Experimental constraints on Li isotope fractionation during the interaction between kaolinite and seawater
Résumé
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.
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