Melting of Tonalite and Crystallization of Andesite Liquid with Excess Water to 30 Kilobars

Abstract

Melting relations of a finely ground, crystalline tonalite with excess water have been studied at pressures between 10 kbar and 30 kbar, to a maximum temperature of 850° C, and these data have been combined with previously published results at lower pressures and higher temperatures. The rock mineralogy of plagioclase, quartz, orthoclase, biotite, and amphibole begins to change just below 10 kbar, and by 30 kbar it is coesite, clinopyroxene, garnet, and kyanite. Runs were planned to determine the curve for the beginning of melting, but combining these with published results for andesites and andesite compositions between 1 kbar and 20 kbar permits delineation of the major features of the phase diagram through the melting interval. The solidus temperatures are: 1 kbar, 765° C; 3 kbar, 690° C; 10 kbar, 635° C; 15 kbar, 628° C; 20 kbar, 670° C; 30 kbar, 735° Orthoclase dissolves in the aqueous vapor phase between 3 kbar and 10 kbar. The quartz out-(coesite-out) phase boundary varies from about 30° to 65° above the solidus. The plagioclase-out phase boundary extends from about 2 kbar, 1,000° to the solidus near 15.5 kbar, 630° C. The liquidus phase is pyroxene up to about 10 kbar, 950° C, and amphibole from there to about 18 kbar, 950° C. Garnet is the liquidus phase at higher pressures. Biotite, stable to 800°-850° C, reacts out between 15 kbar and 17.5 kbar, with the formation of garnet and kyanite. Garnet is reluctant to nucleate, but it appears at 15 kbar and is probably stable at considerably lower pressures. The amphibole reaction boundary extends from 18.5 kbar, 950° to 21.5 kbar, 700° C, but the equilibrium phase boundary could be lower by a few kilobars at 700° according to some published work. No orthopyroxene was found in any runs. The experimental results have applications to high-grade metamorphism and the nature of the lower crust, the generation of batholiths, and the crystallization of andesite magmas rising from subduction zones.