Phase equilibria of a Be, U and F-enriched vitrophyre from Spor Mountain, Utah

The equilibrium melting relations of a vitrophyre representative of topaz rhyolites have been experimentally determined in order to constrain the pre-eruption temperature and H₂O content of topaz rhyolite magmas. The vitrophyre sample is strongly enriched in F and incompatible elements and consists of approximately 70% unaltered volcanic glass and 30% crystals of biotite, quartz, sanidine, plagioclase, magmatic fluorite and a variety of accessory phases. Biotite stability is strongly dependent on temperature, pressure, and a_H2O, in the melt, and is apparently independent of the variations in f{hook}o₂ over the range investigated in this study. The thermal stability of biotite increases relative to that of quartz, sanidine, and plagioclase in water-saturated melts as pressure increases from 1 2 to 2 kbar. Fluorite is stable to temperatures below but within 50° to 75°C of the biotite stability curve, and topaz is present in the products of runs at and near the solidus for all fluid conditions investigated. The 1.2 wt.% F in the vitrophyre results in very low solidus temperatures of ≤ 525°C for all fluid conditions studied. Comparison of the experimentally determined crystallization sequence of the vitrophyre with the observed crystallization sequence of the vitrophyre and of the overlying topaz rhyolites indicates that the phenocrysts in the vitrophyre and in the topaz rhyolites crystallized at a pressure ≥ 1 kb, that the magma contained at least 4 wt.% water during initial crystallization of biotite, and that the magma contained a pre-eruption water content of 5 wt.%.