Potassium isotope fractionation during chemical weathering of basalts

Abstract

Non-traditional stable isotopes (e.g., Li, Mg, and Si) are increasingly used as tracers for studying Earth's surface processes. The isotopes of potassium (K), a highly soluble and mobile element during weathering, could be a promising new tracer for continental weathering; however, the K isotopic variations in weathering profile has not been directly studied due to previous analytical difficulties. Recent high-precision measurements revealed that K isotopes in global river waters are fractionated from the Bulk Silicate Earth (BSE) value, indicating they are influenced by chemical weathering of the crust. Isotopic fractionation during chemical weathering is one of several processes that could ultimately lead to ∼0.6‰ difference of δ41K between the BSE and modern seawater. In order to determine the direction and controlling factors of K isotopic fractionation during basalt weathering, especially under intense weathering conditions, we measured K isotopic compositions in two sets of bauxite developed on the Columbia River Basalts, together with fresh parental basalt and aeolian deposit samples using a recently developed high-precision method. Results show that K isotopic variations among fresh basalts and aeolian dust are limited, close to the BSE value. Extreme K depletion (>99%) and K isotopic fractionation (δ41K up to 0.5 ‰) are observed in bauxite drill cores due to intense chemical weathering. The top of the weathering profiles shows less depletion in K abundances and the δ41K values are closer to those of the fresh basalts and aeolian dusts, likely due to addition of aeolian dust at the tops of both profiles. The weathered products are generally depleted in heavy K isotopes, which is consistent with heavier K isotopic compositions observed in river water and seawater. The δ41K in bauxites displays a positive correlation with K2O contents as well as δ7Li, indicating the behaviors of K and Li isotopes are comparable during chemical weathering. This study shows that K concentrations and its isotopic compositions are sensitive tracers of chemical weathering and could be good weathering proxies over Earth's history.