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We evaluate the factors influencing the abundance, [Li], and isotopic composition of riverine Li delivered to the oceans through analyses and modeling of [Li] and δ7Li in streams and groundwaters draining a single continental lithology, the Columbia River Basalts (CRBs). The streams were sampled in different climate zones that lie east (dry), and west (wet) of the Cascades Mountains, and during two different seasons (summer and late winter) in order to evaluate climatic and seasonal influences on Li isotopes in rivers. Dissolved Li (δLidis7=+9.3 to +30.4) is systematically heavier than that of fresh or weathered CRBs (-4.7 to +6.0, Liu et al., 2013), suspended loads (-5.9 to -0.3), and shallow groundwaters (+6.7 to +9.4), consistent with previous studies showing that Li isotope fractionation is affected by equilibration between stream water and secondary minerals. However, the lack of correlation between δ7Lidis and climate zone, the uniform secondary minerals and bedrock, coupled with the highly variable (>20‰) δLidis7 indicate that other factors exert a strong control on δ7Lidis. In particular, the heavier Li in streams compared to the shallow groundwaters that feed them indicates that continued isotopic fractionation between stream water and suspended and/or bed loads has a major influence on riverine δ7Li. Seasonal δ7Li variation is observed only for streams west of the Cascades, where the difference in precipitation rate between the dry and wet seasons is greatest. Reactive transport model simulations reveal that riverine δ7Li is strongly controlled by subsurface residence times and the Li isotope fractionation occurring within rivers. The latter explains why there is no positive correlation between δ7Li and traditional weathering proxies such as Si or normalized Si in rivers, as riverine Li isotope fractionation drives δ7Li to higher values during transport, whereas the concentrations of major cations and anions are diluted. The varying residence time for groundwaters feeding the western streams in summer (long residence times, higher δ7Li, greater weathering) and winter (short residence times, lower δ7Li, less weathering) explains the observed seasonal variations. A global, negative correlation between δ7Li and Li/Na for streams and rivers draining basaltic catchments reflects the overall transport time, hence the amount of silicate weathering. Based on our results, the increase of δ7Li in seawater during the Cenozoic is unlikely related to changing climate, but may reflect mountain building giving rise to increased silicate weathering.

Understanding the influence of terrestrial soil-atmosphere-biosphere exchanges on Li geochemical behaviors is vital before using Li isotopes as a weathering tracer. We investigated Li geochemistry of the humid and arid regolith profiles formed on the Pololu lavas, the Kohala Mountain, Hawaii. The shallow regolith (0–1 m depth) retains Li (τ Li,Nb > 0) and displays peak Li accumulation in biologically-active, near-surface soil layers (humid, τ Li,Nb = 10.9; arid, τ Li,Nb = 2.8), with heavy Li isotopic compositions (humid, 4.7–9.9‰; arid, 4.0–13.9‰) with respect to the basalt signal (2.2‰). The deep regolith (>1 m) demonstrates δ7Li (ave. 3.3‰) comparable to the composition of the underlying parent basalt (2.2‰). Decoupling of Li abundance and isotopic composition in the shallow regolith from those of the deep regolith implies different regolith controls on Li chemistry at vertical locations. The Li geochemistry in the shallow regolith has been substantially influenced by: (i) atmospheric deposition, (ii) plant cycling, and (iii) secondary mineral formation. In addition to weathering alteration, our data show that dust addition mainly influences δ7Li in the humid regolith, and marine aerosol largely affects δ7Li in the arid regolith. In the humid regolith, downward seepage migration allows for deeper and more advanced weathering, whereas occasional wetting events followed by rapid drying likely dominate in the arid regolith. Thus, biological cycling and the deposition of Asian dust and volcanic ash from the more recent Hawi eruptions are responsible for upward Li enrichment in soils and increases of soil δ7Li composition in the shallow regolith. This is particularly important in the humid site with dense plant coverage and heavy rainfall. By contrast, the deep regolith of the humid and arid sites is mostly affected by pore fluid percolation and accumulation, thus inheriting heavy Li isotope signals from pore fluids. Our study emphasizes climate-regulated and long-neglected biological controls on terrestrial Li cycling during chemical weathering. [ABSTRACT FROM AUTHOR]

The feedback between CO 2 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. [ABSTRACT FROM AUTHOR]

We report [Li] and δ7Li values for a well-characterized suite of 52 geographically (165–184°W), compositionally (SiO 2 = 46–70 wt.%), and temporally (0–38 Ma) diverse lavas and intrusive samples. The δ7Li in these rocks range from −0.7‰ to +14.2‰, with 32 of the 35 lavas and 12 of the 17 intrusive samples falling within the depleted mantle range (δ7Li +1.6 to +5.6‰), as sampled by mid-ocean ridge basalts (MORB). The δ7Li values of Aleutian lavas do not exhibit the spatial trends observed in other slab component tracers, nor do δ7Li values correlate with any slab component indicators, such as radiogenic isotopes, oxygen isotopes, or trace element ratios such as Cs/La and Th/La. The δ7Li values in Aleutian intrusions also do not exhibit temporal trends, however, an overall positive relationship exists between δ7Li and Th/Nd. Mixing models for δ7Li and 143Nd/144Nd values suggest that Aleutian samples within or above the MORB δ7Li range can be explained by addition of <1–2% sediment-derived aqueous fluid and ≤3% sediment melt to depleted mantle; both are required to explain the range in δ7Li that is observed. Sediment-derived fluid exerts a stronger control on Aleutian samples having higher δ7Li values than the MORB range, while sediment melt skews the Li isotopic compositions of MORB-range samples to slightly lower values than if sediment fluid was the only slab influence. Our study demonstrates that a slab signature may be deciphered via modeling even in arcs where spatial trends in δ7Li values and correlations with slab component indicators are lacking. [ABSTRACT FROM AUTHOR]

This study develops and optimizes a new protocol to measure lithium isotope ratios using a single collector quadrupole inductively coupled plasma mass spectrometer (Q-ICP-MS) operated under hot plasma (1550 W) conditions with a sample–standard bracketing method. Our Q-ICP-MS method reduces sample consumption to 2.5 ng of Li and achieves a high long-term precision of 1.1‰ (2SD). This Q-ICP-MS method exhibits high matrix tolerance (Na/Li < 100), suitable for ng-sized and high-matrix geological samples. We also developed a dual-column system for Li separation, with large loading capacity (29.6 meq), complete recovery (∼100%) and satisfactory purification (Na/Li m m−1 < 1), as well as a fixed elution range for Li fractions (28–60 mL). This new chromatography method has been applied to chemically diverse materials, producing consistent results. In addition, we report the Li isotope compositions of 13 geostandards, and our measurements agree well with reported data within analytical uncertainties. This study documents that Li element concentration and Li isotope composition can be routinely measured using a single collector ICP-MS, which is convenient and commercially affordable for future Li isotope research across the fields of Earth and Environmental Sciences. [ABSTRACT FROM AUTHOR]

The lithium isotope system can be an important tracer for various geological processes, especially tracing continental weathering. The key to this application is the accurate and precise determination of lithium isotopic composition. However, some of the previously established column separation methods are not well behaved when applied to chemically diverse materials, due to the significant variations in matrix/lithium ratios in some materials. Here, we report a new dual‐column system for lithium purification to achieve accurate and precise analysis of lithium isotopic compositions using a multi‐collector inductively coupled plasma‐mass spectrometer (MC‐ICP‐MS). Compared with single‐column systems, our dual‐column system yielded a consistent elution range of the lithium‐bearing fraction (7–16 ml) for samples with a large range of lithium loads and matrix compositions, so that column re‐calibration is not required. In addition, this method achieved complete lithium recovery and low matrix interference (e.g., Na/Li ≤ 1) with a short elution time (~ 6 h, excluding evaporation), with the entire procedure completed in 1.5 days. We report high precision Li isotopic compositions in twelve chemically diverse materials including seawater, silicates, carbonates, manganese nodules and clays. New recommended Li isotopic values and associated uncertainties are presented as reference values for quality control and inter‐laboratory calibration for future research and were consistent with previously published data. However, significant lithium isotopic variances (~ 1‰) in BHVO‐2 from different batches suggest Li isotopic heterogeneity in this reference material and that Li isotopic studies using this reference material should be treated with caution. [ABSTRACT FROM AUTHOR]

Past variations in lithium concentration and isotope ratio of seawater recorded in marine carbonates have shown great potential in tracing carbon cycles during critical climate transitions, especially coupled with the radiogenic strontium isotope system. However, acquiring reliable seawater Li isotope compositions from marine carbonates can be technically challenging because non‑carbonate phases co-existed in ancient marine carbonates can easily contaminate carbonate Li signal during inappropriate sample dissolution. Each contaminant phase, including Mn oxides, clays, and silicate minerals, can have different Li isotope compositions than that preserved in the carbonate lattice. More importantly, a significant portion of Li is adsorbed on the Mn oxides and clay minerals. This fraction is considered exchangeable and can be liberated using weak acid without destruction of mineral structure. Cleaning procedures and partial digestion have been applied to remove the exchangeable Li fraction in contaminative phases and selectively dissolve the carbonates. However, previous methods cannot be directly applied to ancient marine carbonates of various carbonate purity, which lack systematic testing on the threshold of sample purity to maintain the efficiency of the precleaning and partial digestion. Here we report an efficient sequential leaching protocol to extract primary seawater Li and Sr isotope compositions from bulk marine carbonates. The procedure consists of pre-cleaning steps using 1 M ammonium acetate and 1 M ammonium carbonate to completely remove the exchangeable fraction, followed by partial digestion using 0.3 M acetic acid to selectively target carbonate-bounded Li. We validate this method by mixing carbonate and contaminant endmembers in different weight percentages. We found this protocol works effectively for carbonates of >80% purity. Additionally, we demonstrate filtration is necessary when separating leachates from the solids to quantitatively avoid contaminations from non‑carbonate phases. We also suggest NIST-SRM-1d to be a cross-lab carbonate standard for Li isotope analyses and report its long-term external precision (two years). To sum, we propose an effective community standard procedure to reconstruct δ7Li sw records using ancient bulk carbonate samples. [Display omitted] [ABSTRACT FROM AUTHOR]

We evaluate the factors influencing the abundance, [Li], and isotopic composition of riverine Li delivered to the oceans through analyses and modeling of [Li] and δ 7 Li in streams and groundwaters draining a single continental lithology, the Columbia River Basalts (CRBs). The streams were sampled in different climate zones that lie east (dry), and west (wet) of the Cascades Mountains, and during two different seasons (summer and late winter) in order to evaluate climatic and seasonal influences on Li isotopes in rivers. Dissolved Li ( δ Li dis 7 = + 9.3 to +30.4) is systematically heavier than that of fresh or weathered CRBs (−4.7 to +6.0, Liu et al., 2013 ), suspended loads (−5.9 to −0.3), and shallow groundwaters (+6.7 to +9.4), consistent with previous studies showing that Li isotope fractionation is affected by equilibration between stream water and secondary minerals. However, the lack of correlation between δ 7 Li dis and climate zone, the uniform secondary minerals and bedrock, coupled with the highly variable ( > 20 ‰ ) δ Li dis 7 indicate that other factors exert a strong control on δ 7 Li dis . In particular, the heavier Li in streams compared to the shallow groundwaters that feed them indicates that continued isotopic fractionation between stream water and suspended and/or bed loads has a major influence on riverine δ 7 Li. Seasonal δ 7 Li variation is observed only for streams west of the Cascades, where the difference in precipitation rate between the dry and wet seasons is greatest. Reactive transport model simulations reveal that riverine δ 7 Li is strongly controlled by subsurface residence times and the Li isotope fractionation occurring within rivers. The latter explains why there is no positive correlation between δ 7 Li and traditional weathering proxies such as Si or normalized Si in rivers, as riverine Li isotope fractionation drives δ 7 Li to higher values during transport, whereas the concentrations of major cations and anions are diluted. The varying residence time for groundwaters feeding the western streams in summer (long residence times, higher δ 7 Li, greater weathering) and winter (short residence times, lower δ 7 Li, less weathering) explains the observed seasonal variations. A global, negative correlation between δ 7 Li and Li/Na for streams and rivers draining basaltic catchments reflects the overall transport time, hence the amount of silicate weathering. Based on our results, the increase of δ 7 Li in seawater during the Cenozoic is unlikely related to changing climate, but may reflect mountain building giving rise to increased silicate weathering. [ABSTRACT FROM AUTHOR]

Abstract: Mineralogical, chemical, and Li and Nd isotopic compositions of two drill cores (8–9m deep) through bauxites developed on the Miocene Columbia River Basalts document the changes associated with basalt weathering, provide insights into the processes involved, and allow us to examine the overall influence of chemical weathering on juvenile (basaltic) crust. Gibbsite, hematite, ±kaolinite, halloysite, goethite, and maghemite are the weathering products in the bauxites. Quartz is observed near the tops of the cores and its abundance decreases progressively with depth; no quartz is observed below five meters depth in either core. Most major and trace elements, including “mobile” and some “immobile” elements are severely depleted in the bauxites. Niobium is less mobile relative to the rare earth elements, thus chemical weathering attenuates the negative Nb anomaly in the continental crust. Li and Nd are strongly depleted relative to fresh basalt, and both increase systematically towards the surface in the quartz-bearing samples while δ7Li and ε Nd values decrease systematically towards the surface in these same samples. Both Li and Nd were likely lost from the bauxites through leaching. The systematic enrichment of Li, Nd, and quartz, as well as the less radiogenic Nd isotopic composition at the tops of both profiles reflects 20–60wt.% addition of an eolian component to the soils. The eolian dust is unlikely to have experienced significant post-depositional weathering due to the relatively high Li contents near the tops of the profiles, and, therefore, the low δ7Li and ε Nd values suggest that the dust came from an old, weathered region of the continent. Our results demonstrate that lithium isotopes are sensitive tracers of chemical weathering, particularly in extreme weathering settings, and support the hypothesis that chemical weathering influences the mass and composition of the continental crust. [Copyright &y& Elsevier]

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 HNO 3) 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‰ (Δ7Li l-s) and from −1.01 to −0.11‰ (Δ41K l-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. [ABSTRACT FROM AUTHOR]

Lithium isotopes have been identified as a promising tracer of subducted materials in arc lavas due to the observable variations in related reservoirs such as subducting sediments and altered oceanic crust. The Tonga–Kermadec arc–Lau back‐arc provides an end‐member of subduction zones with the coldest thermal structure on Earth. Reported here are Li isotope data for 14 lavas from the arc front and 7 back‐arc lavas as well as 12 pelagic and volcaniclastic sediments along a profile through the sedimentary sequence at DSDP Site 204. The arc and back‐arc lavas range from basalts to dacites in composition with SiO2 = 48.3–65.3 wt% over which Li concentrations increase from 2 ppm to 16 ppm. Li/Y ratios range from 0.08 to 0.77 and from 0.24 to 0.65 in the arc and back‐arc lavas, respectively. The majority of the lavas have δ7Li that ranges from 2.5 ‰ to 5.0 ‰ with an average of (3.6 ±0.7) ‰, similar to that reported from other arcs and there is no distinction between the arc front and back‐arc lavas. The pelagic sediments have variable Li concentrations (33–133 ppm) and δ7Li that ranges from 1.2 ‰ to 10.2 ‰ while the volcaniclastic sediments have an even greater range of Li concentrations (3.6–165 ppm) and generally higher δ7Li values (8–14 ‰). However, δ7Li in the lavas does not correlate with commonly used trace element ratio or isotope signatures indicative of slab‐derived fluids or the sediments. This is probably because the range of δ7Li in the lavas and sediments overlap. Calculated sediment mass‐balance models require significantly more sediment than previous estimates based on Th–Nd–Be isotopes. This may indicate that a sizeable proportion of the total Li budget in the lavas is provided by Li‐enriched fluids from the subducting sediments and/or altered oceanic crust. Lithium isotopes have been identified as a promising tracer of subducted materials in arc lavas due to the observable variations in related reserviors such as subducting sediments and altered oceanic crust. Here we report Li isotope data for 14 lavas from the arc front and 7 back‐arc lavas as well as 12 pelagic and volcaniclastic sediments along a profile through the sedimentary sequence at DSDP Site 204. Calculated sediment mass‐balance models require significantly more sediment than previous estimates based on Th–Nd–Be isotopes. This may indicate that a sizeable proportion of the total Li budget in the lavas is provided by Li‐enriched fluids from the subducting sediments and/or altered oceanic crust. [ABSTRACT FROM AUTHOR]

The increase in marine Li isotope composition (δ 7 Li, ∼9‰) since the Cenozoic is possibly related to continental chemical weathering associated with tectonic uplift (e.g., the Tibetan Plateau [TP]). However, the reasons for the spatiotemporal variations of δ 7 Li in the rivers flowing through the TP are still under debate, hindering the understanding of the changes in seawater δ 7 Li. Herein, various geological samples, i.e. river waters, river suspended particulate materials, river sediments, hot springs and silicate rocks, from the Yarlung Tsangpo River basin, the largest river system in the TP, have been analyzed to understand the source and isotopic fractionation of dissolved Li in rivers flowing through high-altitude and tectonic regions. The Yarlung Tsangpo River has an unusually high dissolved Li concentration ([Li] dis) (0.5–313 μg/L, mean 58.4 μg/L, n = 93) and low δ 7 Li dis values (+1.0‰–+14.7‰, mean +6.4‰, n = 92) compared with most river waters. These results can be explained by the input of geothermal water with extremely high [Li] dis (5.5–34.4 mg/L, mean 15.7 mg/L, n = 9) and low δ 7 Li dis (−1.7‰–+3.1‰, mean +1.0‰). Silicate weathering is probably responsible for elevated δ 7 Li dis in river water compared to geothermal water, but the binary mixture model results of silicate weathering and geothermal water do not support this speculation. Ongoing Li isotope fractionation between dissolved loads and secondary minerals has been suggested to be the main reason for the increased δ 7 Li dis in river water. Field study and adsorption experiment results support the view of continuous Li isotope fractionation in rivers. Physical erosion and chemical weathering processes that provide fresh secondary minerals to rivers as well as dissolved Li from geothermal water transported in rivers over long residence times promote Li isotope fractionation. Hence, with the emergence of a continental geothermal system, the tectonic activity may directly or indirectly induce the simultaneous increase in dissolved Li flux and δ 7 Li dis in rivers that eventually flow into the ocean. This can partially explain the increase in seawater δ 7 Li in the Late Cenozoic. • Li isotope composition is analyzed in the largest river system in Tibetan Plateau. • Dissolved load has very high [Li] dis , high Li/Na ratios, and low δ 7 Li dis values. • Geothermal water is responsible for the high [Li] dis and low δ 7 Li dis in river water. • Continental geothermal water increases in the Li flux and δ 7 Li dis in river estuaries. • The results help to explain the increase of seawater δ 7 Li in the late Cenozoic. [ABSTRACT FROM AUTHOR]