In the present study, we report lithium concentrations and Li isotopic compositions for different samples within a granitic watershed (Margeride, France). We investigate unweathered bedrock and samples displaying different stages of weathering in order to characterize Li isotopic fractionation. This was achieved by coupling lithium isotope geochemistry and reactive transport modeling during granite weathering at the scale of this watershed. The first stages of this work were intended to building a robust alteration model considering the initial minerals within the bedrock (wt.% and Li concentration). This weathering model was developed using the PHREEQ-C code and simulates a weathering sequence in agreement with the products observed within the soils, saprolites and sediments for Li concentrations. This first stage was important in order to conserve the mass balance for lithium. During weathering, the isotopic fractionation during the montmorillonite precipitation is therefore considered according to differentiated kinetic reactions between the ⁷Li and ⁶Li-bearing phases and corresponding to the fractionation factor determined for montmorillonite. Due to the Li content in the micas compared to the other phases, lithium chemistry and isotopic composition remain very strongly related to the relative abundance of micas (no isotopic fractionation is likely to occur during mineral dissolution).

Romain Millot is particularly grateful to Tom Bullen for helping him in the first stages of Li isotopes implementation at BRGM more than 15 years ago; this work is dedicated to his memory.