The solubility of calcite in water at 6-16 kbar and 500-800 °C

The solubility of calcite in H2O was measured at 6-16 kbar, 500-800degreesC, using a piston-cylinder apparatus. The solubility was determined by the weight loss of a single crystal and by direct analysis of the quench fluid. Calcite dissolves congruently in the pressure (P) and temperature (T) range of this study. At 10 kbar, calcite solubility increases with increasing temperature from 0.016+/-0.005 molal at 500degreesC to 0.057+/-0.022 molal at 750 degreesC. The experiments reveal evidence for hydrous melting of calcite between 750 and 800degreesC. Solubilities show only a slight increase with increasing P over the range investigated. Comparison with work at low P demonstrates that the P dependence of calcite solubility is large between 1 and 6 kbar, increasing at 500degreesC from 1.8x10(-5) molal at 1 kbar to 6.4x10(-3) molal at 6 kbar. The experimental results are described by: log m(CacO3) = -3.95 + 0.00266T +(32.8 - 0.0280T)log rho(H2O) where T is in Kelvin and rho(H2O) is the density of pure water in g/cm(3). The equation is applicable at 1-20 kbar and 400-800 degreesC, where calcite and H2O stably coexist. Extrapolated thermodynamic data for HCO3- indicates that the dominant dissolved carbon species is CO2,aq at all experimental conditions. The results require that equilibrium constant for the reaction: CaCO3 + 2H(+) = Ca+2 + CO2,aq + H2Ocalcite increases by several orders of magnitude between 1 and6kbar, and also rises with isobaric T increase. Published thermodynamic data for aqueous species fail to predict this behavior. The increase in calcite solubility with P and T demonstrates that there is a strong potential for calcite precipitation during cooling and decompression of water-rich metamorphic fluids sourced in the middle to lower crust.