Distinct pathways of Mg isotope fractionation during biotite weathering in granitoid weathering profiles

Magnesium (Mg) isotopes have emerged as powerful tracer of chemical weathering processes, providing insights into the mobilization of Mg from minerals and its role in regulating seawater chemistry, global biogeochemical cycle, and long-term climate change. However, the relationship between mineral weathering and Mg isotope fractionation remains poorly constrained in natural weathering systems, particularly with respect to the direction and magnitude of isotope fractionation. Here, elemental and Mg isotope geochemistry in two types of biotite collected from in–situ granitoid weathering profiles were investigated, along with their mineralogical properties. While positive correlations between depth and elemental concentrations occur in both types of biotite, Mg isotopic variations are different: 1) oxidized biotites are enriched in 26Mg, up to +0.4 ‰ in δ26Mg, via diffusive release of octahedral 24Mg during iron oxidation; 2) hydrobiotites (1:1 regularly interstratified biotite-vermiculite) have the same δ26Mg as fresh biotite at all depths, indicating that newly formed vermiculite inherits the δ26Mg of biotite. Furthermore, δ26Mg values in both types of biotite display little correlation with the abundance of secondary phases, indicating negligible effects of secondary phases (i.e. vermiculite and kaolinite) on Mg isotope fractionation. This study provides the first evidence that, at the incipient stage of biotite weathering, oxidized biotite releases isotopically light Mg, with a fractionation factor of up to 1.0024 ± 0.0012 (2σ), whereas hydrobiotite retains the δ26Mg signature of fresh biotite regardless of weathering intensity. These findings highlight that diffusive Mg isotope fractionation during iron oxidation in natural weathering systems over geological timescales may dominate over other processes at the onset of silicate weathering.