Potassium isotope fractionation during continental weathering and implications for global K isotopic balance

Potassium isotopic compositions of profiles through saprolites developed on a diabase in South Carolina, U.S.A., and on a granite in Guangdong, China, allow characterization of the behavior of K isotopes during continental weathering. Saprolites from the diabase profile are heavily weathered with chemical index of alteration (CIA) values up to 95; however, their K isotopic variation is limited, with delta K-41 ranging from -0.475 +/- 0.028 parts per thousand in the unweathered diabase to -0.407 +/- 0.021 parts per thousand for the saprolites. The lack of significant K isotope fractionation mainly reflects the conservative behavior of K in the diabase weathering profile, with >50% of the original K remaining in the saprolites. By contrast, K isotopes are fractionated during granite weathering and correlate with sample depth, CIA, and kaolinite abundance, with delta K-41 decreasing from -0.493 +/- 0.030 parts per thousand in the unweathered granites at the bottom to -0.628 +/- 0.021 parts per thousand in the most weathered saprolite close to the surface. These observations suggest the preference of light K isotopes in saprolites relative to fluids, which is further supported by the overall isotopically heavy nearby stream water samples (delta K-41 = -0.709 +/- 0.017 to -0.339 +/- 0.018 parts per thousand). These results demonstrate that continental weathering plays an important role in the global K isotopic budget through the formation of isotopically heterogeneous rivers and weathered regolith. Recycling of K-rich crustal materials with distinct K isotopic signatures may produce distinct mantle K isotopic end members. (C) 2020 Elsevier Ltd. All rights reserved.