Lithium Isotopes Research Articles

Lithium isotope behaviour during basalt weathering experiments amended with organic acids

Lithium (Li) isotopes are a tracer of silicate weathering processes, but how they react to different components of organic and plant-assisted weathering is poorly known. To examine the effect of organic acids compared to a strong mineral acid (HCl) on Li isotope behaviour, basalt-water weathering experiments were amended with different organic acids (glycine, malic acid, cinnamic acid, and humic acid; 0.01 M). The presence of the different acids significantly affects the behaviour of dissolved elemental concentrations (such as Mg, Fe, and Al), both by increasing primary rock dissolution and hindering rates of secondary mineral formation. However, the behaviour of Li isotopes appears unaffected, with all experiments following an almost identical trend of δ7Li versus Li/Na. This observation was consistent with a single fractionation factor during the uptake of Li into secondary minerals, yet both calculated saturation states and leaching experiments on the reacted solids indicated that Li was removed into multiple phases, suggesting that the bulk combined fractionation factor barely varied. Of the Li lost from solution in the organic experiments, we estimated that on average 76% went into neoformed clays, 16% into oxides/oxyhydroxides, and 10% into the exchangeable fraction. The fractionations observed for each phase were Δ7Liexch-soln = −12.7 ± 1.7‰, Δ7Liox-soln = −26.7 ± 0.4‰, and Δ7Liclay-soln = −21.6 ± 3.3‰. These fractionations were identical, within error, to those from experiments with organic-free water, implying that the Li isotope behaviour was unaffected by the presence of organic acids in the weathering reaction. This result has interesting consequences for the interpretation of Li isotopes in terms of plant-assisted weathering and the geological record of terrestrialisation. In particular, it appears to imply that seawater Li isotope records can be expected to resolve the integrated effect of plants on weathering fluxes or weathering congruence, rather than being sensitive to specific organic-mediated weathering mechanisms.

Weathering intensity and lithium isotopes: A reactive transport perspective

Lithium isotopes have emerged as a powerful tool to probe the response of global weathering to changes in climate. Due to the preferential incorporation of ^6^Li into clay minerals during chemical weathering, the isotope ratio δ^7^Li may be used to interrogate the balance of primary mineral dissolution and clay precipitation. This balance has been linked to relative rates of chemical and physical denudation, such that dissolved δ^7^Li (δ^7^Li~diss~) is highest at moderate weathering intensities when chemical and physical denudation are comparable. However, we argue that current theory linking δ^7^Li to weathering regimes through fluid travel times are unable to explain observations of low δ^7^Li and high Li concentrations in rapidly eroding settings. In this study, we re-examine the relationships between δ^7^Li, Li concentration, and weathering regime by incorporating Li isotopes into simulations of weathering profiles using a reactive transport model (CrunchFlow) that includes advective fluxes of regolith to simulate variable erosion rates in response to uplift. In these simulations, fractionation is implemented through a kinetic fractionation factor during clay precipitation, which allows the δ^7^Li of dissolved and suspended loads in the model to vary as a function of Li/Al ratios in primary and secondary minerals. When the model is run over a range of infiltration and erosion rates, simulations reproduce observed global patterns of δ^7^Li~diss~ and suspended load δ^7^Li as a function of weathering intensity, controlled primarily by water travel times and mineral residence times in weathered bedrock. We find that reduced water travel times at low weathering intensity, however, are inconsistent with observations of high Li concentrations. As an alternative, we demonstrate how the rapid weathering of soluble, Li-rich minerals such as chlorite under low weathering intensities may resolve this apparent discrepancy between data and theory. We also suggest that observed patterns are consistent with geothermal Li sources under low weathering intensities. This work offers a foundation guiding future studies in testing potential mechanisms underlying global riverine δ*^7^*Li~diss~.

Effects of contact metamorphism on the lithium content and isotopic composition of kerogen in coal

Lithium isotopes (δ7Li) in coals have been shown to increase with thermal maturity, suggesting preferential release of 6Li from kerogen to porefluids. This has important implications for paleoclimate studies based on δ7Li of buried marine carbonates, which may incorporate Li from porefluids during recrystallization. Here, the Li content and isotopic composition of macerals from two coal seams intruded by dikes, were studied as a function of temperature across a thermal gradient into the unmetamorphosed coal. Samples were collected in Colorado (USA) from a Vermejo Fm. coal seam intruded by a mafic-lamprophyre dike and compared to a Dutch Creek No.2 coal seam intruded by felsic-porphyry dike; a potential source of Li-rich fluids.The Li-content and Li-isotope compositions of coal macerals were measured in situ by Secondary Ion Mass Spectrometry (SIMS). The macerals of the Vermejo coal samples, buried to VRo 0.68% (Tmax = 104 °C), contained <1.5 μg/g Li with an average vitrinite δ7Li of −28.4 ± 1.6‰, while liptinite and inertinite were heavier, averaging −15.4 ± 3.6‰ and − 10.5 ± 3.7‰, respectively. The contact metamorphosed vitrinite/coke showed the greatest change with temperature with δ7Li 18 to 37‰ heavier than the unmetamorphosed vitrinite.The Dutch Creek coal, buried to VRo 1.15% (Tmax = 147 °C), prior to dike emplacement, may have released Li during burial, as less isotopic change was observed between contact metamorphosed and unmetamorphosed macerals. Overall, Li contents were < 1 μg/g, and the vitrinite in metamorphosed coal had δ7Li values 8 to 21‰ heavier than the unmetamorphosed coal. SIMS measurements on macerals near the dike did not show an increase in Li-content indicative of Li derived from dike fluids, however previous bulk measurements that included silicates showed slightly higher (2-3 μg/g) Li-contents near the dike, suggesting possible Li incorporation from dike fluid into metamorphic silicates. A negative correlation was observed between Li-content and 12C+/30Si+ count ratios, indicating that at metamorphic temperatures Li becomes concentrated in silicates.

Seasonal River Chemistry and Lithium Isotopes in the Min Jiang at Eastern Tibetan Plateau: Roles of Silicate Weathering and Hydrology

Riverine lithium (Li) isotopes have been considered as a robust tracer for silicate weathering, but processes controlling riverine δ7Li ratios remain controversial. To address the impacts of weathering and hydrology on riverine δ7Li, the seasonal variation of water chemistry in the Min Jiang at the eastern Tibetan Plateau was investigated over December of 2009 to the end of 2010. The results showed distinct seasonal variations in ionic chemistry and δ7Li. Increased river discharge in the monsoon season diluted dissolved ions, and monsoonal hydrological changes caused frequent δ7Li fluctuations. High discharge caused by monsoonal rainfall reduced Li isotope fractionation by shortened rock–fluid interaction time, resulting in lower δ7Li, whereas the input of high δ7Li groundwater and landslide seepage elevated riverine δ7Li, together with lengthened rock–fluid interaction time in less rain intervals. Based on the high-resolution sampling strategy and dataset over one hydrological year, this study highlights that changes of hydrological conditions can have a significant impact on weathering processes and water sources, and therefore on riverine δ7Li variation.

Lithium in garnet as a tracer of subduction zone metamorphic reactions: The record in ultrahigh-pressure metapelites at Lago di Cignana, Italy

Abstract Lithium is of great interest as a tracer of metamorphic reactions and related fluid-mineral interactions because of its potential to isotopically fractionate during inter- and intracrystalline diffusional processes. Study of its transfer through subduction zones, based on study of arc volcanic and metamorphic rocks, can yield insight regarding ocean-to-mantle chemical cycling. We investigated major- and trace-element concentrations and δ7Li in garnet in ultrahigh-pressure (UHP) Lago di Cignana metasedimentary rocks, relating these observations to reconstructed prograde devolatilization history. In all garnet crystals we studied, heavy rare earth elements (HREEs), Y, and Li showed strong zoning, with elevated concentrations in cores (15–50 ppm Li) and marked high-concentration anomalies (up to 117 ppm Li, 5500 ppm Y; little or no major-element shift) as growth annuli, in which some crystals showed subtle elevation in δ7Li greater than analytical error of ~3% (2σ). Rutile inclusions appeared abruptly at annuli and outward toward rims, accompanied by inclusions of a highly zoned, Ca- and rare earth element–rich phase and decreased Nb concentrations in garnet. These relationships are interpreted to reflect prograde garnet-forming reaction(s), in part involving titanite breakdown to stabilize rutile, which resulted in delivery of more abundant Y and HREEs at surfaces of growing garnet crystals to produce annuli. Co-enrichments in Li and Y + REEs are attributed to mutual incorporation via charge-coupled substitutions; thus, increased Li uptake was a passive consequence of elevated concentrations of Y + REEs. The small-scale fluctuations in δ7Li (overall range of ~9%) observed in some crystals may correlate with abrupt shifts in major- and trace-element concentrations, suggesting that changes in reactant phases exerted some control on the evolution of δ7Li. For one garnet crystal, late-stage growth following partial resorption produced deviation in major- and trace-element compositions, including Li concentration, accompanied by a 10%–15% negative shift in δ7Li, perhaps reflecting a change in the mechanism of incorporation or source of Li. These results highlight the value of measuring the major- and trace-element and isotope compositions of garnets in high-pressure and UHP metamorphic rocks in which matrix mineral assemblages are extensively overprinted by recrystallization during exhumation histories. Lithium concentrations and isotope compositions of the garnets can add valuable information regarding prograde (and retrograde) reaction history, kinetics of porphyroblast growth, intracrystalline diffusion, and fluid-rock interactions. This work, integrated with previous study of devolatilization in the Schistes Lustrés/Cignana metasedimentary suite, indicates retention of a large fraction of the initially subducted sedimentary Li budget to depths approaching those beneath volcanic fronts, despite the redistribution of this Li among mineral phases during complex mineral reaction histories.

Lithium isotope behavior under extreme tropical weathering: A case study of basalts from the Hainan Island, South China

Chemical weathering of silicate rocks (especially basalt) is an important sink of atmospheric carbon dioxide, and has an important impact on long-term carbon cycle and storage. Lithium (Li) isotopes have been regarded as a good tracer of chemical weathering, however, the behavior of Li isotopes in tropical settings is poorly understood. In this study, we studied the Li elemental and isotopic variation in the solid weathering products (soil and saprolite) from a highly-weathered (CIA ∼100%) basalt weathering profile (>15 m thick, including soil, saprolite, semi-weathered rock and fresh basalt) on the tropical Hainan Island in South China. The weathering products have 1.99–58.1 ppm Li, mostly below the average fresh basalt value (4.6 ppm, n = 5). The δ7Li values (−14.3 to 8.9‰) of the solid weathering products exhibit complex stratigraphic variation across the weathering profile. The Li and Mn enrichments coincide spatially, with the Mn-rich samples having the highest Li content and lowest δ7Li value. Lithium is enriched in the semi-weathered basalt sample, but its δ7Li value (−2.7‰) is close to that of fresh basalts (0.7–5.3‰, avg. 3.0‰, n = 5). Strontium isotope data show that the weathering profile is affected by marine aerosol input, and the marine strontium may have been leached to the profile bottom or out of it under extreme weathering. Sequential extraction results indicate that Li in the soil, saprolites (lower part), and semi-weathered basalt occurs mainly in the residual phase, while Li in the saprolites (middle part) occurs largely in Fe–Mn oxides. We suggest that Li isotope fractionation in the upper, middle and lower parts of the profile was controlled by, respectively, marine aerosol input and secondary mineral leaching, dissolution/re-precipitation of Fe–Mn oxides, and neoformation of secondary minerals from heavy pore water. Our results show that the preferential light Li isotope uptake by Fe–Mn oxides may have been the main cause for the major Li isotope fractionation in the Wenchang weathering profile.

Behaviors of lithium and its isotopes in groundwater with different concentrations of dissolved CO2

Lithium (Li) isotopes have shown large potential in tracing weathering in various water bodies, but there is limited study on Li isotopes in subsurface conditions where CO2 has been largely consumed. In this study, we use a thick sandstone aquifer in the Ordos Basin, NW China, as a natural setting to investigate the behaviors of Li isotopes in hydrogeochemical conditions with different concentrations of dissolved CO2. For young groundwater in the recharge area (group R) where CO2 is abundant (mean PCO2 = 10-2.5 atm), clay formation accompanying with weathering leads to the enrichment of 7Li in groundwater. The four deep samples in the recharge area have uniform Li/Na ratios (with a mean of 2.52 μmol/mmol) and δ7Li (with a mean of 25.0‰), corresponding to a mean Li removal rate of 81.2% compared with the sandstone leachate. For groundwater in the shallow part of the discharge area (group D1), Li was firstly removed by clay formation during weathering in the recharge area and was later removed by physisorption when CO2 becomes much lower (mean PCO2 = 10-3.1 atm). Different degrees of weathering lead to a wide range of δ7Li varying from 19.7‰ in the deepest well to 33.0‰ in the shallowest well. The proportion of Li removal caused by physisorption is found to increase with groundwater age. After the stage of Li removal by adsorption, Li was released in the deeper part of the discharge area (group D2), and the positive correlation of δ7Li versus Li/Na is explained by a ternary mixing model. The endmember of water brought by cation exchange is inferred to have a heavier δ7Li than sandstone leachate, demonstrating that cation exchange could cause an enrichment of 7Li in water. This study enhances our understanding of the controlling factors of Li isotopes in deep groundwater with low dissolved CO2, which have implications for the application of Li isotopes in subsurface water.

Source and Enrichment Mechanism of Lithium in the Triassic Argillaceous Marine Sediments from Huangjinkou, Sichuan, China

AbstractIn the Triassic marine sediments, an obvious enrichment of lithium has been found. The source and enrichment mechanism of lithium is unknown. Here, we report trace and rare earth element and isotope analyses for Triassic sedimentary samples from core ZK601, recovered from the Huangjinkou anticline in the Xuanhan basin. Lithium concentrations from the Leikoupo and Jialingjiang formations are much higher than the average concentrations in the crust of eastern China and in other marine sediments. Lithium concentrations are highest at depths of 3300–3360 m (in argillaceous marine sediments), and Li is positively correlated with Rb, Ga, Zr, Nb and other trace elements. The range of δ7Li values in our samples is consistent with that in other Triassic marine carbonate rocks. Lithium concentrations and isotope ratios are negatively correlated in the argillaceous dolomite samples at depths of 3300–3360 m. We compared the results in this study with trace and rare earth elements in the clay from Sichuan and Chongqing, and propose that the clay in the argillaceous marine evaporites from Huangjinkou formed via the hydrolysis of volcanic ash during Early–Middle Triassic volcanic eruptions into brine basins, during which clay adsorbed Li from the brine and formed Li‐rich argillaceous dolomites. The addition and hydrolysis of volcanic ash in the evaporative brine is also related to the formation of a new type of polyhalite.

Lithium Isotope Electromigration Separation in an Ionic Liquid–Crown Ether System: Understanding the Role of Driving Forces

Lithium Isotope Electromigration Separation in an Ionic Liquid–Crown Ether System: Understanding the Role of Driving Forces

Episodic fluid flow in an eclogite-facies shear zone: Insights from Li isotope zoning in garnet

Abstract Episodic fluid overpressure and escape is invoked as a cause or consequence of many subduction-zone seismic phenomena but can be challenging to constrain in exhumed high-pressure metamorphic rocks. In situ measurements of lithium isotopes in garnet reveal evidence of episodic fluid transport in a subduction shear zone now exposed in the Monviso ophiolite (Western Alps). Garnet from an eclogite block and associated metasomatic reaction rind was analyzed by secondary ion mass spectrometry (SIMS). All analyzed garnet preserves core-rim zoning in δ7Li and large negative δ7Li excursions (NEs) in mantles. These excursions cannot be explained by instrumental mass fractionation during analysis, equilibrium fractionation, or intracrystalline diffusion of Li within garnet. Instead, NEs were produced by kinetic fractionation of Li isotopes during bulk diffusion through a pore fluid, and the fractionated isotopic compositions were incorporated into garnet by syn-diffusion growth. Disequilibrium garnet growth textures associated with negative δ7Li support this interpretation and suggest metasomatism drove rapid garnet growth. Four distinct NEs were identified requiring that at least four pulses of fluid were transported within the adjacent shear zone. This evidence of episodic fluid transport along a subduction shear zone at eclogite facies supports models of intermediate-depth seismicity that rely on cyclic fluid overpressure and escape.

Lithium Isotopic Compositions of Mesozoic and Cenozoic Basalts From South-Eastern China: Implications for Extremely Low δ7Li of Continental-Type Eclogites

Lithium isotope geochemical studies of oceanic-type eclogites benefited from previous investigation of altered oceanic crust; however, processes of lithium isotope exchange operating on altered protoliths of continental-type eclogites have remained unclear. Here, the Li concentration and isotopic composition of fresh and altered continental basalts from the South China Block were measured to decipher Li and its isotopic fractionation during subaerial alteration processes. The results show that altered South China basalts have higher Li concentration (11.4–117.7 ppm) and lower δ7Li values (−4.3–+0.5‰) than fresh basalts (Li: 5.0 ppm and δ7Li: ∼3‰). It is found that hydrothermal alteration played an important role in increasing the Li concentration of altered basalts, which is supported by the relation of loss on ignition and Li concentration. Subsequently, continental weathering acted on chlorite-bearing basalts, as evidenced by the occurrence of minor kaolinite and illite and weathering trend of δ7Li values in these basalts. Detailed petrography and in situ geochemical analyses are used to show that chlorite is the chief repository for Li in weathered basalts, supporting that it was responsible for decreasing δ7Li value by Rayleigh distillation during weathering. Because of the lower δ7Li values of continental altered basalts than oceanic altered crust, it is concluded that, in addition to the dehydration of subducted slabs and possible Li diffusion, the extremely light Li isotopic values of continental-type eclogites worldwide partly result from isotope fractionation caused by the continental weathering of their protoliths.

Separation of lithium isotopes: Electromigration coupling with crystallization

6Li and 7Li, two natural isotopes of lithium, play an important role in the nuclear industry. Here, a system of ionic liquid-crown ether was fabricated for lithium isotope separation and the 6Li isotope separation coefficient can reach 1.406 that showing a good prospect for lithium isotope separation by this method of electromigration. Interestingly, the single crystal containing crown ether and lithium, in which 7Li was enriched, was obtained from the experiments. It is contrary to popular wisdom and can be attributed to the steric hindrance effect. This provides a new route to separate lithium isotopes, that is electromigration coupling with crystallization.

Calcium isotopes tracing secondary mineral formation in the high-relief Yalong River Basin, Southeast Tibetan Plateau

Calcium is a critical element in the global carbon cycle due to its role in carbon sequestration via silicate weathering and carbonate formation. Here we apply calcium (δ44/40Ca) and strontium (87Sr/86Sr) isotopes to explore such chemical weathering processes in a river system draining a diverse range of geologic and climatic environments: the Yalong River, China, and its tributaries. This river originates on the Tibetan Plateau and represents one of the upper reaches of the Changjiang River, China. The Ca isotopic composition of the dissolved load of the Yalong River ranges from 0.60‰ to 1.02‰ (relative to the NIST standard SRM 915a). Higher δ44/40Ca values were found in the plateau and lowland rivers, with lower values in the mountainous rivers. Correlations between riverine dissolved δ44/40Ca values, Sr/Ca ratios, and calcite saturation index indicate that the precipitation of secondary carbonates governs the Ca isotopic composition and carbon transformation in most of this river system. However, such correlations are not seen in the lowland tributaries, where the relationship between δ44/40Ca and lithium (Li) isotopes instead suggests a control by topography and climate, via secondary clay mineral formation. Specifically, heavy rainfall in the lowland regions lowers the pH of the soil solution, which inhibits the precipitation of secondary carbonates. In addition, the flat terrain and thick soils increase the time for water-rock interaction, which favours the formation of secondary clay minerals that preferentially incorporate the lighter Ca isotopes. Overall, this study highlights the potential of stable Ca isotopes, when used in combination with other isotope systems (e.g. Sr and Li isotopes), to quantify secondary mineral formation processes in large river basins.

Effect of growth rate and pH on Li isotope fractionation during its incorporation in calcite

Lithium isotope compositions were determined for synthetic calcite and fluid, reported in Füger et al. (2019). Calcite was precipitated at 25 °C, and mineral growth rate and pH varied within the range of 10−8.1 ≤ rp (mol m−2 s−1) ≤ 10−7.2 and 6.3 ≤ pH ≤ 9.5, respectively. At pH = 8.25 ± 0.15, the Δ7Licalcite-fluid = δ7Licalcite - δ7Lifluid value of −2.76 ± 0.22‰ (n = 4) was yielded when calcite growth rate was lower than ∼ 10−7.7 (mol m−2 s−1). As calcite growth rate increased, Δ7Licalcite-fluid values progressively decreased to about −4.5‰. The dependence of Li isotope fractionation on calcite growth rate is described by the surface reaction kinetic model developed by DePaolo (2011). In this model the equilibrium and kinetic isotope fractionation factors obtain values of −2.7 ± 0.1‰ and −8.8 ± 0.1‰, respectively. In addition, for experiments performed under similar surface-normalized growth rate of 10−7.7±0.2 (mol m−2 s−1) a significant decrease in Δ7Licalcite-fluid from pH 9.5 to 6.3 was observed. These variations of Δ7Licalcite-fluid as a function of pH for experiments performed under low degrees of solution saturation with respect to calcite point towards the incorporation of two or more Li-bearing species in the solid phase. Thus, the Li isotope composition of the solid reflects the abundance of these species, which depends on solution pH and fluid composition.Overall the results of this study suggest that both calcite growth rate and pH are parameters that can significantly affect the measured Li isotope fractionation between calcite and fluid. The high sensitivity of Δ7Licalcite-fluid to calcite growth rate observed in this study suggests that a high variability of δ7Licalcite in natural samples, such as foraminifera, brachiopods, and speleothems can be expected in natural environments. The implications for the potential use of elemental and isotope fractionation of Li during calcite formation are discussed in the light of reconstruction of paleo-environmental conditions.

Controls of lithium isotope spatial variability across the Yukon River: Implications for weathering processes in a warming subarctic basin

With ongoing global warming and permafrost thawing, weathering processes will change on the Yukon River, with risks for water quality and ecosystem sustainability. Here, we explore the relationship between weathering processes and permafrost cover using elemental concentration and strontium and lithium isotopic data in the dissolved load of 102 samples collected during the summer across most major tributaries of the Yukon River. The Yukon River basin is dominated by silicate weathering with a high contribution from young volcanic rock units. In glaciated mountainous zones, we observe higher carbonate weathering contribution, low Li/Na ratios and low δ7Li values (<15‰). In these areas, the high denudation rate and high supply of fresh minerals associated with alpine glaciers favor congruent silicate weathering, and sulfide oxidation accelerates carbonate weathering. In floodplains covered by continuous permafrost, we observe a high carbonate weathering contribution, relatively high Li/Na ratios, and low δ7Li values (∼18‰). We argue that the minimal water–rock interactions in this setting inhibit silicate weathering and favor congruent weathering of easily weatherable minerals (i.e., carbonates). Conversely, in areas with discontinuous or sporadic permafrost, we observe a dominance of silicate weathering, with higher and more variable Li/Na ratios and high δ7Li values (11–33‰). In this setting, longer water–rock interactions combined with the high supply of fresh minerals from mountain zones favor more incongruent weathering. The unique history of Pleistocene glaciations on the Yukon River basin also influences weathering processes. Many areas of the basin were never glaciated during the Pleistocene, and rivers draining those regions have higher δ7Li values suggesting more incongruent weathering associated with deeper flow paths and longer water residence time in the regolith. Our work underlines that water–rock interactions, including active layer weathering and groundwater inputs, are highly dependent on climate conditions and glacial processes across the Yukon River basin, with key implications for future water quality in this warming basin.

Secondary Ion Mass Spectrometry Reference Materials for Lithium in Carbonaceous Matrices

Secondary ion mass spectrometry techniques are used to study trace elements in organic samples where matrix compositions vary spatially. This study was conducted to develop calibrations for lithium content and lithium isotope measurements in kerogen. Known concentrations of Li ions (6Li and 7Li) were implanted into organic polymers, with a range of H/C and O/C ratios similar to kerogen, along with glassy carbon (SPI Glas‐22) and silicate glass (NIST SRM 612). Results show that Li content calibration factors (K*) are similar for carbonaceous samples when analysed using a 5 kV secondary ion accelerating voltage. Using a 9 kV secondary ion accelerating voltage, K* factors are negatively correlated with the sample O content, changing ~ 30% between 0 and 15 oxygen atomic %. Thus, to avoid the matrix effect related to O content, using a 5 kV secondary ion accelerating voltage is best for quantification of Li contents based on 7Li+/12C+ ratios. Under these analytical conditions, Li ppm (atomic) = (132 (± 8) × 7Li+/12C+) × 12C atom fraction of the sample measured. Lithium isotope ratio measurements of SPI Glas‐22 and NIST SRM 612 are within uncertainty; however, the organic polymer samples as a group show a 10‰ higher δ7Li than NIST SRM 612.

Biotite as a recorder of an exsolved Li-rich volatile phase in upper-crustal silicic magma reservoirs

Abstract The magmatic-hydrothermal transition is key in controlling the fate of many economically important elements due to the change in partitioning when melt and magmatic fluid coexist. Despite its increasing economic importance, the behavior of lithium (Li) in such environments remains poorly known. We illustrate how compositionally unusual biotites from the rhyolitic Bishop Tuff (California, USA) and Kos Plateau Tuff (Greece) may contain a magmatic volatile phase trapped between layers of biotite crystals. Despite originating in pristine deposits and showing the expected X-ray diffraction spectra, these biotites return low (&amp;lt;95 wt%) analytical totals via electron microprobe (EMP) consistent with the presence of considerable amounts of light elements (non-measurable by EMP). Lithium contents and isotope ratios in these biotites are remarkable, with abundances reaching &amp;gt;2300 ppm, exceptionally light Li isotopic compositions (δ7Li as low as –27.6‰), and large isotopic fractionation between biotite and corresponding bulk samples (Δ7Libt–bulk as low as –36.5‰). Other mineral phases, groundmass glass, and melt inclusions from the same units do not support an extremely Li-rich melt prior to eruption. Biotites from phonolitic systems (Tenerife [Canary Islands] and Campi Flegrei [Italy]) do not show such extreme compositional differences, with biotite and melt showing roughly equivalent Li contents, underscored by significantly reduced Δ7Libt–bulk to a maximum of –10.9‰. We ascribe the difference in behavior to the near-liquidus appearance of biotite in alkaline magmatic suites, before widespread exsolution of a magmatic volatile phase in the magma reservoir, while in rhyolitic suites, biotite crystallizes at low temperature, trapping the coexisting exsolved fluid phase in the reservoir.

Boron and Lithium Isotopic Signatures of Nanometer-Sized Smectite-Rich Mixed-Layers of Bentonite Beds From Campos Basin (Brazil)

AbstractBoron and lithium were analyzed in three nanometer-sized (&lt;20, 20-50 and 50-100 nm) separates of two Santonian (85.8-83.5 Ma) bentonite samples collected closely in the Campos Basin along the southeastern Atlantic coast (Brazil). The B and Li data give various trends that suggest varied crystallization conditions for separates that consist of overwhelming smectite with less than 9% illitic layers. The δ11B of the few illitic tetrahedral sites from one of the samples remains quite constant, while its contents are strictly correlated with those of K, which suggests that illitization proceeded by interaction with pore fluids of the host sediments that supplied the K. In the second sample, the δ11B of the illite layers from the two coarser fractions is indicative of an early volcanic origin, while the smaller size fraction also interacted with sedimentary fluids. Favored by octahedral substitutions of the smectite layers, the δ7Li is more strictly regulated by a volcanic link. In turn, the information of the B and Li isotopic compositions and contents from studied mixed-layers suggests a various origin for the few illite layers of the smectite-rich I-S that contain more B than the smectite layers that host more Li. The difference appears to be sample-site and crystal-size dependent, fueled by pore fluids of the hosting turbidites.

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 (&gt;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.

Electromigration Separation of Lithium Isotopes: The Effect of Electrode Solutions

Both lithium-6 and lithium-7 with high abundance are indispensable materials in nuclear industry. Here, an aqueous solution│organic solution│aqueous solution system was fabricated to separate lithium isotopes. The effects of species and concentration of electrolytes in the electrode solutions on the lithium ions migration and lithium isotope separation with different voltages and migration time was studied. It was found that lithium-7 was enriched in aqueous solutions on both sides at 0 V and 2 V, while lithium-7 was enriched in anode solution and lithium-6 was enriched in cathode solution at 16 V. The weakening stability of the chelate consisted of crown ether and lithium ion with increasing voltage was believed to the possible reason. Meanwhile, the variation of electrolyte in electrode solution led to notable changes in migration ratio of lithium ions and lithium isotope separation effect, which can be attributed to the different degree of both ionization and hydrolysis for various electrolytes in aqueous solutions and the different ability of H+ and NH4 + to replace Li+ of chelate in organic solutions. This work is of great significance for the selection of electrode solutions in electromigration separation of lithium isotopes and even other electrochemical systems.