Calculation of muscovite-paragonite-alkali feldspar phase relations

Experimental data on K-Na exchange between NaCl-KCl-H₂O fluids and alkali feldspars or white micas, and end-member dehydration reactions of white mica (± quartz to alkali feldspar plus corundum (or Al₂SiO₅) and H₂O have been used to evaluate K-Na exchange potentials between coexisting white mica and alkali feldspar. Calculations using these exchange potentials and Margules parameters for excess molar Gibbs energies of alkali feld-spars and white micas have permitted the construction of a projected binary phase diagram for the white mica dehydration. Extrapolation to higher temperatures and pressures gives a topology wholly consistent with field evidence. The subsolidus univariant curves intersect H₂O-saturated minimum-melting curves. At pressures greater than about 6 kbar the common assemblage Ms+Or+Ab+Q could undergo H₂O-saturated minimum-melting at lower temperatures than the disappearance of paragonite or the kyanite-sillimanite isograd in more aluminous rocks. The apparent restriction of anatexis to rocks above the second sillimanite isograd may imply that activity of H₂O is usually less than unity in high-grade regional metamorphism.