POTASSIUM ISOTOPE FRACTIONATION DURING CONTINENTAL WEATHERING

Continental weathering links atmosphere, hydrosphere, biosphere and continents of Earth. It plays a significant role in the evolution of the atmosphere, compositional change of the continents, and chemical variation of the oceans over the geological time. For example, chemical weathering has been considered as one of the main processes that shift the composition of the continental crust from mafic to intermediate. Here we investigate the behaviors of K isotopes during continental weathering by analyzing well-characterized weathering profiles developed in granite and basalt in tropical environment in South China and Hawaii, respectively, as well as adsorption experiments, in order to understand the role of continental weathering on controlling the global K budget. Our results reveal large K isotope fractionation during the formation of clay minerals in both weathering profiles and laboratory experiments. These observations are consistent with the release of heavy K to the hydrosphere and the retention of isotopically light K in the weathered products. Our results suggest that continental weathering can significantly fractionate K isotopes and it plays a major control in the global K isotopic budget. Weathering shifts the K isotopic composition of the continental crust to values lower than the mantle and thus the hydrosphere has a complementary higher δ⁴¹K value.