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Pre-Eruption Vapor in Magma of the Climactic Mount Pinatubo Eruption: Source of the Giant Stratospheric Sulfur Dioxide Cloud

by: Terrence M. Gerlach1, Henry R. Westrich2, and Robert B. Symonds1

1 U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver WA 98661.
2 Sandia National Laboratory, Geochemistry Dept 6118, Albuquerque NM 87185

IN: Newhall, C.G., and Punongbayan, R.S., eds., 1996, Fire and mud: eruptions and lahars of Mount Pinatubo, Philippines: Philippine Institute of Volcanology and Seismology, Quezon City, and University of Washington Press, Seattle, 1126p.

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The climactic June 15, 1991, eruption of Mount Pinatubo injected a minimum of 17 Mt (megatons) of SO2 into the stratosphere--the largest stratospheric SO2 cloud ever observed. This study is an investigation of the immediate source of the sulfur for the giant SO2 cloud. Approximately 100 electron microprobe analyses show no significant differences, at the 95 percent confidence level, in S or Cl contents between glass inclusions and matrix glasses of the erupted dacite. These results indicate that there was no significant degassing of S or Cl from melt during ascent and eruption. Furthermore, the 17-Mt SO2 cloud contained over an order of magnitude more sulfur than could have been dissolved in the quantity of erupted silicate melt at the pre-eruption conditions. A major source of "excess sulfur" is therefore required to account for the SO2 cloud. Degassing of melt in non-erupted dacite as a source of the excess sulfur implies volumes of non-erupted dacite larger than the estimated volume of the magma reservoir beneath the Mount Pinatubo region. Direct degassing of excess sulfur from a basalt source seems unlikely, since the June 15 eruption products lack evidence of mixed or commingled contemporaneous basalt. Anhydrite decomposition rates at atmospheric pressure and expected eruption temperatures are extremely slow and grossly incapable of generating 17 Mt of SO2 by anhydrite breakdown in the eruption cloud. Anhydrite breakdown during ascent decompression is too slow to keep pace with conduit travel times, which were considerably less than 8 minutes. Flash-vaporization of sulfate-rich Pinatubo hydrothermal fluids during the eruption could have caused sulfate mineral deposition but virtually no SO2 production.

It is proposed that the dacite erupted on June 15 was vapor-saturated at depth prior to eruption, and that an accumulated vapor phase in the dacite provided the immediate source of excess sulfur for the 17-Mt SO2 cloud. Investigations based on exploration drilling for geothermal energy suggest that magmatic volatiles were discharged into the Pinatubo hydrothermal system from the vapor-saturated dacite prior to the 1991 eruption. Experimental studies, geobarometer results, and the H2O and CO2 contents of glass inclusions indicate that the Pinatubo dacite was saturated with water-rich vapor before ascent and eruption. Models for the composition of the pre-eruption vapor suggest that it contained a minimum of approximately 96 Mt H2O, 42 Mt CO2, and 3 Mt Cl, in addition to 17 Mt of SO2. The mole fraction composition of the vapor was X(H2O) = 0.80-0.83, X(SO2) = 0.01-0.04, X(CO2) = 0.15, and X(Cl) = 0.01, indicating that the vapor was not excessively SO2-rich. The volume and density of the vapor at depth prior to eruption were at least 0.25 km^3 and about 0.6 g/cm^3, respectively. Vapor comprised at least 5 volume percent of the pre- eruption dacite at depth; the bulk density of the pre- eruption dacite was less than 2.3x10^(12) kg/km^3. Solubility modeling indicates that the total amount of volatiles contained in the pre-eruption vapor and melt of the erupted dacite could not have been dissolved initially in completely molten dacite at the magma reservoir pressures, suggesting some process of pre- eruption vapor accumulation at depth.

The climactic eruption released the accumulated vapor in the estimated 5 km^3 of erupted dacite. About 6.25 wt percent of dissolved water was also degassed from melt during ascent and eruption; scaling to the volume of erupted dacite implies an additional release of 395 Mt of H2O. Additional yields of SO2, CO2, and Cl from degassing of melt were minor to insignificant during ascent and eruption. Thus, the minimum volatile emissions for the climactic eruption--from preeruption vapor phase and degassing of melt--were 17 Mt SO2, 42 Mt CO2, 3 Mt Cl, and 491 Mt H2O.

This study underscores the need for both petrologic measurements and emission measurements to constrain the quantity of dissolved volatiles and pre-eruption vapor in magma at depth. If explosive volcanism commonly involves magmas with substantial accumulated vapor, the volatile contents of glass inclusions alone are not a sufficient basis for inferring the total pre-eruptive volatile contents of magma and for predicting volatile emissions. Consequently, conventional petrologic estimates of SO2 emissions during explosive eruptions of the past may be far too low and significantly underestimate their impacts on climate and the chemistry of the atmosphere.

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