The system albite–H₂O–CO₂: a model for melting and activities of water at high pressures

The melting of albite in the presence of H₂O–CO₂ vapor has been determined in piston-cylinder apparatus from 5 to 25 kbar for mole fractions of H₂O in the vapor $(XH2OV)$ from 1.0 to 0.0. Albite in the presence of CO₂-rich vapor melts as much as 150–300°C lower than indicated by previous theoretical and experimental studies. The albite-CO₂ melting curve is indistinguishable from the vapor-absent melting curve up to 15 kbar, but it is significantly lower above 15 kbar as a result of a substantial increase in the solubility of CO₂ in the liquid. Calculated activity coefficients (γ) for water in H₂O–CO₂ vapor in the range 700–1000°C, 5–15 kbar are everywhere greater than unity and increase isobarically and isothermally toward CO₂-rich compositions. For $XH2OV<0.3$ calculated $γH2OV$ are significantly greater than those predicted by the modified Redlich-Kwong equation.