The structure of water-saturated carbonate melts

Abstract

The structure of water-saturated Ca- and Mg-bearing carbonate melts under reducing and oxidizing conditions was investigated in a series of hydrothermal anvil cell experiments conducted at 400-1100 °C and 442-2839 MPa. Equilibria were investigated in the calcite-H2O, calcite-CaO-H2O, magnesite- H2O, and magnesite-MgO-H2O systems, with redox conditions controlled by Re/ReO2 and Ti/TiO2 assemblages. Melting relationships and the C-O-H speciation of the coexisting aqueous fluid and melt were assessed in situ by Raman vibrational spectroscopy. Hydrous melting of MgCO3-MgO occurred at ~850 °C, 1.5-2 GPa. In the CaCO3-CaO-H2O system, melt was formed at 600-900 °C and pressures of 0.5-1.5 GPa because of melting-point depression imposed by the presence of CaO. The C-O-H speciation of the carbonate melts and coexisting supercritical aqueous solutions was mainly H2O and CO32-, with traces of CO2(aq) and CH4(aq) in the fluid phase. The melt-fluid H2O partition coefficients attained in the Mg-bearing melt (median 0.5) were higher than in the Ca-bearing melt (median 0.3). Under oxidizing redox conditions, dissolved ReO2 - was present in all phases, underscoring the enhanced solubility of metals in carbonate-bearing melts and carbonatites. In effect, the enhanced solubility of H2O along with the ionic nature of the carbonate melts may promote the solvation of ionic species in the melt structure.