Rubidium isotope fractionation during chemical weathering of granite

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 delta Rb-87 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 (tau Rb-TiO2) fluctuates dramatically from -42.0% to 259%, and delta Rb-87 values vary from -0.30 +/- 0.13 parts per thousand in the unaltered bedrock to 0.04 +/- 0.08 parts per thousand in the weathered saprolites. Significant isotopic variations exist among individual mineral species. In the parent rock, biotite, chlorite, K-feldspar and plagioclase have the delta Rb-87 values of -0.42 parts per thousand, -0.30 parts per thousand, -0.23 parts per thousand and +0.01 parts per thousand, respectively. In weathered products, major Rb-bearing minerals are K-feldspar, illite and kaolinite. The delta Rb-87 values of K-feldspar range from -0.23 parts per thousand to 0.14 parts per thousand, 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 delta Rb-87 values of illite vary from -0.28 parts per thousand to -0.10 parts per thousand. As the final product of chemical weathering in this profile, kaolinite has the heaviest isotopic composition, with calculated delta Rb-87 values ranging from -0.07 parts per thousand to 0.20 parts per thousand. Chemical weathering drives the delta Rb-87 values of saprolites to become heavier. Nearby stream water has the average delta Rb-87 value of -0.22 parts per thousand, 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. (C) 2021 Elsevier Ltd. All rights reserved.