluating the General Role of “Vital Effects” in Explaining Element Partitioning in Biogenic Carbonates

AbstractEmiliania huxleyi cells were grown in artificial seawater of different Li and Ca concentrations and coccolith Li/Ca ratios determined. Coccolith Li/Ca ratios were positively correlated to seawater Li/Ca ratios only if the seawater Li concentration was changed, not if the seawater Ca concentration was changed. This Li partitioning pattern of E. huxleyi was previously also observed in the benthic foraminifer Amphistegina lessonii and inorganically precipitated calcite. We argue that Li partitioning in both E. huxleyi and A. lessonii is dominated by a coupled transmembrane transport of Li and Ca from seawater to the site of calcification. We present a refined version of a recently proposed transmembrane transport model for Li and Ca. The model assumes that Li and Ca enter the cell via Ca channels, the Li flux being dependent on the Ca flux. While the original model features a linear function to describe the experimental data, our refined version uses a power function, changing the stoichiometry of Li and Ca. The version presented here accurately predicts the observed dependence of DLi on seawater Li/Ca ratios. Our data demonstrate that minor element partitioning in calcifying organisms is partly mediated by biological processes even if the partitioning behavior of the calcifying organism is indistinguishable from that of inorganically precipitated calcium carbonate.

Fluid-melt trace-element partitioning behaviour between evolved melts and aqueous fluids: Experimental constraints on the magmatic-hydrothermal transport of metals

The fluid-melt partition coefficients of ten trace metals (Li, Mn, Cu, Zn, Rb, Sr, Mo, W, and Pb, along with K) between hydrous evolved melts and coexisting Cl-bearing fluid(s) have been experimentally determined at 850–1050 °C, 0.6–2.2 kbar, and ƒO2 ≈ NNO –0.5 to NNO +2 log units. The measured compositions of the quenched fluid phases vary from low salinity (~2 wt% Cl−) to >19 wt% Cl−, with maximum starting fluid concentrations of >21 wt% Cl. The results of 15 partitioning experiments confirm that Dmetalfluid/melt values depend strongly on the melt and fluid composition, with the greatest metal affinities for the fluid phase shown in the experiments with high-Cl fluids, and the most fractionated melts. Five experiments conducted in Au96Cu4 capsule alloy show DCufluid/melt values ranging from ~40 to >80, and a strong correlation with melt FeO wt% contents at equivalent fluid Cl− contents. In contrast, Li shows only weak partitioning in favour of coexisting fluids across all melt compositions, with DLifluid/melt ranging from 0.16 in low salinity fluids to 1.17 in the highest Cl fluid (19.15 wt% Cl−).A procedure was employed to redissolve and analyse the aluminosilicate precipitates that form during quenching of the experiments. This procedure allows for metal contents of the final quench fluids to be constrained by both mass balance and direct measurement by solution ICP-MS analysis. A comparison of the mass balance calculated vs. directly measured fluid-melt partition coefficients generally gives good agreement, within an order of magnitude for most trace elements. However, discrepancies between these methods arise due to errors inherent in the mass balance calculations for metals with low abundances in the fluids (e.g. Sr), and especially those showing affinities to alloy with the experiment capsule material (e.g. Mo). These discrepancies in constraining experimental fluid trace element budgets also suggest that the precious metal capsule alloys cannot always be considered completely inert containers in experimental investigations, and semi-quantitative LA-ICP-MS analysis of the raw capsule alloys confirms the detectable presence of several of the trace elements investigated here.Integrating these new partitioning data with previous experimental and natural studies confirms that aqueous Cl-bearing fluids exsolved by hydrous calc-alkaline melts stored at mid- to upper-crustal conditions are capable of efficiently sequestering and transporting some economically important trace metals (e.g. Mn, Cu, Zn). However, the new fluid/melt Li partitioning data, in conjunction with evidence of very late (post-eruptive) Li modification, suggests that Li is moderately-strongly fluid-immobile at depth and suffers from significant post-eruptive re-equilibration. This has implications for deciphering the signatures of ‘volatile fluxing’ in arc magmas that are preserved in mineral and melt inclusion trace element concentrations, and the development of mineralised stockwork regions in porphyry-style metal deposits.

Reconciling Li and O diffusion in zircon with protracted magmatic crystal residence

Lithium and O in zircon have strong potential as diffusion chronometers for assessing thermal histories of magmatic and metamorphic systems, but their diffusion mechanisms remain controversial. Zircon populations erupted from El Chichon volcano, Chiapas, Mexico, often contain xenocrysts of detrital origin with juvenile overgrowth forming a natural diffusion boundary within individual crystals. High-spatial resolution O and Li isotope analyses, U–Pb and U–Th geochronology, and Li (along with Al and Y) scanning ion imaging were acquired to compare diffusion model timescales with those derived from radiometric dating. Oxygen isotopes in El Chichon zircon xenocrysts preserve source heterogeneities, with xenocrystic and juvenile domains of zircon being in O-isotopic disequilibrium with the melt at the time of eruption. Preservation of these heterogeneities is consistent with recently determined O-isotope diffusion parameters when crystal residence at magmatic temperatures is assumed over the 103–104 year timescales indicated by U–Th geochronology. Lithium concentration gradients across the same boundaries are sharp within the ~ 2–5 µm lateral resolution of the ion beam, whereas Li isotopic compositions for xenocrystic cores are homogeneous at δ7Li = + 7.8 ± 2.3‰ (1 standard error) and different from their putative source. Diffusive equilibration of Li isotopes in zircon is consistent with protracted magmatic crystal residence, but preservation of Li abundance heterogeneities is difficult to reconcile with evidence for recurrent high-temperature exposure of zircon in a highly active magmatic system. We speculate that Li partitioning in zircon is in part coupled to slow-diffusing trivalent cations, which can cause severe underestimation of diffusion timescales extracted from Li diffusion modeling.

Effect of growth rate and pH on lithium incorporation in calcite

Abstract Carbonates are only a minor sink of oceanic lithium, yet the presence of this element and its abundance relative to other metal cations in natural carbonate minerals is routinely used as a paleo-environmental proxy. To date, however, experimental studies on the influence of physicochemical parameters that may control lithium incorporation in calcite, like pH and precipitation rate, are scarce. Therefore, we experimentally studied Li incorporation in calcite to quantify the apparent partitioning coefficient ( D Li ∗ = c Li / c Ca calcite m Li + / m Ca 2 + solution ) between calcite and reactive fluid as a function of calcite growth rate and pH. The obtained results suggest that D Li ∗ increases with calcite growth rate, according to the expression: L o g D Li ∗ = 1.331 ( ± 0.116 ) × L o g R a t e + 6.371 ( ± 0.880 ) ( R 2 = 0.87 ; 10 - 8 . 1 ≤ R a t e ≤ 10 - 7 . 1 m o l m - 2 s - 1 ) Additionally the experimental results suggest that D Li ∗ values exhibit a strong pH dependence. For experiments conducted at similar growth rates (i.e. Rate = 10−7.7±0.2 mol m−2 s−1), D Li ∗ decreases with increasing pH as described by: L o g D Li ∗ = - 0.57 ( ± 0.047 ) × p H + 0.759 ( ± 0.366 ) ( R 2 = 0.90 ; 6.3 p H 9.5 ) The positive correlation of D Li ∗ with calcite growth rate is consistent with an increasing entrapment of traces/impurities at rapidly growing calcite surfaces, although the incorporation of monovalent cations such as Li+ and Na+ does not necessarily imply a substitution of Ca2+ ions in the calcite crystal lattice. The dependence of D Li ∗ on pH can be considered as an indication that activity of aqueous HCO3− controls the incorporation of Li+ in calcite. The proposed coupled reaction can be explained by charge balance of these monovalent species, which is likely valid at least during the initial step of adsorption on the crystal surface. These new findings shed light on the mechanisms controlling Li incorporation in calcite and have direct implications on the use of Li partitioning coefficients in natural carbonates as an environmental proxy.

Partitioning and isotopic fractionation of lithium in mineral phases of hot, dry rhyolites: The case of the Mesa Falls Tuff, Yellowstone

Studying the inventory and distribution of lithium (Li) in rhyolitic magmas, and the potential Li isotope fractionation during progressive crystallisation is crucial to understand the formation of economic Li deposits. Lithium concentrations and isotopic compositions of co-existing groundmass glass, quartz, sanidine, plagioclase, clinopyroxene, fayalite, and orthopyroxene from the Mesa Falls Tuff (Yellowstone) were determined using LA-ICP-MS and MC-ICP-MS. The highest Li contents were obtained from quartz-hosted melt inclusions (avg. 244 ppm) and groundmass glass (avg. 43 ppm). Lithium concentrations are lower in mineral phases, decreasing from quartz (avg. 18 ppm), plagioclase (avg. 17 ppm), fayalite (avg. 16 ppm), clinopyroxene (avg. 11 ppm), orthopyroxene (avg. 8 ppm) to sanidine (avg. 6 ppm). Lithium isotopic compositions of bulk tuff samples and major mineral phases reveal a range in δ7Li values (per mil deviation of 7Li/6Li ratio relative to L-SVEC reference material), spanning ~10‰ between the co-existing phases and decreasing from groundmass glass (6.5–7.5‰) to feldspars (plagioclase from −2.3 to −1.4‰; sanidine from −0.6 to −0.1‰). Lithium concentration profiles across sanidine and plagioclase crystals show a decrease towards the crystal rims, which is not observed in the other mineral phases of the Mesa Falls Tuff. The variations identified in feldspar are decoupled from both the major and all other trace element compositions, precluding an origin related to mafic recharge and magma mixing. We propose that they result from Li loss from the melt to a vapour or fluid phase taking place immediately prior to, or synchronously with, eruption. This is supported by the observation that Li contents in quartz-hosted melt inclusions are a factor of six greater than those found in groundmass glass while the concentrations of immobile trace elements are identical. The low-Li rims on sanidine and plagioclase crystals observed in the high-silica Mesa Falls Tuff rhyolite are potential candidates to preserve Li isotopic disequilibrium as has been observed in less evolved systems. The variation in Li abundances within feldspar crystals hints at the potential for kinetic isotopic fractionation to play a role in the reported isotopic compositions of bulk separates. The large elemental database of Li concentrations from the MFT allows us to calculate apparent partition coefficients between groundmass glass and co-existing minerals. These data are compared to fifteen other ignimbrites from the Yellowstone-Snake River Plain province to provide a coherent picture of Li partitioning in hot and dry rhyolites with Li most compatible in quartz and olivine, and least compatible in sanidine.

Metasomatism in the sub-continental lithospheric mantle beneath the south French Massif Central: Constraints from trace elements, Li and H in peridotite minerals

Mantle metasomatism by percolating melts/fluids can significantly modify the geochemical and mineralogical compositions of the sub-continental lithospheric mantle (SCLM). We present a detailed study of water contents and Li concentrations and isotopic compositions in mantle minerals from a suite of peridotite xenoliths entrained by a Cenozoic Strombolian volcano in the southern French Massif Central (FMC). Wide ranging clinopyroxene trace element distributions (e.g., (La/Yb)N from 0.25 to 22.21; Ti/Eu ratios from 453 to 4892) suggest that the SCLM has undergone metasomatism by carbonatitic melts/fluids or melts/fluids related to subducted materials. Two amphibole-bearing samples exhibit depletion of light rare earth elements (LREE; (La/Yb)N=0.26 and 0.30, respectively) in amphiboles, similar to that in co-existing clinopyroxenes; these samples indicate that amphiboles grew during a separate modally metasomatic event predating the cryptic metasomatism accounting for LREE enrichment and negative HFSE anomalies in other samples. Mineral Li concentrations are similar to those in the normal mantle, with inter-mineral Li partitioning nearly equilibrated and intragranular Li distributions nearly homogeneous. However, negative δ7Li values of pyroxenes in some samples (as low as −8.8‰ in clinopyroxene of sample MC38) can be attributed to diffusive exchange with a small-volume melt of moderate Li concentration and light Li isotopic composition, originally associated with a recycled component. Preservation of the currently observed large inter-mineral Li isotopic variations indicates that melt percolation occurred shortly before entrainment of the peridotite xenoliths by the host magma. Mineral water contents vary from 41 to 428ppm in clinopyroxenes and from 28 to 152ppm in orthopyroxenes, and their roughly negative co-variation with coexisting olivine Fo contents imply that partial melting was the main control over mineral water content variations in most samples. Varied water contents in LREE-enriched metasomatized samples indicate the involvement of metasomatic agents of different origins. The aqueous agent responsible for generation of amphiboles in two samples did not produce a notable increase in the water contents of coexisting nominally anhydrous minerals.

Constraining late stage melt-peridotite interaction in the lithospheric mantle of southern Ethiopia: evidence from lithium elemental and isotopic compositions

Lithium (Li) elemental and isotopic compositions for mineral separates of coexisting olivine, orthopyroxene and clinopyroxene of mantle xenoliths from the Quaternary volcanic rocks of southern Ethiopian rift (Dillo and Megado) reveal the influence of late stage melt-peridotite interaction on the early depleted and variably metasomatized lithospheric mantle. Two types of lherzolites are reported (LREE-depleted La/Sm(N) = 0.11–0.37 × Cl and LREE-enriched, La/Sm(N) = 1.88–15.72 × Cl). The depleted lherzolites have variable range in Li concentration (olivine: 2.1–5.4 ppm; opx: 1.1–2.3 ppm; cpx: 1.0–1.8 ppm) and in Li isotopic composition (δ7Li in olivine: −9.4 to 1.5‰; in opx: −4.5 to 3.6‰; in cpx: −17.0 to 4.8‰), indicating strong disequilibrium in Li partitioning and Li isotope fractionation between samples. The enriched lherzolites have limited range in both Li abundances (olivine: 2.7–3.0 ppm; opx: 1.1–3.1 ppm; cpx: 1.1–2.3 ppm) and Li isotopic compositions (δ7Li in olivine: −1.3 to +1.3‰; in opx: −2.0 to +5.0‰; in cpx: −7.5 to +4.8‰), suggest that the earlier metasomatic event which lead to LREE enrichment could also homogenize the Li contents and its isotopes. The enriched harzburgite and clinopyroxenite minerals show limited variation in Li abundances and variable Li isotopic compositions. The Li enrichments of olivine and clinopyroxene correlate neither with the incompatible trace element enrichment nor with the Sr-Nd isotopic compositions of clinopyroxene. These observations indicate that the metasomatic events which are responsible for the LREE enrichment and for the Li addition are distinct, whereby the LREE-enrichment pre-dates the influx of Li. The presence of large Li isotopic disequilibria within and between minerals of depleted and enriched peridotites suggest that the lithospheric mantle beneath the southern Ethiopian rift has experienced recent melt-peridotite interaction. Thus, the Li data set reported in this study offer new additional evidence for the existence of late stage metasomatism, which probably occurred at shallow depth briefly before and/or during entrainment and ascent of mantle xenoliths to the surface.

Li partitioning in the benthic foraminifera Amphistegina lessonii

AbstractThe shallow water benthic foraminifer Amphistegina lessonii was grown in seawater of variable Li and Ca concentration and shell Li/Ca was determined by means of LA‐ICPMS. Shell Li/Ca is positively correlated to seawater Li/Ca only when the Li concentration of seawater is changed. If the seawater Ca concentration is changed, shell Li/Ca remains constant. This indicates that Li does not compete with Ca for incorporation in the shell of A. lessonii. A recently proposed calcification model can be applied to divalent cations (e.g., Mg and Sr), which compete for binding sites of ion transporters and positions in the calcite lattice. By contrast, the transport pathway of monovalent cations such as Li is probably diffusion based (e.g., ion‐channels), and monovalent cations do not compete with Ca for a position in the calcite lattice. Here we present a new model for Li partitioning into foraminiferal calcite which predicts our experimental results and should also be applicable to other alkali metals.

Lithium concentration gradients in feldspar and quartz record the final minutes of magma ascent in an explosive supereruption

Pre- and syn-eruptive time-scales of magmatic processes in large-scale explosive eruptions can be quantified from compositional gradients established between (and within) crystals and their host melt using models of diffusive relaxation. The faster the elemental diffusion rate, the shorter the time periods that are measurable. Here we document Li compositional gradients from crystals in the rhyolitic magma of the ~27ka Oruanui supereruption (Taupo, New Zealand). In plagioclase feldspar and quartz from pumices in the first-erupted material, and loose feldspar crystals from late-erupted ignimbrite, the crystal rims display a ~50% increase in Li concentration over equilibrium values in the crystal cores. At appropriate magmatic temperatures, these gradients represent time-scales of 125 to 720s, equivalent to decompression rates of 30–180kPa/s and rise rates of frothy magma of 4–21m/s. We infer that these short time-scales reflect changes in lithium partitioning behaviour during decompression, and cannot be due to changes in Li concentration accompanying degassing and the growth of bubbles (which act in the opposite sense). The Li partitioning changes are due to the breakdown of the liquid–vapour equilibrium at the critical pressure of 22MPa, where the vapour phase transitions from a supercritical to a subcritical state. This breakdown causes Li to partition from melt into crystals, inducing a short-lived increase in effective Li activity of about 50%. The system is then quenched rapidly by cooling in air or water on eruption. These results capture syn-eruptive processes on unprecedentedly short time-scales in a type and size of eruption where information concerning such late-stage processes has never before been obtainable.

Slab-derived lithium isotopic signatures in mantle xenoliths from northeastern North China Craton

In-situ lithium (Li) concentrations and δ7Li of olivine, orthopyroxene (opx) and clinopyroxene (cpx) in mantle xenoliths from the Quaternary Longgang maar provide insights into the nature and evolution of the lithospheric mantle beneath the easterly part of the North China Craton. SIMS analyses show Li partitioning of olivine>opx≥cpx and Li isotopic disequilibrium in each sample. Olivines in harzburgite are generally homogeneous, with Li abundances in the range of normal mantle, but their δ7Li (−0.9 to −4.0) are lower than normal mantle olivine δ7Li values of ~+4±2. Olivines in cpx-rich lherzolite and pyroxenite have higher Li contents and normal mantle-like δ7Li values. The olivine δ7Li values in the harzburgite and cpx-poor lherzolites are much lower than those published for worldwide peridotites, showing the peculiarity of these peridotites. The Li abundances and δ7Li in the olivines show a positive correlation, indicating mixing between a low-δ7Li melt and normal mantle. Collectively, Li elemental and isotopic compositions of the xenoliths suggest that low δ7Li in olivines reflect a slab-derived Li isotopic signature in the lithospheric mantle. These observations, coupled with geochemical and geophysical evidence, suggest that the lithospheric mantle has been modified three times by (1) low-δ7Li melt likely derived from the subducted Pacific plate (leading to the low δ7Li of olivines), (2) high-degree partial melting (decreasing Li abundances of harzburgite and cpx-poor lherzolites close to the normal mantle), and (3) refertilization by melts mainly from the asthenosphere (producing the normal mantle-like δ7Li and high Li contents in cpx-rich lherzolite and pyroxenite). The melts/fluids derived from the subducted Pacific plate could play a crucial role in the destruction of the craton.

Coupled lithium- and iron isotope fractionation during magmatic differentiation

In this study, we investigated Fe and Li isotope fractionation between mineral separates of olivine pheno- and xenocrysts (including one clinopyroxyene phenocryst) and their basaltic hosts. Samples were collected from the Canary Islands (Teneriffa, La Palma) and some German volcanic regions (Vogelsberg, Westerwald and Hegau). All investigated bulk samples fall in a tight range of Li and Fe isotope compositions (δ56Fewr=0.06–0.17‰ and δ7Lima=2.5–5.2‰, assuming δ7Li of the olivine-free matrix is virtually identical to that of the bulk sample for mass balance reasons). In contrast, olivine phenocrysts display highly variable, but generally light Fe and mostly light Li isotope compositions compared to their respective olivine-free basaltic matrix, which was considered to represent the melt (with δ56Feol=−0.24 to 0.14‰ and δ7Liol=−10.5 to +6.5‰, respectively). Single olivine crystals from one sample display even a larger range of δ56Feol between −0.7 and +0.1‰. One single clinopyroxene phenocryst displays the lightest Li isotope composition (δ7Licpx=−17.7‰), but no Fe isotope fractionation relative to melt. The olivine phenocrysts show variable Mg# and Ni (correlated in most cases) that range between 0.89 and 0.74 and between 300 and 3000μg/g, respectively. These olivines likely grew by fractional crystallization in an evolving magma. One sample from the Vogelsberg volcano contained olivine xenocrysts (Mg#>0.89 and Ni>3000μg/g), in addition to olivine phenocrysts. This sample displays the highest Li- and the second highest Fe-isotope fractionation between olivine and melt (Δ7Liol-melt=−13; Δ56Feol-melt=−0.29).Our data, i.e. the variable olivine- at constant whole rock and matrix isotope compositions, strongly indicate disequilibrium, i.e. kinetic Fe and Li isotope fractionation between olivine and melt (for Li also between cpx and melt) during fractional crystallization. Δ7Liol-melt is correlated with the Li partitioning between olivine and melt (i.e. with Liol/Limelt), indicating Li isotope fractionation due to preferential (faster) diffusion of 6Li into olivine during fractional crystallization. Olivine with low Δ7Liol-melt, also have low Δ56Feol-melt, indicating that Fe isotope fractionation is also driven by diffusion of isotopically light Fe into olivine, potentially, as Fe–Mg inter-diffusion. The lowest Δ56Feol-melt (−0.40) was observed in a sample from Westerwald (Germany) with abundant magnetite, indicating relatively oxidizing conditions during magma differentiation. This may have enhanced equilibrium Fe isotope fractionation between olivine and melt or fine dispersed magnetite in the basalt matrix may have shifted its Fe isotope composition towards higher δ56Fe. The decoupling of Li- and Fe isotope fractionation in cpx is likely due to faster diffusion of Li relative to Fe in cpx, implying that the large investigated cpx phenocryst resided in the magma for only a short period of time which was sufficient for Li- but not for Fe diffusion. The absence of any equilibrium Fe isotope fractionation between the investigated cpx phenocryst and its basaltic host may be related to the similar Fe3+/Fe2+ of cpx and melt. In contrast to cpx, the generally light Fe isotope composition of all investigated olivine separates implies the existence of equilibrium- (in addition to diffusion-driven) isotope fractionation between olivine and melt, on the order of 0.1‰.

Melt/rock interaction in remains of refertilized Archean lithospheric mantle in Jiaodong Peninsula, North China Craton: Li isotopic evidence

Li contents and its isotopes of minerals in mantle peridotite xenoliths from late Cretaceous mafic dikes, analyzed in situ by Cameca IMS-1280, reveal the existence of melt/rock interaction in remains of refertilized Archean lithospheric mantle in Qingdao, Jiaodong Peninsula, North China Craton. Two groups of peridotites exist, i.e., low-Mg# lherzolite and high-Mg# harzburgites. The low-Mg# lherzolite has a relatively homogeneous Li concentration (ol: 2.01–2.11 ppm; opx: 1.77–1.88 ppm; cpx: 1.75–1.93 ppm) and Li isotopic composition (δ7Li in ol: 4.2–7.6‰; in opx: 6.0–8.3‰; in cpx: 5.3–8.4‰). The similarity in δ7Li value to the fresh MORB provides further evidence for the argument that the low-Mg# lherzolite could be the fragment of the newly accreted lithospheric mantle. The high-Mg# harzburgites have heterogeneous Li abundances (ol: 0.83–2.09 ppm; opx: 0.92–1.94 ppm; cpx: 1.12–4.89 ppm) and Li isotopic compositions (δ7Li in ol: −0.5 to +11.5‰; in opx: −6.2 to +11.1‰; in cpx: −34.3 to +10.1‰), showing strong disequilibrium in Li partitioning and Li isotope fractionation between samples. The cores of most minerals in these high-Mg# harzburgites have relatively homogeneous δ7Li values, which are higher than those of fresh MORB, but similar to those previously reported for arc lavas. These harzburgites have enriched trace elemental and Sr–Nd isotopic compositions. These observations indicate that in the early Mesozoic the lithospheric mantle beneath the southeastern North China Craton was similar to that in arc settings, which is metasomatized by subducted crustal materials. Extremely low δ7Li preserved in cpxs requires diffusive fractionation of Li isotopes from later-stage melt into the minerals. Thus, the Li data provide further evidence that the Archean refractory lithospheric mantle represented by the high-Mg# harzburgites was refertilized through melt/rock interaction and transformed to the Mesozoic less refractory and incompatible element and Sr–Nd isotopes enriched lithospheric mantle.

Towards a Li barometer for bimineralic eclogites: experiments in CMAS

An eclogite barometer has profound importance in the study of upper mantle processes and potential application to diamond prospecting. Studies on the partitioning of Li between clinopyroxene (cpx) and garnet (grt) in natural samples have shown that this particular element is very sensitive to changes in pressure and could be calibrated as the barometer demanded for bimineralic eclogites. Experiments were performed from 4 to 13 GPa and 1,100–1,400°C in the CMAS (CaO, MgO, Al2O3, SiO2) system with Li added as Li3PO4 to quantify this pressure dependence into a barometer expressed in the following equation: P = (0.00255T – ln K d)/0.2351 where P is in GPa, T is in °C and K d is defined as the partition coefficient of Li (in ppm) between clinopyroxene and garnet. The experimental pressures are reproduced to ±0.38 GPa (1σ) by this equation. This barometer is strictly applicable only to CMAS. Experiments at 1,300°C, 8–12 GPa showed that Henry’s Law is fulfilled for Li partitioning between cpx and grt in the concentration range of approximately 0.01–1 wt% Li. Direct application of the equation to experiments in natural systems performed at 1,300°C from 4 to 13 GPa consistently overestimates pressures by approximately 2 GPa.

Fractionation of lithium isotopes in magmatic systems as a natural consequence of cooling

High-temperature, diffusive fractionation has been invoked to account for striking Li isotopic variability recently observed within individual phenocrysts and xenolith minerals. It has been argued that chemical potential gradients required to drive such diffusion arise from changes in Li partitioning between coexisting phases during cooling. If so, Li isotopic zoning should be a common occurrence but the role of temperature-dependent partition coefficients in generating Li isotopic variability remains to be tested in a quantitative manner. Here we consider a basic scenario of a phenocryst in a cooling lava, using simple parameterisations of the temperature dependence of Li partitioning and diffusivity in clinopyroxene. Our model initially produces an asymmetric isotope profile across the crystal with a δ 7Li minimum that remains close to the edge of a crystal. Such a distinctive shape mimics Li isotopic profiles documented in some olivine and clinopyroxene phenocrysts, which have isotopically normal cores but anomalously light rims. The temperature dependence of both the diffusivity and the partition coefficient of Li are key factors in generating this form of diffusion profile. Continued diffusion leads to an inversion in the sense of isotopic change between core and rim and results in the whole phenocryst attaining markedly light isotopic values. Our calculations show that significant Li isotopic zoning can occur as a natural consequence of cooling magmatic systems. Crystals that have experienced more complex thermal histories (e.g. re-entrained cumulates versus true phenocrysts) will therefore exhibit contrasting isotopic profiles and, as such, these data may be useful for tracing sub-volcanic processes.

The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere

Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budget of these elements we have studied samples of highly serpentinized (>50%) spinel harzburgite drilled at the Mid-Atlantic Ridge (Fifteen–Twenty Fracture zone, ODP Leg 209, Sites 1272A and 1274A). In-situ analysis by secondary ion mass spectrometry reveals that the B, Li and Be contents of mantle minerals (olivine, orthopyroxene, and clinopyroxene) remain unchanged during serpentinization. B and Li abundances largely correspond to those of unaltered mantle minerals whereas Be is close to the detection limit. The Li contents of clinopyroxene are slightly higher (0.44–2.8 μg g −1) compared to unaltered mantle clinopyroxene, and olivine and clinopyroxene show an inverse Li partitioning compared to literature data. These findings along with textural observations and major element composition obtained from microprobe analysis suggest reaction of the peridotites with a mafic silicate melt before serpentinization. Serpentine minerals are enriched in B (most values between 10 and 100 μg g −1), depleted in Li (most values below 1 μg g −1) compared to the primary phases, with considerable variation within and between samples. Be is at the detection limit. Analysis of whole rock samples by prompt gamma activation shows that serpentinization tends to increase B (10.4–65.0 μg g −1), H 2O and Cl contents and to lower Li contents (0.07–3.37 μg g −1) of peridotites, implying that—contrary to alteration of oceanic crust—B is fractionated from Li and that the B and Li inventory should depend essentially on rock–water ratios. Based on our results and on literature data, we calculate the inventory of B and Li contained in the oceanic lithosphere, and its partitioning between crust and mantle as a function of plate characteristics. We model four cases, an ODP Leg 209-type lithosphere with almost no igneous crust, and a Semail-type lithosphere with a thick igneous crust, both at 1 and 75 Ma, respectively. The results show that the Li contents of the oceanic lithosphere are highly variable (17–307 kg in a column of 1 m × 1 m × thickness of the lithosphere (kg/col)). They are controlled by the primary mantle phases and by altered crust, whereas the B contents (25–904 kg/col) depend entirely on serpentinization. In all cases, large quantities of B reside in the uppermost part of the plate and could hence be easily liberated during slab dehydration. The most prominent input of Li into subduction zones is to be expected from Semail-type lithosphere because most of the Li is stored at shallow levels in the plate. Subducting an ODP Leg 209-type lithosphere would mean only very little Li contribution from the slab. Serpentinized mantle thus plays an important role in B recycling in subduction zones, but it is of lesser importance for Li.

RELATION BETWEEN MELT PERCOLATION AND LIGHT ELEMENT CONTENTS (LI, BE AND B) IN PERIDOTITES FROM THE PINDOS OPHIOLITE (GREECE)

The Pindos ophiolite represents non-metamorphic oceanic lithosphere obducted during the Jurassic. The mantle part was hydrothermally altered to different degrees during oceanization, but fresh peridotites are still preserved. The Pindos mantle mainly consists of highly depleted spinel harzburgites, but some plagioclase lherzolites are present in the northwestern part of the body. Textures in thin sections clearly demonstrate a direct relation between plagioclase formation and melt infiltration into the oceanic mantle. Two main textures were observed: a “porous flow” texture where plagioclase is homogeneously distributed in the rock and a channelized texture where plagioclase crystallized in microveins. Melt infiltration also led to crystallization of orthopyroxene and clinopyroxene associated with plagioclase. Similar textures were observed in the Othris ophiolite (Greece), southeast of Pindos, by Dijkstra et al. (2001) and in other Tethyan ophiolites (Piccardo et al., 2004). A study on light element contents of minerals from Pindos harzburgite and serpentinite was conducted in order to evaluate the role of the oceanic mantle as repository for these elements. Results obtained on spinel harzburgites and plagioclase lherzolites by SIMS (Secondary Ion Mass Spectrometry) show no significant difference between Li, Be and B contents of primary mantle minerals and melt impregnation phases. Li contents of olivine (0.68-1.1 ppm) and orthopyroxene (0.2-1.5 ppm) seem to be consistent with values for “normal” mantle minerals (Eggins et al., 1998; Woodland et al., 2004). The Li contents of clinopyroxene (0.2-3.7 ppm) are within the upper range of values published for unmetasomatised mantle clinopyroxene (only xenolith data available; Seitz & Woodland, 2000; Woodland et al., 2004). The Li characteristics of spinel and plagioclase peridotites differ from published data for non-metasomatised mantle by higher values for clinopyroxene than for olivine. This inverse Li partitioning could be explained by a reaction with a mafic silicate melt (Seitz and Woodland, 2000). This would be in agreement with the textures observed in the plagioclase lherzolites. Be abundances are below detection limit and B is low in all minerals from below the detection limit to 0.4 ppm. Li, Be and B contents in plagioclase are extremely low, often below the detection limit. In conclusion, the percolating melt did not add considerable amounts of B, Li and Be, probably because it was depleted in these elements. Melt impregnation is also supported by major element chemistry. Clinopyroxenes (first generation cpx and cpx recrystallized from the melt) of plagioclase lherzolites are enriched in Na, Ti and their XMg is higher compared to those of the spinel harzburgites. The same observation was made on the Othris peridotites by Dijkstra et al. (2001 and 2003). In plagioclase lherzolites, serpentinization textures confirm that hydrothermalism occured after melt infiltration. In highly hydrothermalized peridotites, light element contents of primary mantle minerals are low and similar to those of fresh samples. B is mainly concentrated in serpentine (0.4- 24 ppm) and Li in chlorite (2-3.5 ppm). Values for Be were always below the detection limit. Low contents of Li and B in primary minerals such as olivine, orthopyroxene and clinopyroxene after hydrothermalism can be explained by HT serpentinization.

Behavior of Li and its isotopes during serpentinization of oceanic peridotites

Analyses of Li and Li isotopes in serpentinized peridotites have been performed using Thermo‐Ionisation Mass Spectrometry (TIMS) and Secondary Ion Mass Spectrometry (SIMS) techniques on samples collected from the southwest Indian Ridge (SWIR). In the bulk samples, Li concentrations range from 0.6 to 8.2 ppm, while whole rock δ6Li values range from −2.9 to −14‰. In situ analyses display a greater range in both Li concentration (0.1–19.5 ppm) and Li isotopic composition (−27 to +19‰), with the serpentinized portions having higher Li concentrations than the associated relict phases. These variations may reflect changes in Li partitioning and isotopic fractionation between serpentine and fluid with temperature and water/rock ratio. They may also be explained by changes in the composition of the serpentinizing fluid over the course of serpentinization. As the serpentine forms by interaction with a circulating fluid, it preferentially removes 6Li, causing the Li in the fluid to become isotopically heavier. The isotopic composition of the initial hydrothermal fluid is dominated by basalt‐derived Li, which easily overwhelms the very low Li content originally present in seawater. As this fluid circulates through ultramafic rocks, it induces the formation of serpentine that incorporates this mantle‐derived Li. Hence, Li in serpentine is mainly derived from oceanic crust rather than from seawater and serpentinization involves Li recycling within this crust. Consequently, Li isotopes are good tracers of the hydrothermal contribution in serpentinizing fluid. These results imply that serpentinized peridotites are probably only a minor sink of oceanic Li.

Sodic-ferripedrizite, a new monoclinic amphibole bridging the magnesium-iron-manganese-lithium and the sodium-calcium groups

Sodic-ferripedrizite, ideally \(Na(LiNa)(Fe_{2}^{3+}Mg_{2}Li)Si_{8}O_{22}(OH)_{2}\) , is the second new amphibole end-member found in episyenites formed after cordierite-bearing porphyritic granites in the East Pedriza Massif (Central System, Spain). It is green, vitreous, translucent, non-fluorescent and brittle, and has gray streak, H(Mohs) = 6, splintery fracture, perfect {110} cleavage, D meas = 3.15, D calc = 3.15 g/cm 3 . In plane-polarized light, sodic-ferripedrizite is strongly pleochroic, X = green blue, Y = blue green, Z = yellow green ( X > Y > Z ); Y = b , X ^ c = 4°, Z ^ c = −8°. It is biaxial positive: α = 1.694 (1), β = 1.698(1), and γ = 1.702(1); 2 V Z = 83(2)° and 2 V Z,calc = 85(3)°, dispersion r > v . Sodic-ferripedrizite is monoclinic, space group C 2/ m , a = 9.536(1), b = 17.789(2), c = 5.277(1) A, β = 102.53°, V = 873.8(1) A 3 . The five strongest lines in the X-ray powder-diffraction pattern [ d in A[ I ] ( hkl ) are 3.397[3](131), 3.056[10](310), 2.749[5](330), 2.699[6](151), and 1.639[4](461). Analysis by electron microprobe and flame photometry gave an average chemical formula of \(^{A}(Na_{0.70}K_{0.03})^{B}(Li_{1.34}Na_{0.58}Ca_{0.08}) (Mg_{1.75}Fe_{1.65}^{3+}Li_{0.88}Fe_{0.32}^{2+}A1_{0.21}Ti_{0.11}Mn_{0.07}^{2+}Zn_{0.01})Si_{8.00}O_{22}(OH_{1.35}F_{0.65})\) . Structure refinement of one crystal of pedrizite and of another crystal with higher clinoholmquistite component, both of which were also analyzed by ion microprobe for light and volatile elements, allowed us to assess Li partitioning among the B- and C-group sites and the active crystal-chemical mechanisms; Li is ordered at the [6+2]-coordinated position in the M4 cavity and at the M3 site. Thus, sodic-ferripedrizite encompasses the different site-preferences and crystal-chemical mechanisms observed for Li in amphiboles of the Mg-Fe-Mn-Li group (clinoholmquistite series) and the Na-Ca group (leakeite series). These data and other recent results on synthetic amphiboles suggest that miscibility between the two groups of B-cations is far more extensive than previously expected.