Fractionation Mechanism and Flux Estimation of Strontium Isotopes During Basalt Weathering

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 87Sr/86Sr and d88Sr data from a basaltic weathering regolith on Hainan Island, South China to elucidate Sr isotope fractionation and weathering fluxes. The 87Sr/86Sr ratios vary from 0.703936 to 0.706338 as a result of differential weathering of the minerals. d88Sr values in the weathering regolith range from 0.29 to 0.37%, with the majority of the weathering regolith having lower d88Sr values than the parent rock. Sr is leached into the soil solution during plagioclase decomposition, while 86Sr 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 86Sr into the soil solution. The differential weathering of kaolinite and smectite controls the d88Sr values of the weathering regolith, with pH being an important determinant of isotope fractionation. Furthermore, Sr elemental fluxes (SrFlux) and Sr isotopic fluxes (d88SrFlux) 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 d88SrFlux 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. Plain Language Summary The stable Sr isotopic composition is sensitive to continental weathering processes and offers insights into weathering mechanisms. However, the fractionation mechanism of stable Sr isotopes remains elusive. Significant stable Sr isotope fractionation was observed in a basaltic weathering regolith on Hainan Island, South China. Plagioclase decomposition releases Sr, with isotopically light Sr adsorbed onto the surfaces of secondary minerals during incipient weathering. Smectite decomposition and kaolinite desorption control the stable Sr isotopic composition of the weathering regolith during advanced weathering. The variable pH of the regolith is an important external control factor. We focus on modeling Sr isotopic fluxes and Sr elemental fluxes from weathering regolith. Our results show that within a certain range, an increase in weathering intensity leads to a nonlinear increase in weathering fluxes. The d88 Sr values of the loss component are consistent with those in silicate-dominated river catchments, suggesting that metal cations from the weathering of silicate rocks have a significant influence on the hydrochemical and isotopic composition of river waters in these catchments. Stable Sr isotope fractionation thus provides quantitative constraints on continental weathering fluxes.