THERMODYNAMIC CONTROL OF THE ISOTOPE COMPOSITION OF DIVALENT METAL CATIONS IN AQUEOUS SOLUTIONS AND IN CARBONATE MINERALS

Abstract:

The very contrasting steric and electronic properties of divalent metals dramatically affect the reactivity and composition of their aqueous species and their partitioning between fluids and minerals. These contrasting properties result also in very distinct kinetic and thermodynamic trends of their isotopic composition in aqueous fluids and carbonate minerals. For example, if alkaline earths in calcite are all enriched in light isotopes, only Mg exhibits a decrease of its isotope fractionation with increasing calcite growth rate. Moreover, the Mg²⁺ aquo ion is the only alkaline earth ion whose isotopic composition is markedly affected by the presence in solution of inorganic ligands like bicarbonate, carbonate or sulfate. The distinct behavior of Mg stems from the reduced lability of water molecules in its coordination sphere and from the reduction of its aquo ion coordination sphere when it coordinates to HCO₃^- and CO₃^2-. Ab initio calculations show that the preferred four hydration number of Mg in stable Mg bicarbonate and Mg carbonate monomers results in a strong enrichment in ²⁶Mg of these species compared to Mg(H₂O)₆²⁺ (i.e. 1000lnβ_26/24MgCO3°-1000lnβ_26/24Mg2+ = 5.16 ‰; Fujii, personal communication). The analysis of recent experiments on Mg isotope fractionation between carbonate crystals and solution using density functionnal theory estimation of lnβ values from Fujii i) confirm the marked impact of carbonate and bicarbonate ligands on the isotope composition of Mg in calcite and magnesite and ii) allow to reconcile First-principles and experimental estimates of equilibrium Mg isotope fractionation in carbonate crystals. Recent experiments also confirm that the strong affinity of Zn²⁺ or Cu²⁺ for RO^- ligands results in a marked impact of fluid pH, ΣCO₂(aq) and/or carboxylic ligands concentrations on the isotope composition of these metals in carbonate minerals. These observations provide new insights into the parameters controlling the isotope composition of Me²⁺(aq) as well as new tools to reconstruct paleo-environmental conditions from the isotope composition of Me²⁺ recorded in carbonate sediments.