Petrologic indicators of preeruption magma dynamics

Abstract

New phase-equilibria experiments on dacite from the June 1991 eruption of Mount Pinatubo, Philippines, allow calibration of a geobarometer for matrix glass at pressures of 160–220 MPa. Comparison with natural glass compositions suggests equilibration at lower water fugacity, , than inferred from Al-in-hornblende geobarometry. The disparity between geobarometric techniques may be explained by a decrease in and consequent crystallization several weeks before eruption—a time period too short for hornblende reequilibration. Reduced could have resulted from processes associated with basalt magma injection, including (1) late-stage magma ascent (under H₂O-saturated conditions) or (2) influx of other volatiles (CO₂, SO₂) into the dacite magma system.