H2O–CO2–S fluid triggering the 1991 Mount Pinatubo climactic eruption (Philippines)

The factors that trigger explosive eruptions often remain elusive because of the lack of direct data from representative samples. Here, we report the first micro-Raman spectroscopy measurements of fluid and multiphase inclusions trapped in quartz xenocrysts and microlites from andesitic lavas and basaltic enclaves of the 1991 Mount Pinatubo eruption. Our analyses reveal two-phase H 2 O–CO 2 –S inclusions containing a CO 2 -dominated phase and an aqueous sulfate-bearing liquid phase and, less commonly, anhydrite (CaSO 4(solid) ). The two fluid phases are low-temperature products of a supercritical H 2 O–CO 2 –S fluid which was associated with a hydrous silicate melt prior to eruption. The average density of the CO 2 phase is 0.4 ± 0.2 g/cm 3 at room temperature, corresponding to a supercritical fluid density of 0.6 ± 0.1 g/cm 3 at the conditions of entrapment at 760–1000 °C and up to ∼260 MPa. For the first time, a dense CO 2 -bearing fluid is reported in Mount Pinatubo volcanic samples. We suggest that this hybrid H 2 O–CO 2 –S fluid originated from mixing between sulfur-rich basaltic and hydrous dacitic magmas, as the former was intruded into and interacted with the pre-eruptive Mount Pinatubo dacite magma reservoir, at depths of at least 10 km. Thermodynamic modeling demonstrates that part of the SO 2 liberated from the intruded basaltic magma was consumed via interaction with the aqueous fluid-saturated dacitic magma according to the reaction 4SO 2 basalt + 4H 2 O dacite = 3HSO 4 - + H 2 S + 3H + , yielding early Cu-rich sulfides, late abundant anhydrite, and SO 4 -rich apatites, which are commonly found in the Mount Pinatubo dacites. We suggest that this hybrid H 2 O–CO 2 –S fluid played an important role in triggering the 1991 climactic eruption.