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,

\(\mathit{f}_{H_{2}O}\)
⁠, than inferred from Al-in-hornblende geobarometry. The disparity between geobarometric techniques may be explained by a decrease in
\(\mathit{f}_{H_{2}O}\)
and consequent crystallization several weeks before eruption—a time period too short for hornblende reequilibration. Reduced
\(\mathit{f}_{H_{2}O}\)
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.

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