Re-assessing the effect of differential weathering of minerals on strontium stable isotope behavior during granodiorite weathering

Abstract

The shift in Sr stable isotope ratios (88Sr/86Sr, reported as δ88/86Sr values) in weathering systems provides insights into the transformation of the binding form of Sr between the dissolved and solid phases. However, the mechanism behind the fractionation of Sr stable isotopes during chemical weathering remains poorly constrained. We present mineral and elemental compositions along with the radiogenic Sr isotope ratios (87Sr/86Sr) and δ88/86Sr values from a ∼ 1 m deep horizontal spheroidal weathering profile of granodiorite in southeast China. Strontium was markedly lost during granodiorite weathering and a large amount of plagioclase and hornblende was decomposed in saprolite relative to the parent rock. The 87Sr/86Sr ratios substantially increased from 0.708704 to 0.712532 and δ88/86Sr values sharply decreased from 0.29‰ to −0.12‰ in the corestone-near saprolite as the weathering intensity increased. However, these values were rather constant in the corestone-distant saprolite, albeit with a large increase in the abundance of kaolinite. The Sr in the different chemical fractions indicates that the silicate components dominate the Sr isotopic composition in bulk rock and saprolite samples. Differential weathering of minerals is mainly responsible for the Sr stable isotopic composition of the corestone-near saprolite, while the lower δ88/86Sr values of the corestone-distant saprolite point to the possibility of preferential incorporation of lighter Sr isotopes into the structural sites of secondary clay minerals. In contrast, δ88/86Sr values of the exchangeable and reducible fractions (0.11‰–0.48‰) were more positive than those of the corresponding bulk samples because they were able to inherit the isotopic signatures of the weathered minerals. Further, an acid leaching experiment on the granodioritic parent rock reveals that mineral weathering reactions dominated the Sr isotope composition of the dissolved phase. The substantially higher δ88/86Sr values in the leachates relative to those in the residual solids (Δ88/86Srleachate-residue = 0.14‰–0.42‰) are consistent with the fact that isotopically heavier Sr in the dissolved load of rivers is mostly set by differential weathering of minerals (such as biotite and plagioclase). Combined with classic indices of chemical weathering (such as the chemical depletion fraction (CDF) and chemical index of alteration (CIA)), we suggest that Sr stable isotopes offer a promising proxy of the intensity of silicate weathering in saprolite studies in which isotope fractionation caused by biological cycling is negligible.