The 15 June 1991 eruption of Mount Pinatubo. I. Phase equilibria and pre-eruption P-T-fO(2)-fH(2)O conditions of the dacite magma

Crystallization experiments were carried out on a representative sample of the crystal-rich dacite ejected during the 15 June 1991 eruption of Mt. Pin atubo, to define the pre-eruption conditions of this major volcanic ev ent. Experiments were performed in the temperature and pressure range of 750-90 0 degrees C and 220-390 MPa, respectively. Redox conditions were varied bet ween those of the NNO (nickel-nickel oxide) buffer and 2.7 log fO(2) units above (NNO + 2.7). Melt water contents ranged from 3 to 7 wt% (H2O saturati on). Phase equilibria at 220 MPa reproduce the phase assemblage of the magm a only at temperatures below 780 degrees C and water-rich conditions: melt H2O content > 6 wt%, or XH2Ofluid > 0.80. Phase abundances and the composit ions of hornblende, plagioclase, and melt indicate that the eruption tapped a magma body that was at a temperature of 760 +/- 20 degrees C and a press ure in the range of 220-180 MPa, with melt H2O contents between 6 and 6.5 w t%, that is, XH2Ofluid > 0.88. Thermodynamic calculations constrain CO2 in the melt to be <100 ppm. Comparison between natural and experimental Fe-Ti oxide compositions indicate that fO(2) was in the range NNO + 1.5 to NNO 1.7. Current calibrations of the Fe-Ti oxide thermobarometer are not approp riate for oxidizing conditions, and overestimate both T and fO(2) when fO(2 ) is greater than NNO + 1.5. Compositional zoning of plagioclase reflects c rystallization over 150 degrees C (900-750 degrees C), and implies conditio ns close to, or at, H2O saturation throughout crystallization in the upper- crustal magma chamber. Although continuous magma replenishment probably occ urred, the injected magma must have had the same bulk composition (includin g the same melt H2O content) as the erupted dacite to preserve the linear r elationship observed between An and Or contents of the plagioclase. Composi tional zoning of hornblende is consistent either with an early crystallizat ion event at similar to 400 MPa and 840-900 degrees C, or with mixing event s before eruption. The experiments show that addition of S leads to an incr ease in the mg-number of hornblende in the redox range where pyrrhotite occ urs (less than or equal to NNO + 1.4). The lack of Mg-rich overgrowth on ho rnblende shows that if sulfur was introduced into the magma in the course o f its crystallization, such as by volatile infiltration from an underlying, triggering basalt magma, then it must have occurred when the dacite was al ready oxidized.