Melting relations of muscovite with quartz and sanidine in the K_2O-Al_2O_3-SiO_2-H_2O system to 30 kilobars and an outline of paragonite melting relations

Abstract

Mixtures of synthetic muscovite with synthetic sanidine or natural quartz, with and without water, were reacted in piston-cylinder apparatus between 10 and 35 kb; metastable corundum persisted in runs not seeded with sillimanite and kyanite. Phases produced include muscovite (Ms), KAlSi_3O_8 • H_2O (OrH), sanidine (Or), kyanite (Ky), sillimanite (Si), quartz (Qz), coesite (Ct), liquid (L), and vapor (V). Melting reactions determined include: (A) Ms + Qz ⇔ Or + Ky + L, (B) Ms + Qz + V ⇔ Ky + L, (C) Or + Qz + V ⇔ L, and (D) OrH + Ct + V ⇔ L. At 30 kb, the vapor-absent melling reaction (A) is about 140°C higher than the corresponding reaction (B) with excess vapor. Melting reactions (A) and (B) extrapolate downward to meet the invariant point for the assemblage Ms + Or + Qz + Si + L + V at 5.8 kb and 730°C, as defined by Storre and Karotke (1972). This point lies just below melting reaction (C). This invariant point is the high pressure limit for the subsolidus dehydration reaction: Ms + Qz ⇔ Or + Si + V, and its position facilitates selection from among the varied dehydration reactions previously published. These results are combined with published results for melting reactions of muscovite without quartz, to provide a P-T projection of muscovite melting and dehydration reactions in the quaternary system. By analogy with the system K_2O-Al_2O_3-SiO_2-H_2O, a similar diagram for paragonite dehydration and melting reactions is obtained by combining published paragonite dehydration reactions with albite-quartz-water melting reactions. Comparison of these mineral stability grids with published estimates of temperature distribution in subducted lithosphere slabs suggests that muscovite in metamorphosed subducted sediment dissociates or melts at relatively shallow depths, and it seems unlikely that muscovite can contribute water for magmatic processes much beyond the arc-trench gap or influence chemical variations in lavas across an arc complex.