Pinatubo pre-eruption dynamics

Two independent petrologic studies determined preeruptive magma storage at 220±50 MPa (Rutherford and Devine, 1996; Scaillet and Evans, 1999). However, in attempting to investigate magma ascent and degassing processes, we discovered that this assessment of the equilibration pressure is incomplete. By virtue of their fast equilibration rates, glass compositions record partial equilibration at a lower partial pressure of H₂O (P_H2O) at least three weeks before the magma’s appearance at the surface. Differences in the rates of mineral-melt reactions may be similarly exploited at other volcanic systems to set limits on the timing and extent of preeruptive magma degassing.

Phase equilibria experiments on dacite from the June 1991 eruption of Mt. Pinatubo were used to calibrate a geobarometer for matrix glass at pressures of 160-220 MPa. Comparison with natural glass compositions suggests equilibration at substantially lower f_H2O than is indicated by Al-in-hornblende geobarometry. The disparity between geobarometric techniques may be explained by a decrease in f_H2O and consequent crystallization several weeks before eruption—a time period too short for hornblende re-equilibration. Reduced f_H2O could result 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.

Hammer, J.E., and M.J. Rutherford, Petrologic indicators of preeruption magma dynamics, Geology, 31 (1), 79-82, 2003.