Behaviour of Li isotopes in leachate during granite weathering: the Xunwu profile, southeastern China

Introduction Lithium (Li) isotopes are powerful tracers of silicate weathering processes. However, the geochemical behavior of lithium isotopes in granite leachates remains unclear. Methods Here we report Li isotope compositions of leachates from a granite weathering profile in southeastern China. The parameter tau (τ Si and τ Al ) and the Chemical Index of Alteration (CIA) were also used to characterize weathering intensity and related geochemical processes. Results The Li concentration in the leachate varied from 0.05 to 1.03 mg/kg, and the δ 7 Li leachate values were −15.0‰ to +6.0‰ (mean = −0.85‰, n = 28). Below 0.8 m depth, the leachate had similar Li isotopic composition (−0.80‰ to +6.0‰, mean = +2.5‰, n = 19) with parent granite (+3.7‰). Discussion The leachate δ 7 Li values exhibit distinct vertical variations, reflecting contrasting geochemical processes along the profile. Below 0.8 m, δ 7 Li values are comparable to those of the parent granite, indicating limited isotopic fractionation during early weathering process. In contrast, markedly lower δ 7 Li values above 0.8 m suggest the release of 6 Li from the dissolution of secondary minerals. This interpretation is supported by increased τ Si and τ Al values and their negative correlations with δ 7 Li leachate , implying co-migration of 6 Li, Si, and Al during mineral dissolution. A positive correlation between δ 7 Li and CIA in the upper profile further indicates enhanced secondary mineral dissolution under intensified weathering. Our results suggest that as weathering progressed, the Li isotopic composition of the leachate from the upper weathering profile became gradually heavier toward the top, positively correlating with weathering intensity and indicating the dissolution of surface secondary minerals under intense weathering conditions.