Volcano Emissions Project

ABSTRACT - 1994 American Geophysical Fall Meeting

Total SO2 Emissions and Pre-Eruption Vapor-Saturated Magma at Mount St. Helens, 1980-88

SO2 from explosive volcanism can cause significant climatic and atmospheric impacts, but the source of the sulfur is controversial. TOMS, COSPEC, and ash leachate data for the recent eruption of Mount St. Helens give a total SO2 emission of 2 Mt from 18 May 1980 to 6 September 1988. The rate of SO2 emission dropped sharply from the time of the climactic eruption on 18 May 1980 to the late stages of nonexplosive degassing in 1988, apparently controlled by a decreasing magma supply. In contrast, a total SO2 emission of only 0.08 Mt is estimated from melt inclusion data and the conventional assumption that the main source is dissolved sulfur in pre-eruption melt, thus indicating that melt inclusions can significantly underestimate SO2 emissions and environmental impacts of explosive volcanism. Commonly invoked sources of "excess sulfur" (anhydrite decomposition, basaltic magma, and degassing of non-erupted magma) are not viable for the 2-Mt emission. Observed CO2 emissions during the eruption, the water content of melt inclusions, and experimental data for CO2 and H2O solubility in evolved silicate melt indicate the St. Helens magma was vapor-saturated at depth prior to ascent, and suggest that a vapor phase was the main source of sulfur for the 2 Mt of emitted SO2. A vapor phase source is consistent with phase equilibria studies on the St. Helens dacite, and it removes the need for a much-debated proposal of shallow magma emplacement shortly before or after 18 May 1980.

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