Equilibrium Mg isotope fractionation among fluids and minerals: Insights from first-principles molecular dynamics simulations

Mg isotopes have been widely applied to investigate the continental weathering, the Mg global cycle, and the paleoenvironmental reconstruction. Equilibrium Mg isotope fractionations between aqueous Mg²⁺ and minerals are key parameters for Mg isotope geochemistry. Previous studies have performed precipitation experiments and theoretical calculations to determine the equilibrium fractionation factors of Mg isotopes between solid phases and solutions, but large controversies still exist among between experimental data, and between theoretical results.

In this study, we firstly conducted first-principles molecular dynamics simulations (FPMD) based on the density functional theory (DFT) for aqueous Mg²⁺ , and calculated the reduced partition function ratio (10³lnβ) of solutions. Our results suggest large equilibrium Mg isotope fractionations between minerals and aqueous Mg²⁺, which are dominantly controlled by the average Mg-O bond length in these phases. Compared to aqueous Mg²⁺, carbonates are enriched in light Mg isotopes but brucite and lizardite show enrichment in heavy Mg isotopes. Notably, 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 Mg and the reliability of our calculations. Our calculations provide fundamental data for applications of Mg isotopes in the Mg global cycling and paleoenvironmental reconstruction.