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

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. Our results show that 10(3)ln alpha(aragonite-Ba_aq) and 10(3)ln alpha(witherite-Ba_aq) are 0.36 parts per thousand and -0.02 parts per thousand at 300 K, respectively, consistent with results of experimental studies at equilibrium. The depletion of heavy Ba isotopes observed in natural corals relative to seawater suggests that kinetic effects play an important role in Ba isotope fractionation during coral growth. The 10(3)ln alpha(bariteBa_aq) is only -0.17 parts per thousand at 300 K, indicating limited Ba isotope fractionation caused by the Ba removal stemmed from inorganic barite precipitation. Overall, the equilibrium Ba isotope fractionation factors between minerals and aqueous Ba2+ calculated in this study provide a guideline for the applications of Ba isotopes in low-temperature geochemistry. (c) 2020 Elsevier Ltd. All rights reserved.

Automated summary

This study calculated the equilibrium barium isotope fractionation between minerals and aqueous solution using density functional theory, providing insights into the applications of barium isotopes in low-temperature geochemistry. Kinetic effects were found to play an important role in barium isotope fractionation during coral growth.