Equilibrium carbon, magnesium and position-specific oxygen isotope fractionations in the magnesium carbonate – water – carbon dioxide system based on quantum mechanical calculations and laboratory precipitation experiments

Mg-carbonate minerals form in and help constrain our understanding of low-temperature surface processes in terrestrial environments and on Mars and the parent bodies of some primitive meteorites. Knowledge of isotopic fractionations among Mg-carbonates, water, and CO₂ can contribute to interpretation of the environmental significance of natural magnesium carbonate minerals, but we presently lack a comprehensive body of experimentally calibrated fractionations among these phases. In this study, we use quantum mechanics (density functional theory) to evaluate equilibrium mineral-fluid fractionation factors for C, O, and Mg isotopes, including selected site-specific fractionations, for a series of hydrous and anhydrous Mg-carbonate minerals and amorphous magnesium carbonate (AMC). We also revisit the existing calibrations for the temperature-dependent abundance of multiply-substituted (‘clumped’) species for hydrous Mg-carbonates. Because magnesite (anhydrous MgCO₃) precipitation is kinetically hindered under laboratory conditions in the temperature range of interest, we use measurements of the isotope effects associated with AMC precipitation to assess the accuracy of our calculations. Using these fractionation factors, we predict ²⁶Mg/²⁴Mg of magnesite detected on Mars. These results can be used to investigate the formation of Mg-carbonates and the compositions of their coexisting fluids in numerous natural settings.