Melting Reactions in the System NaAlSi_3O_8-KAlSi_3O_8-SiO_2 to 35 Kilobars, Dry and with Excess Water

Abstract

Piston-cylinder apparatus was used to determine the univariant minimum melting reaction in the dry system Ab-Or-Qz to 30 kb, and to extend the corresponding curve with excess water from 20 kb to 35 kb. At 30 kb, temperatures of the two univariant curves are 1,235°C and 680°C, respectively. The new data and other published data are combined to produce a complete P-T diagram for univariant reactions in the system Ab-Or-Qz-H_2O above 500°C, and phase diagrams comparing the effect of pressure on field boundaries in the systems Ab-Or-Qz-H_2O and Ab-Or-Qz. The slopes (dP/dT) of the excess water reactions change from negative to positive at pressures where the feldspars are replaced by denser minerals: about 17 kb where jadeite is formed from albite, and 27-30 kb where sanidine is replaced by sanidine hydrate. The liquidus volume for the primary crystallization of quartz increases with pressure, and above 10 kb the effect is greater in the dry system than in the water-saturated system. This accounts for the increased melting interval of granites with increasing pressure, and the wider interval beneath the liquidus of granites for the crystallization of quartz (or coesite) without accompanying feldspar. Partial fusion of metamorphosed feldspathic sediments in deeply subducted oceanic crust would produce a liquid with only a few per cent normative quartz; the initial liquid would be closer to a trachyte than a normal rhyolite. The results are consistent with the conclusion from other experiments that primary granites and ryolites cannot be derived from subduction zones at depths of 100 km or so.