Equilibrium Mg isotope fractionation among aqueous Mg2+, carbonates, brucite and lizardite: Insights from first-principles molecular dynamics simulations

Equilibrium Mg isotope fractionation properties between aqueous Mg2+ and minerals are the key for the applications of Mg isotopes in geochemistry. This study conducts first-principles molecular dynamics (FPMD) simulations for aqueous Mg2+ based on the density functional theory (DFT). Thirty-five snapshots are extracted from the FPMD trajectories after equilibration for the calculations of the reduced partition function ratio (beta factor or 10(3)ln beta). Combining with the beta factors of minerals, we found that the beta factor decreases in the sequence of lizardite > brucite > aqueous Mg2+ > dolomite > magnesite > calcite > aragonite. Our calculations also confirm the effect of Mg concentration on the beta factor of calcite, and further show that the concentration effect is negligible when Mg/(Mg + Ca) (atomic ratio hereafter) is lower than 1/32. Our results depict significant equilibrium Mg isotope fractionations between minerals and aqueous Mg2+ (10(3)ln alpha(minerals-)(Mg_aq)), which are dominantly controlled by the average force constant of Mg in these phases. Compared to aqueous Mg2+, carbonates are enriched in light Mg isotopes but brucite and lizardite show enrichment in heavy Mg isotopes. Among all minerals, 10(3)ln alpha(aragonite-Mg_aq) is the largest, which is up to -14 parts per thousand at 300 K. Notably, 10(3) ln alpha(calcite-Mg_aq) is not only controlled by temperature, but also significantly affected by Mg content in calcite. 10(3)ln alpha(calcite-Mg_aq) is negatively correlated with Mg content in calcite. The experimentally measured Mg isotope fractionations between minerals (dolomite, magnesite, brucite) and solution at equilibrium are consistent with our predicted results, showing the accurate description of FPMD simulations for aqueous Mg2+ and the reliability of our calculations. Overall, the calculated equilibrium Mg isotope fractionations between minerals and aqueous Mg2+ provide a guideline for applications of Mg isotopes in aqueous geochemical processes. (C) 2019 Elsevier Ltd. All rights reserved.