Lithium Isotope Fractionation Research Articles

Lithium isotopic fractionation during weathering and erosion of shale

Clay weathering in shales is an important component of the global Li budget because Li is mobilized from Li-rich clay minerals and shale represents about one quarter of the exposed rocks on Earth. We investigate Li isotopes and concentrations to explore implications and mechanisms of Li isotopic fractionation in Shale Hills, a first-order catchment developed entirely on shale in a temperate climate in the Appalachian Mountains, northeastern USA. The Li isotopic compositions (δ7Li) of aqueous Li in stream water and groundwater vary between 14.5 and 40.0‰. This range is more than half that observed in rivers globally. The δ7Li of aqueous Li increases with increasing Li retention in secondary minerals, which is simulated using a box model that considers pore fluid advection to be the dominant transport process, silicate dissolution to be the source of Li to the pore fluid, and uptake of Li by kaolinite, Fe-oxides, and interlayer sites of clays to be the sinks. The simulations suggest that only those deep groundwaters with δ7Li values of ∼15‰ are explainable as steady state values; those fluids with δ7Li values > 18‰, especially near-surface waters, can only be explained as time-dependent, transient signals in an evolving system. Lithium is highly retained in the residual solid phase during chemical weathering; however, bulk soils (0.5 ± 1.2‰ (1 SD)) and stream sediments (0.3‰) have similar, or higher, δ7Li values compared to average bedrock (−2.0‰). This is attributed to preferential removal of clay particles from soils. Soil clays are isotopically depleted in 7Li (δ7Li values down to −5.2‰) compared to parental material, and δ7Li values correlate with soil Li concentration, soil pH, and availability of exchangeable sites for Li as a function of landscape position (valley floor versus ridge top). The strong depletion of Li and clay minerals in soils compared to bedrock is attributed at least partly to loss of Li through export of fine-grained clay particles in subsurface water flow. This process might be enhanced as the upper weathering zone of this catchment is highly fractured due to former periglacial conditions. The Li isotopic composition of vegetation is similar to soil clay and both are distinct from mobile catchment water (soil pore water, stream and groundwater). Extrapolating from this catchment means that subsurface particle loss from shales could be significant today and in the past, affecting isotopic signatures of soils and water. For example, clay transformations together with removal of clay particles before re-dissolution support weathering conditions that lead to a low aqueous Li flux but to high δ7Li values in water.

Lithium and potassium isotope fractionation during silicate rock dissolution: An experimental approach

Lithium and potassium isotope fractionation during silicate rock dissolution: An experimental approach

Lithium and its isotopes behavior during incipient weathering of granite in the eastern Tibetan Plateau, China

Lithium (Li) isotopes have been shown to be fractionated significantly during chemical weathering of silicate rocks. However, previous research has mainly been focused on basalt weathering, whereas the behavior of Li isotopes during the weathering of granite remains unclear. The Li isotopic compositions in the saprolite profile developed on granite was analyzed, as well as several individual primary minerals (K-feldspar, plagioclase, quartz and biotite) in the parent granite. Lithium concentration and the isotopic compositions of the individual primary minerals appear to be heterogeneous. Biotite is rich in Li (369.4 mg/kg), but the Li concentration is very low in other minerals (1.3–11.3 mg/kg). Quartz has the highest δ7Li value (+19.9‰); other minerals contain δ7Li within a fairly narrow range (+5.8‰ to +9.0‰). The saprolite samples were mainly composed of primary minerals and were characterized by low weathering intensity (CIA = 52–60). Lithium was distinctly lost in the saprolites relative to parent granite. Meanwhile, δ7Li values of saprolites were all lower than those of parent granites (−2.0‰ to +3.9‰ vs. +7.1‰, respectively), which systematically decreased with increasing weathering intensity. The mineralogical composition and 87Sr/86Sr ratio in the saprolites suggest that the biotite was preferentially weathered below 120 cm depth in the examined profile, whereas plagioclase weathering mainly occurred above 120 cm depth. The Li released during granite weathering was characteristically different from the isotopes in the primary minerals. Lithium isotopic fractionation during granite weathering is affected by adsorption and/or incorporation of Li by secondary minerals (or clay minerals), and the release of Li from biotite. This study highlights the fact that significant Li isotope fractionation occurs during Li leaching (relatively higher 7Li/6Li ratios) from biotite in the incipient stage of granite weathering, which may help to explain the higher δ7Li values found in the dissolved load from the catchments of low weathering intensity.

Experimental investigation of lithium isotope fractionation during kaolinite adsorption: Implications for chemical weathering

The feedback between CO2 sources and sinks through chemical weathering is one of the important reasons why Earth has maintained a habitable climate for over four billion years. The lithium (Li) isotopic system is a promising tracer of silicate weathering, but the mechanisms causing its isotope fractionation during weathering remain ambiguous. Here, we performed batch experiments of Li adsorption to one of the common clay minerals -kaolinite in three sets, including the time-series, pH-dependent, and concentration-control sets. Our results demonstrate that the liquid-solid Li isotope fractionation reaches up to 36‰, with up to 99% initial Li being adsorbed on kaolinite. The magnitudes of Li adsorption and isotope fractionation increase with reaction time, and reach the steady-state after ∼1000 min. The magnitude of Li isotope fractionation increases with the adsorption ratio of Li, in positive relationships with solute pH and ionic strength. At constant solute pH = 8.5 and ionic strength of 0.001 M, the adsorption ratio and isotope fractionation of Li on kaolinite reach the maximum with the lowest initial Li concentration of 2 μM. In experiments, Li is removed by kaolinite as the inner-sphere and/or outer-sphere complexes, likely followed by structural occupation as supported by incomplete desorption. We model the Li isotope fractionation in all three sets, which can be best described by Rayleigh distillation models. In summary, significant Li isotope fractionation occurs following a kinetic law in closed-systems during adsorption on kaolinite. Adsorption-driven isotope fractionation conforms to a kinetic solid-liquid isotope fractionation factor α ∼ 0.992, consistent with theoretical ion-desolvation between complexed and dissolved ions (α = 0.9925, Hofmann et al., 2012). This study reveals a vital kinetic role of clay uptake in Li isotope fractionation during chemical weathering, suggesting rapid forward adsorption reaction versus relatively slow backward reaction. The dominance of the kinetic mechanism over the equilibrium mechanism further explains field observations from rivers worldwide. Given above, the outcome of this study calls for additional attention on low-temperature kinetic Li isotope fractionation at Earth’s surface and further refinement of quantitative models using geological Li records to trace weathering and reconstruct climate.

Experimental investigation of lithium isotope fractionation during kaolinite adsorption: Implications for chemical weathering

Experimental investigation of lithium isotope fractionation during kaolinite adsorption: Implications for chemical weathering

Lithium isotope fractionation during fluid exsolution: Implications for Li mineralization of the Bailongshan pegmatites in the West Kunlun, NW Tibet

Granitic pegmatites are considered significant host rocks for the rare metal deposits. However, the enrichment mechanisms for the rare metal elements (e.g., Li, Be, Nb and Ta) remain debated. Here we report the whole-rock and mineral (e.g., muscovite and spodumene) major, trace elements and Li isotopic compositions of the Bailongshan granitic pegmatites in the West Kunlun, NW Tibet, to address this issue. The Bailongshan pegmatites (hosting a super-large Li–Rb polymetallic deposit) are composed of Li-poor and Li-rich pegmatites. The Li-poor pegmatites display high δ7Li values in the whole rock (2.30–4.94‰) and muscovite (2.22–7.55‰). In contrast, the Li-rich pegmatites show low δ7Li values in the whole rock (−1.89 to 0.35‰), muscovite (−3.39 to 4.49‰) and spodumene (−2.83 to 1.87‰). These results, together with the variations of the elements, suggest that the Li-poor and Li-rich pegmatites were produced by the melt-fluid separation during the late stage of granitic magma evolution. This means that the Li-poor pegmatites were formed in a H2O-poor silicate-rich melt system, while the Li-rich pegmatites were generated in a H2O-rich silicate-poor melt (supercritical fluid) system. The melt-fluid separation can lead to significant Li isotope fractionation, with 7Li enriched in the strongly-bonded residual silicate melt, and 6Li preferring to weaker hydrated bonds in the fluid. Alternatively, the Li-rich pegmatites enriched in 6Li could have been induced by preferential accumulation of the faster-diffusing 6Li in the supercritical or near critical fluid. The Li-rich pegmatites experienced extensive crystal-fluid interaction in a relatively closed magmatic-hydrothermal system, which resulted in the leaching and reprecipitation of ore-forming elements, but produced restricted Li isotope fractionation. The exsolved supercritical fluid is more enriched in alkali-, fluxing components (Li, Rb, Cs, P and F) by the continuous interaction with the residual silicate melt. Therefore, the supercritical fluid exsolution plays an important role in the Li isotope fractionation and rare metal mineralization.

Modelling the riverine δ7Li variability throughout the Amazon Basin

The present study investigates the processes controlling the elementary and isotopic cycle of the lithium over the Amazon basin. A numerical model is developed to simulate two major processes that have been proposed as key controls of the river lithium isotopic composition: weathering reactions inside the regolith, accounting for secondary phase formation, and interactions between riverine water and secondary phases in floodplain. Both processes generate fractionation of lithium isotopes (“batch” fractionation and “Rayleigh” distillation respectively) that potentially control the riverine isotopic composition of the Amazon and its tributaries.A study of the model parameters shows that two different regimes are impacting the lithium isotopic composition of the rivers within the Amazon catchment. In the South (Madeira and its tributaries), the lithium isotopic signature of river waters can be explained by lithium release and fractionation during weathering reactions in the regolith, followed by “Rayleigh distillation” in the floodplain increasing progressively the lithium isotopic composition, in agreement with a previously published hypothesis. In contrast, the lithium isotopic composition of rivers located in the northern part of the Amazon watershed (Solimoes and tributaries) cannot be simulated by the model assuming the same processes than in the southern part. Model optimization suggests than the nature of the material being eroded and weathered is important. In the North, fresh source rocks of volcanic origin releases large amount of Li and promotes rapid smectite precipitation, allowing the riverine δ7Li to rise before flowing through floodplains. This result suggests that the environments able to generate high riverine δ7Li (higher than 25‰) are complex and not firmly identified yet.

Lithium Behaviour and Isotope Fractionation During Fluid–Rock Interactions in Variscan Oceanic Suture Zones: Limousin Ophiolite and Ile de Groix High-pressure Terrane (France)

AbstractOphiolites and high-pressure/low-temperature (HP–LT) terranes are important sites for the study of geochemical cycling in ancient oceanic lithosphere. We have analysed Li abundances and isotope composition in a series of ultrabasic and basic rocks from the Variscan Limousin ophiolite, as well as in basic and pelitic rocks from the Ile de Groix HP–LT terrane. Both bulk and in situ analyses are employed to evaluate Li mobility and isotope fractionation in the oceanic lithosphere during fluid–rock interactions related to seafloor and sub-seafloor hydrothermal alteration, subduction and exhumation processes. In the Limousin ophiolite, early stages of high-temperature (high-T) hydrothermal alteration of oceanic ultrabasic rocks produced serpentine with low Li abundances (0·9–4·6 ppm) and low δ7Li (–8·9‰). The δ7Li increase from –2·2 to +4·2‰ in the following generations of serpentine during late-stage hydrothermal alteration results from changes in the fluid composition and temperature conditions. Therefore, even if dehydrating subducted serpentinites generate high amounts of fluids during subduction, abyssal serpentinites do not constitute an important source of Li for Li-rich metabasic rocks. In the associated amphibolites, hornblende displays typical Li contents (3·1–8·2 ppm) and isotopic compositions (+3·5 to +12·5‰) similar to hydrothermally altered sheeted dykes and gabbros. In contrast, the low Li abundances and extremely high δ7Li values recorded by omphacite and pargasitic amphibole in the ultrahigh-pressure (UHP) zoisite-eclogite from the Limousin probably reflect interaction with a heavy-Li sediment-derived fluid. The HP–LT metabasites of the Ile de Groix record different Li behaviour, with high Li abundances and low δ7Li. They contain Li abundances significantly higher than fresh mid-ocean ridge basalts (MORB) (16–124 ppm), indicating a metasomatic overprint by fluids derived from the neighbouring Li-rich mica-schist (15–52 ppm) in addition to seawater during the early stages of subduction. Lithium is mainly hosted by (1) glaucophane and omphacite in blueschists and eclogites, (2) chlorite and albite in retrograde greenschists, and (3) phengite and chlorite in mica-schists. The metabasites have δ7Li values of –4·8 to +3·2‰ that are generally lower than those of fresh and altered MORB. The intercalated mica-schists display δ7Li values ranging from –1·7 to +0·2‰ that are typical of subducted sediments. The δ7Li decrease from blueschists to eclogites from +1·8 to –4·8‰, as well as the rimward δ7Li decrease in glaucophane from MORB-like δ7Li values to negative values in blueschists (core: –2·4 to +8·8‰; rims: –7·1 to +2·2‰), reveals that significant fluid-induced Li isotope fractionation occurred at the transition from the lawsonite-blueschist facies to the epidote-blueschist facies, and may be triggered by prograde lawsonite breakdown. In eclogites, the low δ7Li measured in whole-rocks (–4·8 to –2·5‰), omphacite (–22·4 to +3·3‰) and glaucophane (–6·9 to +1·4‰) indicates that Li isotope kinetic fractionation had stronger effects under eclogite-facies conditions. The δ7Li increase toward positive values in the most retrogressed greenschist samples suggests Li mineral–fluid isotopic exchange during rehydration reactions and interaction with a Li-heavy fluid that is probably derived from the dehydrating metabasites. Thus, lithium isotope fractionation in the HP–LT rocks of the Ile de Groix highlights migration of heavy-Li fluids along the oceanic crust–mantle interface in the subduction zone.

Geochemistry, Monazite U–Pb Dating, and Li–Nd Isotopes of the Madi Rare Metal Granite in the Northeastern Part of the North China Craton

The Madi rare metal granite is a complex massif, which contains a variety of rare metals, such as Nb, Ta, Li, and Be. In this paper, the geochemical characteristics of the granite were obtained by multi‐collector inductively coupled mass spectrometry (MC‐ICP‐MS). The precise crystalline age of the granite was obtained from monazite U‐Pb dating, and the source of the granite was determined using Li‐Nd isotopes. The Madi rare metal granite is a high‐K (calc‐alkaline), peraluminous, S‐type granite. The U‐Pb monazite age indicates that the crystalline age of the granite is 175.6 Ma, which is Early Jurassic. The granite is characterized by a relatively wide range of δ7Li values (+2.99‰ to +5.83‰) and high lithium concentrations (181 ppm to 1022 ppm). The lithium isotopic composition of the granite does not significantly correlate with the degree of magmatic differentiation. An insignificant amount of lithium isotope fractionation occurred during the granitic differentiation. The lithium isotopic composition of the granite significantly differs from that of the wall rock, but it is very similar to that of a primitive mantle peridotite xenolith (mean δ7Li value +3.5‰). The plot of Li concentration versus δ7Li indicates that the Li isotopic composition of the granite is similar to that of island arc lavas. Based on the above‐described evidence, the granite was mainly derived from the crust, but it was contaminated by a deep granitic magma.

Temperature-dependent variations in mineralogy, major element chemistry and the stable isotopes of boron, lithium and chlorine resulting from hydration of rhyolite: Constraints from hydrothermal experiments at 150 to 350 °C and 25 MPa

Rhyolite-hosted hydrothermal systems in the continental crust contain valuable energy and mineral resources that make them of special interest across several scientific disciplines. Despite extensive research on these systems, the temperature-dependence of chemical reactions between host rocks and aqueous-rich fluids and the mineralogical transformations resulting from these reactions are not well quantified. To expand our understanding of the controlling processes operating in these systems, we carried out seven laboratory experiments in which rhyolite was reacted with deionized water at 150 °C to 350 °C and 25 MPa. An additional experiment at 200 °C was carried out to examine the effect of dissolved CO 2 on the reactions. The overarching goal of this experimental study was to provide new insights on the temperature-dependence of water-rock interaction in continental hydrothermal systems. We applied a wide range of chemical, isotopic and mineralogical methods to analyze the reacted rhyolite and waters, and the major observations are: (1) the rhyolite progressively hydrates with increasing temperature between 150 °C to a maximum of 8.2 wt% H 2 O at 275 °C; hydration then decreases until 350 °C in conjunction with the destruction of the rhyolite glass and crystallization of secondary mineral phases; (2) the ratio of molecular water (H 2 O m ) to hydroxyl (OH − ) of the water that is dissolved in the reacted rhyolite decreases from ∼7 at 150 °C to ∼4 at 250 °C; (3) the main secondary minerals formed are the zeolite ferrierite (T ≥ 275 °C); biotite, albite and cristobalite mainly form at higher experimental temperatures (T ≥ 300 °C); (4) the reacted waters are nearly saturated with respect to amorphous silica; (5) at temperatures ≥ 275 °C nearly all the chlorine is leached into solution; (6) fluorine leaching from the rhyolite gradually increases between 150 °C and 250 °C, but then gradually decreases at higher temperatures and is incorporated into a secondary mineral phase; (7) dissolved CO 2 in the water enhances alkali metal cation leaching from the rhyolite; and (8) calculated Na-K and silica geothermometer temperatures differ from the experimental temperatures by varying amounts. In addition, apart from some small lithium isotope fractionation at temperatures ≤ 250 °C, the stable isotopes of boron, lithium and chlorine do not fractionate during rhyolite-water reactions, and the stable isotope compositions of these species in the reacted water are similar to those in the reactant rhyolite. These results provide new insights for a broad range of applications, including quantifying processes involving rhyolite glass hydration (obsidian hydration dating, perlite formation and discriminating secondary from magmatic water in rhyolitic matrix-glass of volcanic pyroclasts), for geothermal energy and mineral deposit exploration and for monitoring volcanoes.

Experimental constraints on Li isotope fractionation during clay formation

Abstract Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth system models. This study refines our understanding of isotope fractionation during clay formation with essential implications for the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopic fractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions were all enriched in 6 Li compared to the initial solution. On average, the exchangeable solutions had δ 7 Li values 7‰ lower than the initial solution. The average difference between the residual solid and initial solution δ 7 Li values ( Δ 7 Li residue - solution ) for the synthesised layer silicates was −16.6 ± 1.7‰ at 20 ° C, in agreement with modelling studies, extrapolations from high temperature experimental data and field observations. Three bonding environments were identified from 7 Li-NMR spectra which were present in both bulk and residual solid 7 Li-NMR spectra, implying that some exchangeable Li remains after treatment with ammonium chloride. The 7 Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the 7 Li-NMR data with mass balance constraints we calculated a fractionation factor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are −21.5 ± 1.1‰, −0.2 ± 1.9‰ and 15.0 ± 12.3‰ for octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1 σ ). The bulk fractionation factor ( Δ 7 Li bulk - solution ) is dependent on the chemistry of the initial solution. The higher the Na concentration in the initial solution the lower the bulk δ 7 Li value. We suggest this is due to Na outcompeting Li for interlayer sites and as interlayer Li has a high δ 7 Li value relative to octahedral Li, increased Na serves to lower the bulk δ 7 Li value. Three experiments conducted at higher pH exhibited lower δ 7 Li values in the residual solid. This could either be a kinetic effect, resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in the residual solid and/or a change in Li speciation in solution. This study highlights the power of 7 Li-NMR in experimental studies of clay synthesis to target site specific Li isotope fractionation factors which can then be used to provide much needed constraints on field processes.

Investigation on Lithium Isotope Fractionation with Diffusion, Electrochemical Migration, and Electrochemical Isotope Effect in PEO-PC Based Gel Electrolyte

Lithium isotope fractionation, based on the difference of the diffusion coefficients, migration mobility, as well as the electrochemical isotope effect, can be achieved in the appropriate solvents. The Propylene Carbonate-Polyethylene Oxide gel electrolyte was utilized to separate Li isotopes by eliminating the fluid flow mixing and part of the solvation shell around Li ions. A higher Li isotope fractionation (△7Li = −4.53‰) was achieved with the effects of both diffusion and electrochemical migration in a long 10-cm cell test. Also, a short 2-cm cell gel electrolyte short duration test yielded a very high separation factor of 1.033, which is a very promising separation factor for Li isotope separation. Similarly, the electrochemical intercalation test into the graphite plate electrode associated with the diffusion and migration assisted separation shows a separation factor of 1.033. Influence of the electrochemical cell length was studied with an electrochemical model, and results show that an appropriate cell length needs to be determined by considering both the separation time and the separation factor.

Lithium isotope fractionation during magmatic differentiation and hydrothermal processes in rare-metal granites

Lithium isotope composition is potentially an effective geochemical tracer for hydrothermal processes and magmatic differentiation associated with rare-metal granitic rocks. The Yashan and Xihuashan plutons in South China are extraordinarily Li-F rich rare-metal granites that contain niobium-tantalum and tungsten deposits, respectively. As a moderately incompatible trace element in a felsic melt system, Li notably increases from protolithionite granite and Li-mica granite (88.7–175 μg/g) to topaz-lepidolite granite (7430–8080 μg/g) in the Yanshan pluton. Despite a large variation in Li concentrations, the δ7Li values of the Yashan pluton vary within in a narrow range from −1.5‰ to 1.5‰. In contrast, the δ7Li values of the Xihuashan pluton notably increase from biotite granite and two-mica granite (−0.2‰ to +0.7‰) to muscovite granite (+1.9‰ to +4.4‰) with much less variation in the Li concentrations (37.8–209 μg/g), which is best explained by the high diffusion rate of 6Li relative to 7Li during disequilibrium fluid-rock interaction. The Xihuashan greisen has negative δ7Li values (from −2.7‰ to −2.1‰), which are attributed to extensive fluid-rock interaction in an open system.Lithium isotope fractionations are consistent with a diversity of mineralization in the rare-metal granitic rocks. Tungsten mineralization is likely associated with an open hydrothermal process. Fluid-rock interaction has a much stronger effect on Li isotope fractionation than does magmatic differentiation in a highly evolved magmatic system. Ta-Nb mineralization is related to the magmatic differentiation in a closed magmatic-hydrothermal system. The exsolution of a supercritical fluid during magmatic differentiation and fluid-rock interaction in a closed magmatic-hydrothermal system is insufficient for producing notable Li isotope fractionation.

Lithium isotope fractionation by diffusion in minerals Part 2: Olivine

Recent experiments have shown that lithium isotopes can be significantly fractionated by diffusion in silicate liquids and in augite. Here we report new laboratory experiments that document similarly large lithium isotopic fractionation by diffusion in olivine. Two types of experiments were used. A powder-source method where lithium from finely ground spodumene (LiAlSi2O6) diffused into oriented San Carlos olivine, and piston cylinder annealing experiments where Kunlun clinopyroxene (∼30ppm lithium) and oriented San Carlos olivine (∼2ppm lithium) were juxtaposed. The lithium concentration along traverses across the run products was measured using both laser ablation asa source for a Varian 820-MS quadrupole mass spectrometer and a CAMECA 1270 secondary ion mass spectrometer. The CAMECA 1270 was also used to measure the lithium isotopic fractionation across olivine grains recovered from the experiments. The lithium isotopes were found to be fractionationed by many tens of permil in the diffusion boundary layer at the grain edges asa result of 6Li diffusing significantly faster than 7Li. The lithium concentration and isotopic fractionation data across the olivine recovered from the different experiments were modeled using calculations in which lithium was assumed to be of two distinct types – one being fast diffusing interstitial lithium, the other much less mobile lithium on a metal site. The two-site diffusion model involves a large number of independent parameters and we found that different choices of the parameters can produce very comparable fits to the lithium concentration profiles and associated isotopic fractionation. Because of this nonuniqueness we are able to determine only a range for the relative diffusivity of 6Li compared to 7Li. When the mass dependence of lithium diffusion is parameterized as D6Li/D7Li=(7/6)β, the isotope fractionation for diffusion along the a and c crystallographic direction of olivine can be fit by β=0.4±0.1 while the fractionation in the b direction appears to be somewhat lower. Model calculations were also used to fit the lithium concentration and isotopic fractionation across a natural olivine grain from a peridotite xenolith from the Eastern North China Craton. The isotopic data were fit using β values (0.3–0.36) similar to that of the laboratory experiments. This, along with the fact that the isotopic fractionation is restricted to that part of the mineral with a gradient in lithium concentration, is strong evidence that the lithium zoning of this mineral grain is the result of lithium loss by diffusion and thus that it can be used, as illustrated, to constrain the cooling history.

First Lithium Isotope Data from Rivers and Subsurface Water in the Pamirs

Lithium and its isotopes has generally been used as a proxy for silicate weathering and only in a few case studies investigated the link between Lithium isotope systematics of surface and ground waters with respect to residence times in crustal reservoirs and aquifers. However, the processes controlling the Lithium isotope fractionation in such systems are not fully understood, yet. In order to gain a better understanding of how Lithium isotopes are fractionated in large catchments we analyzed Lithium concentrations and Lithium isotope compositions of different water reservoirs (rivers, groundwater, thermal water and lakes) of a high alpine granitic catchment in the Pamir Mountains. We also determined the Lithium concentration and Lithium isotope composition of the (granitic) basement as well as the suspended particulate matter (SPM) in the waters. We observe large isotopic variations in the water samples (from δ7Li = +3 to +36‰) and light isotopic compositions for the bedrock and SPM (δ7Li = -14 to +1.5‰). The wide span of Li isotope composition in the water samples reflects the weathering of silicate rock along the flow path of the (sub) surface water within the catchment.

Lithium isotope fractionation during incongruent melting: Constraints from post-collisional leucogranite and residual enclaves from Bengbu Uplift, China

Lithium (Li) elemental and isotopic compositions of the Jurassic Jingshan leucogranites, including garnet-rich mafic enclaves and wall rock Wuhe gneisses from the southeast margin of North China Craton (NCC) were investigated to understand the behavior of Li isotopes during post-collisional magmatism. The Jingshan leucogranites have distinct U-shape REE patterns with Y and REE concentrations significantly lower yet Sr/Y ratios higher than their presumed source rocks, i.e., the Dabie-Sulu gneisses. Trace element modeling of REE and Sr/Y suggests these elemental signatures of the Jingshan leucogranites can be consistently explained by a fluid-present crustal incongruent partial melting: Bt+Qz+Pl+H2O=Grt+melt, leaving mainly Grt+Bt with minor allanite in the residuum. The mafic enclaves show identical Sr-Nd isotopic compositions with their host leucogranites, contrasting with the Wuhe gneiss and the exposed regional lower crust. The garnet-rich mafic enclaves are thus interpreted as entrained residual phases formed by this incongruent partial melting.The Jingshan leucogranites show relatively high δ7Li values (+4.0‰ to +9.0‰) and low Li concentrations (4.7–11.3ppm) in comparison to published data for worldwide granites. In contrast, the residual enclaves show low δ7Li values (as low as +0.6‰) and high Li concentrations (as high as 118ppm). Garnet separated from residual enclaves is characterized by a narrow range of low δ7Li values (−1.5‰ to −0.1‰) with high Li concentrations from 32.9 to 81.7ppm. By contrast, coexisting quartz shows relatively high δ7Li values (+15.0‰ to +16.6‰) with very low Li concentrations (~1ppm). Biotite from both leucogranite and residual enclaves shows high Li concentrations (195–382ppm) and relatively heavy Li isotope compositions (+3.2‰ to +7.5‰). The Li elemental and isotopic signatures of the residual enclaves can be modeled as a Grt-Bt rich residuum mixed with leucogranite melt in various proportions. This work indicates that the Li isotopic compositions for magmatic rocks that are derived from anatexis of mid to lower crustal gneisses may not be a faithful source indicator as commonly suggested.

Evidence for high‐temperature fractionation of lithium isotopes during differentiation of the Moon

AbstractLithium isotope and abundance data are reported for Apollo 15 and 17 mare basalts and the LaPaz low‐Ti mare basalt meteorites, along with lithium isotope data for carbonaceous, ordinary, and enstatite chondrites, and chondrules from the Allende CV3 meteorite. Apollo 15 low‐Ti mare basalts have lower Li contents and lower δ7Li (3.8 ± 1.2‰; all uncertainties are 2 standard deviations) than Apollo 17 high‐Ti mare basalts (δ7Li = 5.2 ± 1.2‰), with evolved LaPaz mare basalts having high Li contents, but similar low δ7Li (3.7 ± 0.5‰) to Apollo 15 mare basalts. In low‐Ti mare basalt 15555, the highest concentrations of Li occur in late‐stage tridymite (>20 ppm) and plagioclase (11 ± 3 ppm), with olivine (6.1 ± 3.8 ppm), pyroxene (4.2 ± 1.6 ppm), and ilmenite (0.8 ± 0.7 ppm) having lower Li concentrations. Values of δ7Li in low‐ and high‐Ti mare basalt sources broadly correlate negatively with 18O/16O and positively with 56Fe/54Fe (low‐Ti: δ7Li ≤4‰; δ56Fe ≤0.04‰; δ18O ≥5.7‰; high‐Ti: δ7Li >6‰; δ56Fe >0.18‰; δ18O <5.4‰). Lithium does not appear to have acted as a volatile element during planetary formation, with subequal Li contents in mare basalts compared with terrestrial, martian, or vestan basaltic rocks. Observed Li isotopic fractionations in mare basalts can potentially be explained through large‐degree, high‐temperature igneous differentiation of their source regions. Progressive magma ocean crystallization led to enrichment in Li and δ7Li in late‐stage liquids, probably as a consequence of preferential retention of 7Li and Li in the melt relative to crystallizing solids. Lithium isotopic fractionation has not been observed during extensive differentiation in terrestrial magmatic systems and may only be recognizable during extensive planetary magmatic differentiation under volatile‐poor conditions, as expected for the lunar magma ocean. Our new analyses of chondrites show that they have δ7Li ranging between −2.5‰ and 4‰. The higher δ7Li in planetary basalts than in the compilation of chondrites (2.1 ± 1.3‰) demonstrates that differentiated planetary basalts are, on average, isotopically heavier than most chondrites.

Reassessing the cooling rate and geologic setting of Martian meteorites MIL 03346 and NWA 817

Lithium concentration and isotopic fractionation profiles across augite grains from two Martian meteorites – MIL 03346 and NWA 817 – were used to determine their thermal history and implications for their geologic setting. The iron–magnesium zoning and associated magnesium isotopic fractionation of olivine grains from NWA 817 were also measured and provide a separate estimate of the cooling rate. The observed correlation of concentration with isotopic fractionation provides the essential evidence that the zoning of these grains was in fact due to diffusion and thus can be used as a measure of their cooling rate. The diffusion rate of lithium in augite depends on the oxygen fugacity, which has to be taken into account when determining a cooling rate based on the lithium zoning. The Fe–Mg exchange in olivine is much less sensitive to oxygen fugacity, but it is significantly anisotropic and for this reason we determined the direction relative to crystallographic axes of the line along which the Fe–Mg zoning was measured. We found that the cooling rate of NWA 817 determined from the lithium zoning in augite grains and that based on the Fe–Mg zoning of olivines are in good agreement at an oxygen fugacity close to that of quartz–fayalite–magnetite oxygen buffer. The cooling rate of MIL 03346 was found to be resolvably faster than that of NWA 817 – of the order of 1°C/h for the former and of the order of 0.2°C/h for the latter. An important observation regarding the history of MIL 03346 and NWA 817 is that the lithium and Fe–Mg zoning are only observed where the augite or olivine is in contact with the mesostasis, which implies that they were already about 80% crystallized at the time diffusion began. The augite and olivine core compositions while very homogeneous are not in equilibrium with each other, which we interpret to imply that prior to the rapid cooling there must have been a protracted period of the order of years above the solidus, during which the much faster Fe–Mg exchange in olivine compared to that in augite allowed the olivine to maintain equilibrium with a changing melt composition while the augite was not significantly affected. We suggest two possible geological settings for the origin and evolution of MIL 03346 and NWA 817: (1) a slow cooling stage in a crystallization front in a crustal magma chamber, followed by eruption of melt plus portions of the crystallization front onto the surface where the final fast cooling took place at the bottom of a lava flow or melt pond, and (2) eruption of a crystal laden melt as a thick long-lived lava flow where the crystals continued to grow as a cumulate and were rapidly cooled when the overlying lava layer was suddenly drained.

Extreme lithium isotopic fractionation in three zircon standards (Plešovice, Qinghu and Temora).

To understand the behavior of Li in zircon, we have analyzed the abundance and isotopic composition of Li in three zircon standards (Plešovice, Qinghu and Temora) widely used for microbeam analysis of U-Pb ages and O-Hf isotopes. We have mapped Li concentration ([Li]) on large grains, using a Cameca 1280HR Secondary Ion Mass Spectrometer (SIMS). All zircons have a rim 5–20 μm wide in which [Li] is 5 to 20 times higher than in the core. Up to ~20‰ isotopic fractionation is observed on a small scale in the rims of a single zircon grain. The measured δ7Li values range from –14.3 to 3.7‰ for Plešovice, –22.8 to 1.4‰ for Qinghu and –4.7 to 16.1‰ for Temora zircon. The [Li] and δ7Li are highly variable at the rims, but relatively homogenous in the cores of the grains. From zircon rim to core, [Li] decreases rapidly, while δ7Li increases, suggesting that the large isotopic variation of Li in zircons could be caused by diffusion. Our data demonstrate that homogeneous δ7Li in the cores of zircon can retain the original isotopic signatures of the magmas, while the bulk analysis of Li isotopes in mineral separates and in bulk-rock samples may produce misleading data.

Lithium isotope fractionation in the Ganges–Brahmaputra floodplain and implications for groundwater impact on seawater isotopic composition

Lithium isotopes are a promising proxy for reconstructing past weathering processes, but unraveling the seawater record requires a comprehensive understanding of the magnitude and isotopic composition of Li fluxes to the oceans, and of how these change over time. Little information is available on the role of floodplain sediments and groundwater systems in setting the Li isotope signature of the dissolved flux delivered from the continents to the oceans. Here we investigate the Li dissolved fluxes of river waters and groundwaters in the Ganges–Brahmaputra floodplain. The data suggest that a maximum of 3.1×108 and 1.5×108 moles Li/yr are carried to the Bay of Bengal by Ganges–Brahmaputra rivers and groundwaters, respectively. The riverine flux has a significantly heavier Li isotope composition (average δ7Li: 26‰) than the groundwater flux (average δ7Li: 16‰) and increases downstream across the floodplain. δ7Li in both river waters and shallow groundwater can be explained by Li scavenging by Quaternary floodplain sediments following a Rayleigh fractionation process, with preferential removal of 6Li. On the other hand, deep groundwaters (>40 m) contributing to submarine groundwater discharge to the Bay of Bengal are enriched in 6Li at depth, likely due to the dissolution of floodplain sediments releasing Li with a light isotope composition. Similarly low δ7Li has been reported in other large sedimentary aquifers. The deep groundwater values are close to the average isotope composition of the global Li inputs to the ocean (∼15‰), so groundwater submarine discharge has only a minor influence on the assessment of the modern Li isotope budget of the ocean. Our analysis further suggests that groundwater discharge of Li has probably played at most a small and secondary role in past changes in the isotope composition of the total continental flux of Li to the ocean.