Magnesium isotope fractionation between calcite and aqueous solutions under elevated temperatures of 98–170 °C

Abstract

The incomplete understanding of Mg isotope fractionation behavior between calcite and aqueous solutions has limited the interpretation and application of Mg isotope data in natural carbonates. In this study, we performed a series of aragonite-to-calcite conversion experiments under hydrothermal conditions to investigate the effects of temperature and organic ligands on elemental and isotopic partitioning of Mg between calcite and aqueous solutions. The experiments produced magnesian calcite via a dissolution-and-precipitation process. The Mg distribution coefficients between solid and aqueous solution (DMg, 0.08 to 0.21) showed negative correlations with Sr distribution coefficients (DSr, 0.04 to 0.48), as well as the proportion of calcite in the bulk solid, which indicates mixing of calcite (Mg-rich and Sr-poor) and aragonite (Sr-rich and Mg-poor) in the solid phases. The apparent Mg isotope fractionation factors between solid and aqueous solutions (Δ26Mgsolid-soln) show a positive correlation with reaction temperatures, where Δ26Mgsolid-soln increases from ca. −1.84 ‰ at 98 °C to −1.35 ‰ at 170 °C. In addition, the presence of oxalate contributed to slightly lower Δ26Mgsolid-soln values, which is interpreted to be related to the isotopic effect of Mg-oxalate speciation. Based on the experimental results, we propose a temperature-dependent function for Mg isotope fractionation between calcite and free Mg-aquo ions (i.e., Mg(H2O)62+): Δ26Mgcal-Mg(aq) = (−0.17 ± 0.01) × 106/T2- (0.52 ± 0.08). This equation is applicable to a wide range of temperatures, especially at hydrothermal conditions, thus enabling interpretations of Mg isotope data from calcite of hydrothermal and burial origin.