Melting relations of muscovite to 30 kilobars in the system KAlSi 3 O 8 -Al 2 O 3 -H 2 O

Mixtures of synthetic muscovite, sanidine, and muscovite-sanidine, with without water, were reacted in piston-cylinder apparatus between 10 and 35 kb. 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 + + L, (b) Ms + V ⇔ Co + L, (c) Ms + Or + V ⇔ L, (cl) Or + V⇔ L. of univariant curves are, at 15 kb, (a) 885° (b) 855° (c) 780° (d) 795°; at 30 kb, (a) 1010° (b) 935 ° (c) 720° (d) 755 °. At 30 kb the vapor-absent melting (a) is 75° higher than the corresponding reaction (b) with excess vapor. of these melting reactions downward places close limits on the invariant at 9.5 kb and 825°C for the coexistence of Ms + Or + Co + L + V and defines high pressure limits for reactions Ms ⇔Or + Co + V, and Or + Co + ⇔ L. This facilitates selection from among the varied muscovite dehydration reaction previously published. of the muscovite stability range with published estimates of temperature in subducted lithosphere slabs suggests that muscovite in subducted neither contributes water for magmatic processes beyond the arc-trench gap influences the chemical variations in lavas across an arc complex. It dissociates melts at shallower depths.