Variation in elemental and Li isotope geochemistry during the weathering of two types of biotite

Lithium (Li) isotopes have been widely used as powerful tracers of chemical weathering processes, providing insights into the coupling between climate and silicate weathering. Although Li isotope fractionation does not occur under equilibrium conditions but rather during kinetically controlled mineral dissolution, the relationship between incipient mineral weathering and Li isotope fractionation remains poorly constrained in natural weathering systems, particularly with respect to the direction and magnitude of fractionation. Here, we investigate elemental and Li isotope geochemistry in two types of biotite—oxidized biotite and hydrobiotite (a 1:1 regularly interstratified biotite–vermiculite)—collected from in situ granitoid weathering profiles. Both biotite types exhibit negative correlations between elemental concentrations and depth; however, Li shows the most pronounced depletion. Elemental loss reaches up to ~70% for Li, with more extensive depletion observed in hydrobiotite compared to oxidized biotite, despite the progressive transformation of biotite into secondary phases such as vermiculite and kaolinite. Lithium isotope analyses are currently underway. By integrating elemental geochemistry with Li isotope compositions, we aim to constrain Li isotope behavior during the initial stages of silicate weathering and to quantify potential Li isotope fractionation associated with distinct biotite alteration pathways. These results will provide new constraints on kinetic controls of Li isotope fractionation during incipient weathering and improve the interpretation of Li isotope signatures in natural weathering systems, including glacial and weathering-limited environments.