Magnesium Isotope Variations in Granite Regoliths From Two Contrasting Climates.

Magnesium (Mg) isotopes have been utilized to constrain continental weathering; however, to date, little is known about the climate effects on Mg isotope fractionation during weathering. In this study, we measured δ26Mg values of bulk regolith and exchangeable fraction in two granite regolith profiles developed under temperate, semiarid and tropical, humid climate conditions, respectively. Combined with mineralogy and element composition, we aimed to investigate how climate influences fractionation patterns of Mg isotopes during chemical weathering. At the temperate site, δ26Mg values of regolith are slightly higher than that of the bedrock and negatively correlated with τMg,Th. Correspondingly, the exchangeable Mg is characterized by low δ26Mg values. These results can be explained by the formation of small number of clay minerals. For the tropical regolith profile, δ26Mg values decrease toward the surface, and the regolith has either lower δ26Mg values above −250 cm or higher δ26Mg values below −250 cm relative to the bedrock. The δ26Mg value of exchangeable Mg is markedly lower than that of the regolith and varies significantly. These results can be explained by the mixing of Mg from solid weathering products and atmospheric deposition. The Mg from rainwater and/or marine aerosol deposit on the regolith and some may enter the crystal structure of the illite. The deposited Mg can overprint the granitic Mg, and the δ26Mg value of shallow regolith samples will reflect mixing between granitic and atmospheric sources. The compilation of our and previously published Mg isotopic data reveals the potential control of climate on Mg isotope fractionation during continental weathering. Plain Language Summary: The isotopes of magnesium (Mg) can serve as a useful tracer in understanding the chemical weathering processes of silicate rocks. Previous studies have mainly focused on Mg isotope fractionation during silicate weathering by investigating single weathering profiles that have developed under constant climate conditions; however, the impact of climate on Mg isotope fractionation during silicate weathering remains poorly understood. Here, we analyzed Mg isotope compositions in two granite regolith profiles developed under temperate (semiarid) and tropical (humid) climate conditions. Our study suggests that the patterns of Mg mobilization and Mg isotope fractionation during granite weathering differ significantly in distinct climate zones. The compilation of our and previously published Mg isotopic data reveals that climate may have a huge impact on Mg isotope fractionation during terrestrial weathering, which is of significance to interpret the Mg isotope compositions of weathering products and river waters at a global scale. Key Points: Magnesium isotope fractionation differs significantly during granite weathering in temperate and tropical climate zonesThe formation of illite results in heavy magnesium isotope enrichment in solid weathering productsAtmospheric deposition contributes a mass of light magnesium isotopes to shallow regolith under extremely weathered conditions [ABSTRACT FROM AUTHOR]