Kinetic and Thermodynamic Controls of Divalent Metals Isotope Composition in Carbonate: Experimental Investigations and Applications

The very contrasting steric and electronic properties of divalent metals (i.e. Ba, Ca, Mg, Sr, Zn, Cu, Cd, Mn, Co, Ni) dramatically affect the reactivity of their aqueous ions in solution and their partitioning between fluids and minerals including calcium carbonates. In this study we show that these contrasting properties result in very distinct kinetic and thermodynamic behaviors of their isotopic fractionation between aqueous fluids and carbonate minerals. For example, because of steric effects, the light isotopes of Ca, Mg, Sr, and Ba, are enriched in precipitated calcite but the extent of Mg isotopes fractionation decreases with increasing calcite growth rate whereas that of Ca, Sr and Ba increases with calcite growth rate. The distinct behavior of Mg stems from the reduced lability of water molecules in its coordination sphere compared to Ca, Ba and Sr. In contrast, the heavy isotopes of Zn (and probably Cu, Cd, Ni) are slightly enriched in precipitated calcite in accord with the great affinity of these metals for the solid (partition coefficient KD > 1), and the extent of their fractionation decreases with increasing calcite growth rates. Moreover, transition metals, especially Cu which is affected by the Jahn-Teller effect, exhibit a strong affinity for RO− ligands and thus a marked dependence of their equilibrium isotope distribution among aqueous fluids and calcite on solution pH, ΣCO2(aq) and the presence of aqueous inorganic and organic ligands. These observations provide new insights into the mechanisms controlling the incorporation of divalent metals into calcite as well as new tools to reconstruct paleo-environmental conditions from their isotope composition recorded in carbonate sediments.