Melting relations of muscovite to 30 kilobars in the system KAlSi_3O_8-Al_2O_3-H_2O

Mixtures of synthetic muscovite, sanidine, and muscovite-sanidine, with or without water, were reacted in piston-cylinder apparatus between 10 and 35 kb. Phases produced include muscovite(Ms), KAlSi_3O_8H_2O(OrH), sanidine(Or), corundum(Co), liquid(L), and vapor(V). Melting reactions determined are: (a) Ms ⇔ Or + Co + L, (b) Ms + V ⇔ Co + L, (c) Ms + Or + V ⇔ L, (cl) Or + V⇔ L. Temperatures of univariant curves are, at 15 kb, (a) 885° (b) 855° (c) 780° (d) 795°; and at 30 kb, (a) 1010° (b) 935 ° (c) 720° (d) 755 °. At 30 kb the vapor-absent melting reaction (a) is 75° higher than the corresponding reaction (b) with excess vapor. Extrapolation of these melting reactions downward places close limits on the invariant point at 9.5 kb and 825°C for the coexistence of Ms + Or + Co + L + V and defines the high pressure limits for reactions Ms ⇔Or + Co + V, and Or + Co + V ⇔ L. This facilitates selection from among the varied muscovite dehydration reaction curves previously published. Comparison of the muscovite stability range with published estimates of temperature distribution in subducted lithosphere slabs suggests that muscovite in subducted sediments neither contributes water for magmatic processes beyond the arc-trench gap nor influences the chemical variations in lavas across an arc complex. It dissociates or melts at shallower depths.