First-principles estimates of equilibrium magnesium isotope fractionation in silicate, oxide, carbonate and hexaaquamagnesium(2+) crystals

Equilibrium mass-dependent magnesium isotope fractionation factors are estimated for a range of crystalline compounds including oxides, silicates, carbonates, and salts containing the Mg(H2O)62+ solvation complex. Fractionation factors for the gas-phase species Mg and MgO are also presented. Fractionation factors are calculated with density functional perturbation theory (DFPT), using norm-conserving pseudopotentials. The results suggest that there will be substantial inter-mineral fractionation, particularly between tetrahedrally coordinated Mg2+ in spinel (MgAl2O4) and the more common octahedrally coordinated Mg2+-sites in silicate and carbonate minerals. Isotope fractionations calculated for Mg2+ in hexaaquamagnesium(2+) salts are in good agreement with previous fractionation models of Mgaq2+ based on large molecular clusters (Black et al., 2007), but show possibly more significant disagreement with a more recent study (Rustad et al., 2010). These models further suggest that solvated Mgaq2+, in the form of Mg(H2O)62+, will have higher 26Mg/24Mg than coexisting magnesite and dolomite. Calculated fractionations are consistent with Mg-isotope fractionations observed in peridotite mineral separates and inorganic carbonate precipitates. Predicted large, temperature-sensitive spinel-silicate fractionations, in particular, may find use in determining equilibration temperatures of peridotites and other high-temperature rock types.