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

Abstract

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 first-principles calculations based on the density functional theory (DFT) to determine the equilibrium Ba isotope fractionation between minerals and aqueous solution (103lnαmineral-Ba_aq of 137Ba/134Ba). 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 (β factor or 103lnβ of 137Ba/134Ba) of aqueous Ba2+. The 103lnβ decreases in the sequence of aragonite > calcite > aqueous Ba2+ ∼ witherite > barite. The β factor is dominantly determined by the force constant, which is affected by both the average BaO bond length and the coordination number.Our results show that 103lnαaragonite-Ba_aq and 103lnαwitherite-Ba_aq are 0.36‰ and −0.02‰ 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 103lnαbarite-Ba_aq is only −0.17‰ 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.