Crystallization of basalt and andesite under high pressure hydrous conditions

Abstract

Experimental crystallization of synthetic high-alumina quartz tholeiite and basaltic andesite glasses at 9–10 kb under conditions of PH2O <Pload demonstrates that sub-silicic clinopyroxene, orthopyroxene and amphibole are the near-liquidus phases above 1000°C. Amphibole is the dominant phase at temperatures of about 960°C and less. Calcic plagioclase occurs nearer the solidus. In an andesite composition plagioclase and clinopyroxene are the near-liquidus phases at 940°C, joined by orthopyroxene, amphibole and garnet at lower temperatures. Electron microprobe analysis of phases crystallizing from the basalt allows calculation of the composition of liquid fractionates obtained under these conditions. The calculated liquid fractionate compositions follow the calc-alkaline trend and demonstrate that the calc-alkaline igneous rock suite may be derived by the fractional crystallization of basalt under hydrous conditions at 30–40 km depth (PH2O <Pload) or alternatively by the partial melting of amphibolite under similar conditions. In both cases the crystalline residuum will consist dominantly of amphibole, with subordinate clinopyroxene and orthopyroxene, and possibly minor calcic plagioclase and garnet, depending on the degree of fractionation.