Magnesium Isotope Variations in Granite Regoliths From Two Contrasting Climates

Abstract

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 τ_Mg,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.