Fractionation Mechanism and Flux Estimation of Strontium Isotopes During Basalt Weathering
Kai Luo, Jinlong Ma, Zhibing Wang, Guanhong Zhu, Ti Zeng, Gangjian Wei
Abstract
Abstract The fluxes of metal cations and isotopes released by weathering of silicate rocks are crucial and a prerequisite for constraining geochemical fluxes to rivers and oceans. This study presents mineral and elemental compositions along with 87 Sr/ 86 Sr and δ 88 Sr data from a basaltic weathering regolith on Hainan Island, South China to elucidate Sr isotope fractionation and weathering fluxes. The 87 Sr/ 86 Sr ratios vary from 0.703936 to 0.706338 as a result of differential weathering of the minerals. δ 88 Sr values in the weathering regolith range from −0.29 to 0.37‰, with the majority of the weathering regolith having lower δ 88 Sr values than the parent rock. Sr is leached into the soil solution during plagioclase decomposition, while 86 Sr is preferentially adsorbed on the surface of secondary minerals. As weathering progresses, smectite decomposes and kaolinite desorbs under weakly acidic conditions, releasing the previously adsorbed 86 Sr into the soil solution. The differential weathering of kaolinite and smectite controls the δ 88 Sr values of the weathering regolith, with pH being an important determinant of isotope fractionation. Furthermore, Sr elemental fluxes (Sr Flux ) and Sr isotopic fluxes (δ 88 Sr Flux ) of this weathering regolith were calculated using a mass balance model, yielding mean values of 0.20 (mg cm −3 Myr −1 ) and 0.052 (‰ (mg cm −3 Myr −1 )), respectively. The δ 88 Sr Flux exhibits a nonlinear positive correlation with the Chemical Index of Alteration (CIA), indicating that enhanced weathering leads to significant stable Sr isotope fractionation at CIA values below 95%. Our research promotes the understanding of Sr recycling and the fractionation behavior of stable Sr isotopes during consecutive weathering.