The system NaAlSi2O6-H2O-argon: Total pressure and water pressure in metamorphism

Phase equilibrium in metamorphic rocks is affected by temperature, pressure, the proportions of nonvolatile components, and the chemical potentials of the reacting volatile components. Theory interrelating these variables has been tested by studying the reaction analcite → albite + nepheline + water in the presence of mixtures of water and argon. New data on the system Ar-H2O permit calculation of the composition of the water-argon mixture, which should equilibrate with the phase assemblage analcite + albite + nepheline. Experimental determination of this composition as a function of pressure at constant temperature is in good agreement with the theory.