In this study, to better understand the factors controlling the concentration and isotope composition of lithium (Li) in the ocean, we investigated the behaviour of Li during interaction of kaolinite with artificial seawater. Dissolution of kaolinite in Li-free seawater at acidic conditions (exp. 1) results in a strong preferential release of light Li isotopes, with △7Liaq-kaol ∼ −19‰, likely reflecting both the preferential breaking of 6Lisingle bondO bonds over 7Lisingle bondO bonds and the release of Li from the isotopically lighter AlO6 octahedral sites. Sorption experiments on kaolinite (exp. 2) revealed a partition coefficient between kaolinite and fluid of up to 28, and an isotopic fractionation of −24‰. Thermodynamic calculation indicates authigenic smectites formed from the dissolution of kaolinite in seawater at pH 8.4 (exp. 3). The formation of authigenic phase strongly removed Li from the solution (with a partition coefficient between the solid and the fluid equal to 89) and led to an increase of ca. 25‰ in seawater δ7Li. This fractionation can be described by a Rayleigh fractionation model at the early stage of the experiment during rapid clay precipitation, followed, at longer reaction time, by equilibrium isotope fractionation during the much slower removal of aqueous Li via co-precipitation and adsorption. Both processes are consistent with a fractionation factor between the solid and the aqueous solution of ∼−20‰. These experiments have implications for interpreting the Li isotopic composition of both continental and marine waters. For instance, the preferential release of 6Li observed during kaolinite far-from-equilibrium dissolution could explain the transient enrichments in 6Li observed in soil profiles. With regard to the evolution of seawater δ7Li over geological time scales, our experimental results suggest that detrital material discharged by rivers to the ocean and ensuing “reverse chemical weathering” have the potential to strongly impact the isotopic signature of the ocean through the neoformation of clay minerals.

中文翻译：

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高岭石与海水相互作用过程中锂同位素分馏的实验约束

在这项研究中，为了更好地了解控制海洋中锂 (Li) 浓度和同位素组成的因素，我们研究了高岭石与人工海水相互作用过程中锂的行为。高岭石在酸性条件下（实验 1）在无锂海水中的溶解导致轻锂同位素的强烈优先释放，△7Liaq-kaol ∼ -19‰，可能反映了 6Lisingle 键的优先断裂超过 7Lisingle 键 O Li 从同位素较轻的 AlO6 八面体位点释放。高岭石的吸附实验（实验 2）显示高岭石和流体之间的分配系数高达 28，同位素分馏为 -24‰。热力学计算表明自生绿土由高岭石在 pH 8.4 的海水中溶解形成（实验 3）。自生相的形成强烈地从溶液中去除了锂（固体和流体之间的分配系数等于 89）并导致大约增加。海水中δ7Li 25‰。这种分馏可以在实验的早期在快速粘土沉淀过程中通过瑞利分馏模型来描述，然后在较长的反应时间，在通过共沉淀和吸附缓慢去除含水锂的过程中平衡同位素分馏。这两个过程都与固体和水溶液之间的分馏因子一致，约为-20‰。这些实验对解释大陆和海水的 Li 同位素组成具有重要意义。例如，在高岭石远离平衡溶解过程中观察到的 6Li 优先释放可以解释在土壤剖面中观察到的 6Li 瞬时富集。关于海水δ7Li在地质时间尺度上的演化，我们的实验结果表明，河流排放到海洋中的碎屑物质和随之而来的“反向化学风化”有可能通过粘土的新形成强烈影响海洋的同位素特征矿物质。