Calculation of muscovite-paragonite-alkali feldspar phase relations

Abstract

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.