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.

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