Effect of sulfate on the kinetic and equilibrium Magnesium isotope fractionation between low Mg-calcite and fluid

Abstract

Experimentally defined metal isotope fractionation factors between carbonate minerals and fluids are routinely used to interpret the isotopic compositions of natural samples. The majority of experimental works, however, have been conducted in simplified and rather diluted background electrolyte solutions although most carbonate proxies were applied in seawater. Thus the impacts of important inorganic ligands such as SO₄²⁻ on metal isotope fractionation between carbonate minerals and fluids are poorly known. It is however nowadays well accepted that metal–ligand interactions strongly affect the incorporation and isotope composition of cations in the mineral lattice by either the formation of aqueous complexes or their direct incorporation in the solid. In order to shed light on the role SO₄²⁻(aq) holds on the Mg isotope fractionation between calcite and fluid, in this study we have measured the δ²⁶Mg composition of Mg-calcites and forming fluids reported by Goetschl et al. (2019). The obtained results suggest that all calcites formed in the presence of sulfate exhibit systematically smaller Mg isotope fractionation values (i.e. Δ²⁶Mg_{calcite-fluid} = δ²⁶Mg_calcite-δ²⁶Mg_fluid) compared to those formed in the absence of this ligand. The increase of Δ²⁶Mg_{calcite-fluid} values can be assigned to two processes. The first is the formation of aqueous MgSO₄^o complexes that enhance the exchange rate of water molecules between Mg²⁺ hydration sphere and bulk water. The formation of MgSO₄^o results in a reduction of the absolute value of Δ²⁶Mg_{calcite-fluid} by allowing the incorporation of less hydrated Mg in calcite. The second is the incorporation in the solid of a significant fraction of Mg as MgSO₄ which reduces the average Mg-O bond length in the crystal lattice, making the presence of ²⁴Mg less abundant compared to solids that do not contain sulfate. The significance of these results is discussed with respect to the interpretation of Mg isotope compositions of natural carbonates and our understanding of the mechanisms controlling isotope fractionation of trace metals in CaCO₃ phases.