Magnesium Isotope Fractionation During Basalt Weathering: An Index of Weathering Fluxes and CO2 Consumption

Abstract

The weathering of silicate rocks exerts a significant control on the weathering fluxes of metals and atmospheric CO₂ consumption. In this study, we present new magnesium (Mg) isotope data from a basalt weathering profile in Hainan Island, South China, to investigate Mg isotope fractionation and calculate weathering fluxes and CO₂ consumption. The Mg mobility (τ_Mg,Ti) in saprolites decreases from −34.1% to −95.7%. The δ²⁶Mg values in saprolites vary from −0.25 ± 0.07‰ to 0.43 ± 0.07‰, higher than those of the parent rock (−0.25 ± 0.07 ‰). The significant Mg loss during the formation and decomposition of clay minerals influences Mg isotope fractionation, particularly with changes in kaolinite structure under different pH conditions, which prefer heavy Mg isotopes. By applying a mass balance model, we have developed a novel method to calculate weathering fluxes based on the weathering profile, yielding Mg elemental fluxes (Mg_Flux) of 2.45–5.85 mol/cm²/Myr, Mg isotopic fluxes (δ²⁶Mg_Flux) of −0.44 to −0.04‰/mol/cm²/Myr, and CO₂ consumption of 2.3 × 10¹² mol/yr for the weathering outputs of basaltic rocks. This highlights the crucial role of basalt weathering in global carbon sequestration. Our findings improve the understanding of Mg cycling and isotope fractionation in epigenetic environments and facilitate the quantification of weathering fluxes and atmospheric CO₂ consumption during basalt weathering.