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

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 Delta Li-7(aqkaol) similar to -19 parts per thousand, likely reflecting both the preferential breaking of Li-6-O bonds over Li-7-O 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 parts per thousand. Thermodynamic calculation indicates authi-genic 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 parts per thousand in seawater delta Li-7. 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 similar to-20 parts per thousand. These experiments have implications for interpreting the Li isotopic composition of both continental and marine waters. For instance, the preferential release of Li-6 observed during kaolinite far-from-equilibrium dissolution could explain the transient enrichments in Li-6 observed in soil profiles. With regard to the evolution of seawater delta Li-7 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. (C) 2020 Elsevier Ltd. All rights reserved.