The control of carbonate mineral Mg isotope composition by aqueous speciation: Theoretical and experimental modeling

The magnesium isotope compositions of sedimentary carbonates are widely used to investigate the geochemical cycling of this element in seawater. Density Functional Theory techniques and experimental data from this study and literature on Mg isotope fractionation between sedimentary minerals and fluids have been used to derive equilibrium fractionation factors and to quantify the impact of aqueous magnesium speciation on the isotopic composition of its aquo ion and Mg in the precipitated carbonates. Although aqueous Mg^{2 +} undergoes hydrolysis to a lesser extent than divalent transition metals, it nevertheless forms relatively strong complexes with inorganic and organic ligands including bicarbonate/carbonate, sulfate and carboxylate. Furthermore, aqueous Mg^{2 +} undergoes a significant contraction of its coordination sphere when it reacts with bicarbonate and carbonate ions to form the MgHCO₃ ⁺ and MgCO₃° complexes which favors the preferential partitioning of heavy Mg into these species. Calculated values of the reduced partition function ratios for Mg^{2 +}(aq) and Mg^{2 +} bound to a number of inorganic and organic ligands show a strong enrichment of ²⁶Mg in MgCO₃°, MgHCO₃ ⁺ and MgSO₄° compared to Mg^{2 +} (i.e. 1000lnβ_26/24MgCO3°-1000lnβ_26/24Mg ^{2 +} = 5.2 at 25 °C), and either a significant enrichment (Mg(oxalate)^{2 −}, Mg(oxalate)₂ ^{2 −}, Mg(citrate)⁻) or depletion (Mg(EDTA)^{2 −}, Mg(citrate)₂ ^{4 −}) of ²⁶Mg in Mg-carboxylate complexes compared to Mg^{2 +}. Analysis of experiments from this study and the literature on Mg isotope fractionation between aqueous solution and Mg carbonate and hydroxide minerals validate the reduced partition functions for Mg²⁺, Mg bicarbonate/carbonate and carboxylate couples generated in the present work and confirms the significant impact of carbonate and carboxylic ligands on the isotope composition of precipitated Mg-bearing minerals. This study thereby provides new insights into the parameters controlling the isotope composition of aqueous Mg²⁺ in natural fluids as well as improved tools to reconstruct paleo-environmental conditions from the magnesium isotope compositions recorded in carbonate sediments.