Equilibrium barium isotope fractionation between minerals and aqueous solution from first-principles calculations

摘要

Barium isotopes could be a novel tracer in low-temperature geochemical processes such as the Ba cycle in rivers and oceans. Equilibrium Ba isotope fractionation between Ba-hosting minerals and aqueous solution is of great importance for the applications of Ba isotopes in geochemistry, but it remains poorly constrained. In this study, we performed firstprinciples calculations based on the density functional theory (DFT) to determine the equilibrium Ba isotope fractionation between minerals and aqueous solution (10(3)ln alpha(mineral-Ba_aq) of Ba-137/Ba-134). The structural properties of aqueous Ba2+ are well predicted by the first-principles molecular dynamics (FPMD) simulation and 121 snapshots are extracted from FPMD trajectories to estimate the reduced partition function ratio (beta factor or 10(3)ln beta of Ba-137/Ba-134) of aqueous Ba2+. The 10(3)ln beta decreases in the sequence of aragonite > calcite > aqueous Ba2+ similar to witherite > barite. The beta factor is dominantly determined by the force constant, which is affected by both the average BaAO bond length and the coordination number.