COMPLEX MG ISOTOPE SYSTEMATICS DURING CHEMICAL WEATHERING

Mg isotope of siliciclastic component in sediments or sedimentary rocks is thought to be a useful proxy of chemical weathering intensity, i.e. the ratio of chemical weathering rate and total weathering rate, due to a large isotopic fractionation (up to 2%) and its insensitive to diagenesis alterations and source rock compositions. Previous study indicates that Mg isotope compositions of weathering profile increase gradually from unweathered bedrock to the most weathered saprolites in the surface. In addition, experimental and field studies show that the weathering process would preferentially remove ²⁴Mg by pore fluid, and can be simulated by a Rayleigh distillation model. Given a constant fractionation factor, intense chemical weathering would cause a more enrichment of ²⁶Mg in weathering residues. However, siliciclastic components in carbonate rock samples, whose Mg isotopes theoretically record the intensity of chemical weathering, display differences in both Mg isotope and fractionation factor in the Rayleigh distillation model. We suggest that the isotopic fractionation between saprolites and pore fluid in soil is not a constant and might be affected by the fraction of Mg lost through fluid. Higher fraction of Mg loss through pore fluid would reduce the amount of Mg sequestration into secondary clay minerals and result in an enrichment of ²⁶Mg in clays. Thus Mg isotope of weathering residues or siliciclasts in sediments/sedimentary rocks is not only controlled by the intensity of chemical weathering, but also affected by some local factors, such as rainfall and erosion.