H_2O-CO_2-S fluid triggering the 1991 Mount Pinatubo climactic eruption (Philippines)

摘要

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_2O-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_2O-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 ～260MPa. For the first time, a dense CO_2-bearing fluid is reported in Mount Pinatubo volcanic samples.We suggest that this hybrid H_2O-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-eruptiveMount Pinatubo dacitemagma 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 fluidsaturated dacitic magma according to the reaction 4SO_2~(basalt)+ 4H_2O~(dacite)=3HSO_4~-+H_2S+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_2O-CO_2-S fluid played an important role in triggering the 1991 climactic eruption.