Effect of Water on the Melting of Enstatite

Abstract

Melting relations for the MgSiO 3 composition have been studied at pressures between 5 and 30 kb under hydrous conditions. The beginning of melting of enstatite is about 1360°, 1280°, and 1270° C at 10, 20, and 30 kb, respectively. The incongruent melting behavior of enstatite, which disappears at pressures higher than about 5 kb under anhydrous conditions, persists to at least 30 kb water pressure. The melting interval where forsterite, liquid, and vapor co-exist is 30° to 50° C in the pressure range 10 to 30 kb, compared with about 20° at 1 atm. Below the solidus the assemblage enstatite + forsterite + vapor is obtained from the MgSiO 3 composition, because the vapor probably contained silica in excess of that in stoichiometric MgSiO 3 . The maximum content of H 2 O that dissolves in the initial H 2 O-saturated melt, determined by the phase-assemblage method, is about 11 weight percent at 10 kb. It is suggested that silica-saturated magmas such as a quartz tholeiite magma may be generated at considerable depths (at least up to 100 km) under hydrous conditions, either by direct partial melting of peridotites or by fractional crystallization of olivine tholeiitic magmas. Even if the H 2 O content is small, its effect cannot be disregarded.