Lithium Isotope Fractionation Research Articles

Heavy metal migration and lithium isotope fractionation under extreme weathering of basalt on tropical Hainan Island, China

Basalt weathering can cause heavy metal enrichment in soils, and lithium isotopes serve as an effective tracer for this weathering process. However, the relations among weathering intensity, heavy metal migration, and Li isotope fractionation remain unclear. We conducted systematic examinations of a basalt weathering profile on tropical Hainan Island in China. The characteristics of mineralogy, elemental and Li isotopic geochemistry indicate that the profile has undergone changes from initial to advanced and ultimately extreme weathering stages. The corresponding dominant minerals are primary minerals, clay minerals such as kaolin-group, and the Al–Fe–Ti (hydro)oxides. The variations of Fe-group heavy metals within the profile are very similarly to that of Fe, exhibiting a pattern of loss-gain-loss. Newly formed secondary minerals preferentially incorporate lighter 6Li, leading to a gradual increase in Li content and a decrease in δ7Li. Due to the high δ7Li value of rainwater input, the ongoing impact of secondary mineral adsorption is partially counteracted. This results in the δ7Li of laterite and topsoil being maintained within a narrow range (−3.2‰ ∼ −1.9‰), despite the broader variation observed across the entire profile (−3.2‰ ∼ +2.8‰). Comparing basalt weathering profiles in different climatic zones reveals a very complex Li isotope behavior and its controlling factors. Therefore, it is essential to carefully evaluate the influence of atmospheric input, secondary mineral adsorption, dissolution, and redeposition processes on Li isotopes. Throughout the entire profile, Cr, Ni, and Cu exhibit enrichment relative to soil background values, indicating that the weathering of tropical basalt may pose potential environmental risks for these elements.

Experimental and theoretical constraints on lithium isotope fractionation during brine evaporation and halite precipitation

Marine evaporitic halite, precipitating from the advanced stage of extremely evaporated seawater, is the most common and abundant evaporitic mineral. The extent to which lithium (Li) is incorporated into evaporite halite during geological periods of massive halite deposition, and the mechanisms of incorporation, are both unknown. These are each important, however, for the quantification of the isotope effect of the evaporitic sink on δ7Liseawater, and for the potential use of halite as a recorder of seawater δ7Li. Here, new experimental data are presented for the Li fractionation factor for halite from an isothermal evaporation experiment (25∘C for 100 days) using marine-derived brine. The resultant solids were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to confirm the mineralogy and crystallinity of the product. Our results show that Li isotopes are not constant during the evaporation process implying that there must be a process that fractionates Li isotopes during halite precipitation. We suggest that this results from Li ionic substitution for sodium (Na) in crystalline halite, a scenario supported by ab-initio calculations, but we also show that Li within halite is predominantly present within fluid inclusions. Thus, the Li isotopic composition of halite is controlled by a mixture of Li within the fluid inclusions and Li that is incorporated into the halite crystalline structure. Overall, the brine becomes enriched in Li7 during evaporation due to the preferential incorporation of Li6 into chloride precipitates, and the evolution of δ7Librine is controlled by the precipitated mineral assemblage. The theoretical equilibrium fractionation factor 1000lnαmineral−fluid is -7.8▪ for Li-halite (LiNa26Cl27) at 25 C∘ at equilibrium, which is at the lower limit of Li isotope fractionation factors. Rayleigh fractionation models were fitted to the experimental data yielding a fractionation factor of -25▪3▪. The difference in Li isotope fractionation factor between brine and structurally bound Li may result from kinetic fractionation effects. This study presents a scenario highlighting that extensive halite deposits, formed over geological time in environments with relatively high Li concentration, could potentially impact the δ7Li values of seawater during that period, and place constraints on their utility as an archive for δ7Liseawater.

Lithium isotope fractionation during submarine hydrothermal alteration processes

River water and submarine hydrothermal fluids are the two primary sources of lithium (Li) input to the ocean. However, compared to well-documented weathering processes, the behavior of Li isotopes during hydrothermal alteration processes remains obscure. In this research, we investigated the Li isotopic compositions (δ7Li) of hydrothermally altered sediments from two IODP drilling cores (C0013 and C0014) in the Iheya North Knoll of the middle Okinawa Trough. The sediments, primarily altered from volcanic clasts, exhibit a pristine δ7Li value (5.2 ± 0.7 ‰) similar to that of MORB (3.7 ± 1 ‰). Intense hydrothermal activity led to the complete conversion of primary feldspars into phyllosilicates, predominantly Mg-chlorite, at site C0013 and in the middle and deep parts of C0014. The altered samples showed a depletion of 7Li (ranging in δ7Li from −10.0 ‰ to 4.2 ‰) compared to background sediment, indicating the preferential incorporation of 6Li during the formation of secondary minerals. Furthermore, both drilling sites exhibited increasing trends in δ7Li values with depth, with the deepest samples approaching the pristine δ7Li values of unaltered volcanic rock. These trends were attributed to the significant thermal gradients present at both sites. The temperature-dependent fractionation factor of Li isotopes allowed us to estimate the alteration temperature, which yielded temperature values consistent with those obtained using the δ18O geothermometer. This finding underscores the potential of Li isotopes as a novel geothermometer tool, although further calibration work is required. Based on our research findings, we developed a dissolution-precipitation model (based on mass balance) that establishes a relationship between the δ7Li values of altered solids and hydrothermal fluids, with the aim of elucidating the relatively limited variation observed in global submarine hydrothermal fluids (ranging from 1.6 ‰ to 11.0 ‰, with an averaged value of 5.9 ± 3.0 ‰, mean ± SD). This model quantitatively depicted the strong restriction of the Li concentration in hydrothermal fluids on their isotopic compositions, as high concentrations of Li in the fluids necessitate a substantial release of Li from the solid phase, causing the fluids' δ7Li values to approach that of the bedrock. Additionally, limited fractionation factors at high temperatures further constrain the variations in the fluids' δ7Li values. Model predictions also suggest that the reported δ7Li values of hydrothermal fluids have encompassed the potential δ7Li variation, providing supporting evidence for the assumption that δ7Li values of submarine hydrothermal fluids have exhibited insignificant variations throughout Earth's history.

The genesis of the Chakabeishan Li-(Be) pegmatite deposit in the northern Tibetan Plateau: Evidence from fluid inclusion and lithium isotope

The Chakabeishan (CKBS) deposit is a newly discovered pegmatite-type lithium-beryllium deposit in the northern Qaidam tectonic belt of the northern Tibet Plateau. In recent years, some studies have discussed the genesis of the deposit, but the topic remains unclear. This study constrains the genesis of CKBS deposit based on detailed field observations and mineralogical studies, and presents fluid inclusion data and Li isotopic compositions of Li-rich pegmatites and Li-poor pegmatites. The mineralization characteristics of CKBS deposit shows a trend from weak Be mineralization to Li mineralization in the south to north direction, which shows the magmatic differentiation feature. Four types of fluid inclusions are identified: (1) triphase crystal-bearing inclusions (type-1), (2) biphase CO2-rich inclusions (type-2), (3) biphase aqueous inclusions (type-3), and (4) monophase liquid inclusions (type-4). The Li-rich pegmatites mainly contain type-1 inclusions and type-2 inclusions, while Li-poor pegmatites contain type-3 inclusions and some type-4 inclusions. Microthermometric and Laser Raman spectroscopy analyses show that Li-rich pegmatites formed in a low to medium temperature (210.5 ∼ 381.3 ℃), low to medium salinity (5.23 ∼ 17.82 % NaCl equiv.) NaCl-H2O-CO2 system, whereas Li-poor pegmatites formed in a low to medium temperature (206.4 ∼ 413.4 ℃), low to medium salinity (1.05 ∼ 10.49 % NaCl equiv.) NaCl-H2O system, which indicates a fluid immiscibility between them. In addition, lithium is greatly enriched in Li-rich pegmatites under the positive effects of CO2. The Li-rich pegmatites display high Li content (8362.4 ∼ 15509.6 ppm) but low δ7Li values (1.61 ∼ 1.80 ‰). In contrast, the Li-poor pegmatites display low Li content (23.9 ∼ 231.9) but high δ7Li values (0.79 ∼ 12.32 ‰). This means that the lithium isotopic fractionation is produced by the melt-fluid immiscibility in CKBS deposit, in which a Li-rich system with rich water and poor silicate and a Li-poor system with poor water and rich silicate were formed during the process. Therefore, the CKBS deposit may have formed under the combined effects of magmatic differentiation, melt-fluid immiscibility, and fluid immiscibility.

Lithium isotope fractionation in highly evolved granites and associated magmatic-hydrothermal process: the case of the Xintianling granites in the Nanling Range, South China

ABSTRACT The highly evolved granites are closely associated with rare-metal mineralization. It is still debated whether the enrichment of rare metals is mainly controlled by magmatic or hydrothermal processes. Lithium is a fluid-mobile element that preferentially partitions to the fluid phase when the melt reaches H2O saturation, making the Li isotope system a powerful geochemical tool to record the potential melt-fluid interaction during magma evolution. This study addresses the issue of Li isotope fractionation in highly evolved fine syenogranites (FSGs) from the Xintianling pluton in the Nanling Range, South China. The Xintianling pluton consists of the coarse biotite monzogranites (CBMGs) and evolved FSGs. The identical Nd isotope data of the CBMGs (εNd(t) = -8.2–-8.8) and FSGs (εNd(t) = -7.9–-8.9) suggest that they were derived from the same crustal magma. The FSGs were derived from the highly evolved melts extracted from CBMGs through the fractionation of major rock-forming minerals. This is supported by the compositional shifts and validated by Rb, Sr, and Ba trace element modelling. Li isotopic analysis shows that highly evolved FSGs have heavier Li isotope compositions (δ7Li = 2.6–7.9 ‰) than the CBMGs (δ7Li = 0.1–1.9 ‰). Isotopic modelling, coupled with variations in Li concentrations in FSGs, suggests that diffusive kinetic fractionation by disequilibrium fluid exsolution during the late stage of the granitic magma evolution controls the Li isotope fractionation of the Xintianling granites. Rapid fluid exsolution not only modified the Li isotope composition of the FSGs but also transported the rare metals (e.g. W) and fluxing components (Li, F, and Cl), which resulted in the formation of the large Xintianling W deposits.

The influence of crystal chemistry on lithium isotopic fractionation in pegmatite minerals

The understanding of lithium (Li) isotopic fractionation controlled by mineral species, especially those containing trace or minor amounts of Li (e.g., a few to tens of ppm), is hindered by lack of knowledge about the crystal chemistry of Li-bearing minerals. In this study, we performed solid-state Nuclear Magnetic Resonance (NMR) experiments, employing the Li7 Magic Angle Spinning (MAS) to investigate the local coordination environments of Li in minerals and their impact on Li isotopic fractionation. Our investigation focused on four coexisting minerals sourced from the Koktokay No. 3 pegmatite dike in the Xinjiang, China, namely quartz, K-feldspar, muscovite, and tourmaline, which has Li content and δLi7 value of 14.6 ppm and 3.90‰, 19.3 ppm and 7.09‰, 430 ppm and − 14.2‰, and 108 ppm and 13.42‰, respectively. Muscovite displayed the lowest δLi7 value, whereas tourmaline exhibited the highest value, and a considerable variation of 27.62‰ in δLi7 value was observed between muscovite and tourmaline. The analysis of Li7 MAS-NMR spectra further revealed distinct Li coordination environments among these minerals. A notable counterintuitive negative correlation exists between the δLi7 value and the chemical shift for the four Li-bearing minerals. This negative correlation contrasts to the expected Li isotopic fractionation under equilibrium conditions, in which enrichment of heavy Li isotopes is accompanied with large chemical shifts in Li-bearing minerals. This unexpected observation can be potentially attributed to nonequilibrium conditions of crystallization of the investigated minerals, coupled with fast diffusion of Li6 in minerals, boasting abnormally high coordination numbers due to weak LiO chemical bonds. Essentially, our study suggests that Li isotopic fractionation among Li-bearing minerals in pegmatite can be significant and is intricately influenced by both crystal chemistry and chemical diffusion.

Lithium partition and isotopic fractionation for cation exchange in clay: Insights from molecular simulations

Chemical weathering is one of the most important processes on the Earth’s surface, and lithium isotopes have been used as a promising tracer of silicate weathering, as the secondary clay minerals precipitated during silicate weathering are thought to preferentially incorporate the lighter 6Li isotope. However, to utilize lithium isotopes as a geochemical tracer, it is essential to quantitatively understand the partitioning of Li in clay minerals, including structurally bound Li and exchangeable interlayer ions, as well as the associated isotope fractionation. With classical molecular dynamic simulations and thermodynamic calculations, the cation exchange between Na-/K-montmorillonite and solvated Li in environmental solution are disclosed under different aqueous activity conditions. The Li+ ion, whose hydration ability is stronger than Na+ and K+ ions, is easier to swell the montmorillonite and shows less affinity for the clay. And the calculated cationic selectivity coefficients between Na+/K+ and Li+ ions are Kc (Li → Na) = 2.0, Kc (Li → K) = 10.0 for Arizona-type montmorillonite and Kc (Li → Na) = 1.0, Kc (Li → K) = 4.0 for Wyoming-type montmorillonite, which are comparable to reported experiments. The cationic partition of Li in the clay interlayer decreases when the hydration states of montmorillonite transform from triple-layer to bilayer and then to monolayer. Based on the density functional theory, the Li isotope fractionations for Li in clay interlayer sites and structural octahedral sites are further derived by thermodynamic calculations with path integral molecular dynamic simulation, which deals with the quantum anharmonicity. The behaviors of Li isotope in the bilayer hydration state closely resembles that in the aqueous solution, and only a slight Li isotope fractionation is found between the monolayer hydrated octahedral-substituted clay and solution phase (Δ7Limonolayer-aq = −2.0‰). While Li isotope fractionation between aqueous solution and mica structural sites is significant (Δ7Listructural-aq = −10.4‰) and shows an obvious quantum anharmonicity, which indicates the importance of the anharmonic treatment for Li isotope fractionation calculations. The newly calculated Li equilibrium isotope fractionation between clay and aqueous solution deviates from the experimentally determined fractionation factor, which can be attributed to the kinetic effects. Moreover, on the basis of the surface kinetic reaction model, the Li isotope fractionation becomes more pronounced with higher clay growth rate. By integrating the cationic selectivity coefficient and Li isotope fractionation, it is possible to constrain the fluid composition during Li-clay deposit and diagenesis, and to evaluate the intensity of chemical weathering processes as well as the related carbon cycle.

Biological fractionation of lithium isotopes by cellular Na+/H+ exchangers unravels fundamental transport mechanisms

Lithium (Li) has a wide range of uses in science, medicine, and industry, but its isotopy is underexplored, except in nuclear science and in geoscience. 6Li and 7Li isotopic ratio exhibits the second largest variation on earth's surface and constitutes a widely used tool for reconstructing past oceans and climates. As large variations have been measured in mammalian organs, plants or marine species, and as 6Li elicits stronger effects than natural Li (∼95% 7Li), a central issue is the identification and quantification of biological influence of Li isotopes distribution. We show that membrane ion channels and Na+-Li+/H+ exchangers (NHEs) fractionate Li isotopes. This systematic 6Li enrichment is driven by membrane potential for channels, and by intracellular pH for NHEs, where it displays cooperativity, a hallmarkof dimeric transport. Evidencing that transport proteins discriminate betweenisotopes differing by one neutron opens new avenues for transport mechanisms, Li physiology, and paleoenvironments.

Origin of lithium-rich salt lakes on the western Kunlun Mountains of the Tibetan Plateau: Evidence from hydrogeochemistry and lithium isotopes

Lithium-rich salt lakes have attracted much attention as important lithium resources. The source and enrichment mechanisms of lithium in salt lakes are fundamental to uncovering the genesis of brine-type lithium deposits. However, the origin of lithium in salt lakes has been controversial, especially for lithium-rich brines associated with hard rock lithium ore has not been concerned by scholars. In this study, for the first time, we investigated the lithium isotope and hydrochemical composition characteristics of a lithium-rich salt lake (Kushui Lake) and a series of surrounding water samples in the lithium pegmatites district of West Kunlun, Xinjiang, China. Hydrochemical composition shows that different samples in the basin are rich in Li, Na, and K and poor in Mg and Ca. The water chemistry of the Kushui River, the largest river within the catchment, changes from carbonate type to sulfate subtype through evaporation. The precipitate of sulfate and chloride minerals is also gradually increasing. In addition, Li, B, Sr, Rb, Cs, U, Ti, V, Mn, Cu, Zn, Cr, Co, Ni, and REE are abnormally enriched in the rivers and lakes of the basin, especially in the spring water, which is several times higher than in the rivers. Likewise, dissolved Li and δ7Li showed ranges of 0.01–287.86 mg/L and 3.28–16.44 ‰, respectively. These data were used to determine the origin of lithium and its enrichment process in salt lakes. The results show that the source of Li in Kushui Lake is closely related to the surrounding felsic volcanic rocks, pegmatite-type lithium ore and deep fluids, whereas the water–rock interaction in the deep crust and the chemical weathering in the supergene environment play an influential role during the formation of Li-rich fluids. In addition, the spring samples in the northwestern of Kushui Lake have inherited the lithium isotopic signature of lithium-bearing source rocks from the water–rock reaction process in the deep crust, allowing us further to isolate the source of Li in the target waters. With the movement of water, lithium exhibits a trend of continuous enrichment under the influence of evaporation and mixing. At different stages of evaporation, the Li/Na ratio in water in this catchment shows a trend of increasing, decreasing, and increasing due to the adsorption of 6Li by secondary minerals and the crystallization of Na. It suggests that strong evaporation under arid conditions is the main mechanism responsible for the rapid enrichment of Li in salt lakes. Moreover, the positive relationship between Li content and the δ7Li value of rivers along the discharge path indicated that the adsorption of 6Li by secondary minerals still existed even at a minor watershed scale, resulting in the fractionation of lithium isotopes in river waters. This study may contribute to a deeper understanding of lithium sources and their enrichment processes in Li-rich salt lakes. In addition, it is also beneficial to search for new lithium resources based on the source relationships among the various lithium reservoirs.

Lithium Isotope Fractionation During Intensive Felsic Magmatic Differentiation

AbstractThe Xihuashan and Yaogangxian granitic plutons in South China comprise highly evolved multiphase Li‐rich granites and host quartz‐vein‐type tungsten deposits. The δ7Li values of Phase A (early stage), B (middle stage), and C (late stage) from the Xihuashan pluton are 1.0–1.2‰, 1.1–3.0‰, and 2.4–2.8‰ respectively, increasing through chemical evolution. The granites from the Yaogangxian pluton also display gradually enriched in heavy Li isotopes in a later stage, although systematically lighter than those of the Xihuashan pluton. In both plutons, the δ7Li shows good correlations with SiO2 and Li concentrations as well as Rb/Sr, Nb/Ta, and Zr/Hf ratios, indicating Li isotopic fractionation most likely caused by magmatic differentiation. In situ analyses show that the minerals of Xihuashan pluton record a continuous elemental spectrum, reflecting the results of progressive magmatic differentiation. The δ7Li values of quartz, feldspar, mica, and zircon all correlate well with the chemical evolutions of granitic magma, systematically elevated in Phases B and C relative to Phase A. The Li isotope data of the mineral separates further document that the enrichment of 7Li in the residual melt was most likely due to the equilibrium fractionation between the mineral and melts. The data are interpreted to reflect that intense magmatic differentiation was responsible for Li isotopic variations coupled with the enrichment in the Li, F, P, and rare metals in the late‐phase granites of the Xihuashan pluton. The lithium isotope behavior documented in this study provides new insights into magmatic differentiation and associated rare‐metal mineralization.

Increased primary mineral dissolution control on a terrestrial silicate lithium isotope record during the middle Miocene Climate Optimum

Lithium isotopes have great potential to fractionate at subaerial environment, so they are promising proxies for tracing chemical weathering history. However, it remains uncertain how to relate lithium isotopic compositions (δ7Li) in sediments to silicate weathering and past climate in terrestrial setting. To tackle this question, we choose the warm and wet middle Miocene climate optimum (MMCO, 17–14 Ma) with large climate variation amplitude and the following cold and dry middle Miocene climate cooling period (MMCC, 14–11 Ma) with damped climatic variation amplitude, as two climate contrasting intervals to study lithium isotopes and content ([Li]) variations of terrestrial mudrocks. Our study site is located at the eastern Qaidam Basin of the northeastern Tibetan Plateau and the precipitation pattern of this part of the basin is similar to the Chinese Loess Plateau over the two studied intervals. The study site is suitable for clarifying link between climate and δ7Li variations because previous studies have demonstrated no provenance shift over the studied interval and short transport distance from source to the study site suggests little chemical weathering over the transportation process. Both δ7Li and [Li] values show larger amplitude over the MMCO (spanning from −1.2 to 3.2 ‰ and from 32.8 to 93.9 ug/g, respectively) than the MMCC interval (−1.8 to 1.2 ‰ and 41.7–80.8 ug/g, respectively), consistent with general feature of global and Chinese Loess Plateau climate. However, δ7Li data are higher over the MMCO in comparison with the MMCC, contrary to expectation of more lithium isotope fractionation at warm interval. We interpret the more positive δ7Li values over the MMCO using increased ratio of primary mineral dissolution over secondary mineral precipitation. Qaidam mudrock and benthic Li records show complementary changes, suggesting their possible link. Furthermore, past research reveals lack of focused uplift of the northeastern Tibetan Plateau over the studied 18–11 Ma. So we conclude that global climate, rather than Tibetan uplift, played an important role in modulating inland Qaidam Basin climate variations over the middle Miocene and that increased chemical weathering over warm climate can lead to a small magnitude of lithium isotope fractionation. Therefore, interpreting chemical weathering history, and hence climate, using lithium isotope data in terrestrial setting requires considering relative importance of primary mineral dissolution versus secondary mineral precipitation.

Lithium isotope fractionation during the weathering of granite: Responses to pH

The weathering of silicate rock is a major sink of atmospheric CO2 in the global carbon cycle and thus controls global climate change. Lithium (Li) isotopes are considered to be a sensitive proxy to trace the continental weathering, but factors controlling the fractionation behaviors of Li isotopes during chemical weathering of granite have not been addressed explicitly. Here, we present new Li isotope data of bulk samples, primary minerals, and secondary minerals from a granite weathering profile in South China to constrain the mechanisms of Li isotope fractionation during chemical weathering. The bulk samples of weathering products have relatively negative τLiTiO2 values ranging from −6.0% to −73.8% and δ7Li values ranging from −11.4‰ to +2.4‰. From the bottom to the top of the weathering profile, the pH gradually decreases (from 9.54 to 4.88). With decreasing pH, the Li concentrations of the weathered saprolites gradually decrease, while the δ7Li values of the saprolites decrease first and then increase. Li concentrations and Li isotope compositions of the constituent minerals vary considerably. Interestingly, Li isotope ratios of both the bulk samples and minerals show two distinct fractionation behaviors during chemical weathering under different pH conditions. The heavy isotope (7Li) is preferentially released into the liquid phase under basic and weakly acidic pH conditions and the light isotope (6Li) is preferentially released into the liquid phase under acidic pH conditions. This finding helps to explain the inconsistent fractionation behaviors of Li isotopes between laboratory experiments and field studies. The influence of pH on Li isotope fractionation during silicate weathering implies that the evolution of oceanic pH over geological time scales may partly account for the variations in oceanic Li isotope compositions.

Mineralogy and fluid chemistry controls on lithium isotope fractionation during clay adsorption

Our current understanding of controls on δ7Li variability and fractionation mechanisms is limited, complicating the interpretation of chemical weathering. The role of clay adsorption in Li isotope fractionation during chemical weathering has been confirmed. However, clay assemblage and fluid chemistry are not simple and often variable in weathering settings, potentially modulating Li isotope fractionation on Earth's surface. Here, this research investigated the patterns and processes of Li isotope fractionation during adsorption on kaolinite and smectite with fluid chemistry of 0.001 M NaCl, 0.5 M NaCl, and 0.001 M Na2HPO4. Specifically, the time-dependent experiments with the reaction period up to 15 days revealed that the steady state can be achieved within one day under neutral conditions. The concentration-dependent (initial Li concentration of 2 to 1000 μM) experiments confirmed the accumulation of Li+ in smectite interlayers and adsorption of Li+ only at the external surfaces of kaolinite. Using 0.5 M NaCl solution and the desorption experiments, we hypothesize that outer-sphere Li may exist in the interlayer sites, which can be replaced by excess Na+. In comparison, inner-sphere Li+ (unexchangeable) potentially dominates at the edge surface of clays. The presence of Na2HPO4 increases the binding capacity for Li+ adsorption, in particular for kaolinite. In all cases, 6Li is enriched on clay surfaces and interlayer spaces, consistent with field observations. Fluid chemistry may affect the degree of clay Li adsorption but exerted negligible impacts on isotope fractionation. For kaolinite, a wide variation (up to 30 ‰) in isotopic fractionation between adsorbed and aqueous Li (Δ7Liaq-ad) exists, conforming to a kinetic fractionation mechanism with a constant fractionation factor αad-aq of ~0.992. By contrast, the isotopic fractionation between Li adsorbed on smectite and Li+ left in solutions keeps constant (Δ7Liaq-ad of ~5 ‰), likely following an equilibrium isotope fractionation law with an αad-aq of ~0.995.

“The role of ignimbrites and fine sediments in the lithium distribution and isotopic fractionation in hyperarid environments: Insights from Li-isotopes in the Atacama Desert”

The Atacama region in Northern Chile hosts the driest desert on Earth and provides the second highest global Li production, containing the largest reported lithium reserves in the world. The occurrence of extremely elevated Li concentration is restricted to brine deposits contained in the halite nucleus of the Salar de Atacama (SDA). The origin of Li and other solutes in the Andean salt flats has been attributed to different processes, where weathering of Neogene and Quaternary volcanic rocks has been considered to be the most important. Despite recent advances, there is still limited geochemical and isotopic Li data in ignimbrites, particularly surrounding the SDA, hence the potential of these units as a source has remained elusive. In this study, we provide new Li concentration and isotopic data in ignimbrites from the Western Cordillera, fine sediments from the marginal zone of the SDA, and suspended load in shallow groundwater. A moderate enrichment (between 20 and 50 ppm) is recorded in volcanic rocks, averaging 33 ppm, while fine sediments present Li contents one order of magnitude higher, with a mean concentration of ~290 ppm. Due to the Li contents recorded in ignimbrites, which is slightly higher than the upper crust average (24–30 ppm; Rudnick and Gao, 2004), we propose that the potential of these units as the main source of Li resides in their large areal extent, volume, and hydraulic gradient with respect to the SDA. The δ7Li values in ignimbrites vary from −1.5 ‰ to +12.8 ‰. In contrast, marginal zone sediments show negative values of δ7Li (from −1.9 ‰ to −5.3 ‰), except for fine sediments from the Peine Block where a δ7Li is +4.4 ‰. Our results reveal that the measured δ7Li in ignimbrites would indicate volatile exsolution as the main syn-eruptive factor controlling the isotope Li distribution. Regarding post-eruptive processes, water-rock interactions trigger partition of 7Li from ignimbrite to water. Both syn-eruptive and post-eruptive proposed were tested by numerical modeling, supporting our interpretation. We propose that the predominant constituent in ignimbrites, crystals or glass, impacts the subsequent distribution and fractionation of Li in inflowing waters and fine sediments covering the marginal zone. The analysis presented here allows us to define three subzones where different processes are controlling Li fractionation. These factors together with the geological, hydrologic, and climatic history of the region have led to the world-renowned accumulation of Li in the SDA.

Lithium and boron diffusivity and isotopic fractionation in hydrated rhyolitic melts

Lithium and boron are trace components of magmas, released during exsolution of a gas phase during volcanic activity. In this study, we determine the diffusivity and isotopic fractionation of Li and B in hydrous silicate melts. Two glasses were synthesized with the same rhyolitic composition (4.2 wt% water), having different Li and B contents; these were studied in diffusion-couple experiments that were performed using an internally heated pressure vessel, operated at 300 MPa in the temperature range 700–1250 °C for durations from 0 s to 24 h. From this we determined activation energies for Li and B diffusion of 57 ± 4 kJ/mol and 152 ± 15 kJ/mol with pre-exponential factors of 1.53 × 10–7 m2/s and 3.80 × 10–8 m2/s, respectively. Lithium isotopic fractionation during diffusion gave β values between 0.15 and 0.20, whereas B showed no clear isotopic fractionation. Our Li diffusivities and isotopic fractionation results differ somewhat from earlier published values, but overall confirm that Li diffusivity increases with water content. Our results on B diffusion show that similarly to Li, B mobility increases in the presence of water. By applying the Eyring relation, we confirm that B diffusivity is limited by viscous flow in silicate melts. Our results on Li and B diffusion present a new tool for understanding degassing-related processes, offering a potential geospeedometer to measure volcanic ascent rates.

Lithium isotope fractionation during magmatic differentiation and hydrothermal processes in post-collisional adakitic rocks

To investigate the behavior of Li isotopes during magmatic differentiation and the petrogenesis of Cu-bearing ore deposits, a suite of post-collisional adakitic rocks from Qulong region, southern Tibet, was studied. Their lithologies range from diorite through granodiorite to granite porphyry with the latter containing giant Cu deposits. Detailed evidence of field observation and geochemical signature suggest that these three sets of rocks were most likely formed by various degrees of partial melting and fractional crystallization from the same source. The dioritic enclave, granodiorite and granite porphyry have δ7Li values ranging from 0.2 to 8.2‰, 3.1 to 6.8‰, and 3.9 to 7.4‰, respectively. Most of these samples are overlapping in Li isotope composition, comparable to other granitoids worldwide, indicating insignificant Li isotope fractionation during partial melting and magma differentiation in adakite-like rocks. By contrast, their Li concentrations are mainly controlled by fractional crystallization as suggested from different modal mineralogy. This process does not lead to further enrichment of Cu although they have initial high concentrations (an average of ∼ 104 ppm). In comparison, granite porphyry has extremely high Cu contents (up to 2000 ppm) and their δ7Li values are positively correlated with Cu content, suggesting the involvement of magmatic fluids that most likely exsolved from deep magma chamber. Such fluids not only modified the Li isotopic compositions of granite porphyries, but also extracted metal elements from the highly evolved magma, eventually resulting in the Cu mineralization. Our work here provides new insight into the formation and evolution of the porphyry Cu-bearing deposit.

Secondary Minerals Drive Extreme Lithium Isotope Fractionation During Tropical Weathering

AbstractLithium isotopes are used to trace weathering intensity, but little is known about the processes that fractionate them in highly weathered settings, where secondary minerals play a dominant role in weathering reactions. To help fill this gap in our knowledge of Li isotope systematics, we investigated Li isotope fractionation at an andesitic catchment in Puerto Rico, where the highest rates of silicate weathering on Earth have been documented. We found the lowest δ7Li values published to date for porewater (−27‰) and bulk regolith (−38‰), representing apparent fractionations relative to parent rock of −31‰ and −42‰, respectively. We also found δ7Li values that are lower in the exchangeable fraction than in the bulk regolith or porewater, the opposite than expected from secondary mineral precipitation. We interpret these large isotopic offsets and the unusual relationships between Li pools as resulting from two distinct weathering processes at different depths in the regolith. At the bedrock‐regolith transition (9.3–8.5 m depth), secondary mineral precipitation preferentially retains the lighter 6Li isotope. These minerals then dissolve further up the profile, leaching 6Li from the bulk solid, with a total variation of about +50‰ within the profile, attributable primarily to clay dissolution. Importantly, streamwater δ7Li (about +35‰) is divorced entirely from these regolith weathering processes, instead reflecting deeper weathering reactions (>9.3 m). Our work thus shows that the δ7Li of waters draining highly weathered catchments may reflect bedrock mineralogy and hydrology, rather than weathering intensity in the regolith covering the catchment.

Temperature dependent lithium isotope fractionation during glass dissolution

Understanding the mechanisms by which borosilicate glasses corrode in contact with aqueous solutions remains a challenge to the safety case for the geological disposal of vitrified high-level nuclear waste. Here, lithium isotope fingerprinting techniques were applied to the leachates of a simulant Magnox waste glass to probe the mechanisms of aqueous corrosion at both short and long timescales (6 hours to 464 days). Experiments took place at 40 and 90 °C to assess the consistency of the dissolution mechanisms across a range of commonly employed temperatures and the legitimacy of applying higher temperature experimental datasets to understand glass corrosion within a disposal facility at lower temperatures.Two competing release mechanisms were observed for lithium (diffusion and hydrolysis), and the relative proportions of these mechanisms changed through time. Leachates initially had lower δ7Li values than the pristine glass (−2.7‰ at 40 °C and −1.1‰ at 90 °C relative to the pristine glass) at both temperatures due to lithium leaching incongruently through diffusive processes. The greater offset between solution and solid at lower temperatures indicates a larger rate of diffusion (incongruent dissolution) relative to the rate of hydrolysis (congruent dissolution) at lower temperatures. The fraction of lithium released through diffusion relative to the fraction of lithium released through hydrolysis then increased at both temperatures with time up to 126 days, increasing from 0.47 and 0.22 at 6 hours to 0.66 and 0.41 at 126 days at 40 and 90 °C respectively. Subsequently, the fractions of lithium released through diffusion sharply decreased to 0.36 at 40 °C and 0.22 at 90 °C after 464 days, consistent with network hydrolysis coupled with secondary phase precipitation later controlling the long-term release of Li at both temperatures. Throughout the duration of the experiments (464 days) the δ7Li values in solution increased to 9.0‰ at 40 °C and 10.0‰ at 90 °C due to the formation of talc and montmorillonite phases at 40 °C and additional smectite phases at 90 °C. Further, no evidence for the formation of a diffusive barrier to the transport of Li within the alteration layers became apparent during the later stages of dissolution at either temperature. However, the fraction of lithium leached through diffusion was still significant throughout all stages of dissolution. Lithium isotope ratios in solution were correlated with the transition from a system which was increasingly dominated by lithium diffusion as the dissolution rate slowed to one which was controlled by hydrolysis coupled with secondary phase precipitation at long durations. Alongside elemental ratios in solution, these results were consistent with the same set of mechanisms governing dissolution across the temperature range studied.

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

This study experimentally investigates the isotopic behaviors of Li and K during the dissolution of silicate rocks (i.e., basalt and granite). Proton-driven dissolution (in 0.8 M HNO3) and ligand-driven dissolution (in 5 mM critic acid or oxalic acid) experiments were performed in batch-closed systems over 15 days. We provide a time-series interpretation of Li and K isotope fractionation during silicate dissolution in ultra-acidic (unidirectional) and near-natural (biologically affected) environments. As the reaction progressed, we measured large isotope fractionation between the liquid (l) phase and the pristine silicate (s) phase, ranging from −10.3 to 0.1‰ (Δ7Lil-s) and from −1.01 to −0.11‰ (Δ41Kl-s) through the early stage of dissolution (<24 h). The enrichment of lighter Li and K isotopes in the solutions rapidly diminished as rock dissolution continued and gradually approached equilibrium to the end of experiments. In contrast, resorption of pre-leached isotopically lighter Li on silicate residuals during ligand-driven dissolution produced lighter isotope enrichment in the solutions compared to the initial rock by up to 2.8‰. Despite the preferential dissolution of specific minerals, the isotope fractionation patterns of Li and K do not vary with lithology, indicating limited inter-mineral isotopic differences. During the experiments, the Li and K isotopic pattern could be divided into two-to-three stages. The initial enrichment of light isotopes in the liquids can be ascribed to the kinetic isotope effect, confounded by diffusion and ion solvation. A later transition towards no isotope fractionation of Li and K may be explained by (i) the masking effect from dissolution, and (ii) an imprint from the destruction of 7Li/41K-enriched surface layers. Lateral resorption of solute Li after ~100 h reaction could be facilitated by the electrostatic attraction from increasing surface negative charges and active hydroxyls with increasing pH during ligand-driven dissolution (pH ~ 4) relative to proton-driven dissolution (pH ~ 0.2). Therefore, the presence of organic ligands impacts dissolution stoichiometry, and potentially modifies Li isotope fractionation in natural weathering environments. In comparison, K isotope fractionation driven by rock dissolution stops immediately (within days) after starting the experiments. This research helps to understand the mechanisms of Li and K isotope fractionation during chemical weathering and trace long-term climate change using geological records.

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.