Rubidium isotope fractionation during chemical weathering of granite

Abstract

Rubidium is an active element on Earth, but whether and how its isotopes fractionate during geological processes are not known yet. In this study, we present data showing large variations in δ87Rb in bulk samples and individual mineral species from a granite weathering profile in South China as well as in surrounding stream water. The bulk samples show that the mobility of Rb (τRbTiO2) fluctuates dramatically from −42.0% to 259%, and δ87Rb values vary from −0.30 ± 0.13‰ in the unaltered bedrock to 0.04 ± 0.08‰ in the weathered saprolites. Significant isotopic variations exist among individual mineral species. In the parent rock, biotite, chlorite, K-feldspar and plagioclase have the δ87Rb values of −0.42‰, −0.30‰, −0.23‰ and +0.01‰, respectively. In weathered products, major Rb-bearing minerals are K-feldspar, illite and kaolinite. The δ87Rb values of K-feldspar range from −0.23‰ to 0.14‰, which are likely influenced by the secondary clay minerals adhered on the surface. Illite is the intermediate secondary mineral during biotite and K-feldspar decomposition into kaolinite. The δ87Rb values of illite vary from −0.28‰ to −0.10‰. As the final product of chemical weathering in this profile, kaolinite has the heaviest isotopic composition, with calculated δ87Rb values ranging from −0.07‰ to 0.20‰. Chemical weathering drives the δ87Rb values of saprolites to become heavier. Nearby stream water has the average δ87Rb value of −0.22‰, which is slightly heavier than the value of parent rock but lighter than the saprolites at the upper part of the profile. Our results demonstrate significant Rb isotope fractionation at the Earth’s surface and that adsorption and desorption of clay minerals play a crucial role in triggering Rb isotope fractionation.