Influence of water on melting of silicates at high pressure

Abstract

The conditions of solidus in water-silicate systems are analysed from thermodynamic considerations. In the region of low H 2O pressures a molar volume of water is considerably greater than that of silicates. It is shown that this fact must determine the inevitable temperature decrease of silicate melting in the presence of water. A substantial decrease of the water molar volume with pressure increase leads to the fact that thermodynamic conditions are created under which the melting temperature of water-silicate systems will augment, i.e. with respect to pressure the water-silicate systems will behave similarly to dry systems. This fact is of considerable petrological interest for the solution of problems of Earth material transitions to the molten state. When passing from low pressures to high ones the change of the d T/d P sign from negative to positive may occur either in the point of extremum for temperature or in the invariant point which lies on the intersection of the T- P curve of the monovariant reaction of melting with the line of solid phase transformations.