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Total sulfur dioxide emissions and pre‐eruption vapor‐saturated magma at Mount St. Helens, 1980–88
Author(s) -
Gerlach T. M.,
McGee K. A.
Publication year - 1994
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/94gl02761
Subject(s) - magma , melt inclusions , geology , dacite , explosive eruption , volcano , sulfur , volcanism , magma chamber , geochemistry , volatiles , sulfur dioxide , mineralogy , andesite , volcanic rock , chemistry , seismology , inorganic chemistry , organic chemistry , tectonics
SO 2 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 Mount St. Helens from the time of the climactic eruption on 18 May 1980 to the final stages of non‐explosive degassing in 1988 give a total SO 2 emission of 2 Mt. COSPEC data show a sharp drop in emission rate that was apparently controlled by a decreasing rate of magma supply. A total SO 2 emission of only 0.08 Mt is estimated from melt inclusion data and the conventional assumption that the main sulfur source was pre‐eruption melt; commonly invoked sources of “excess sulfur” (anhydrite decomposition, basaltic magma, and degassing of non‐erupted magma) are unlikely in this case. Thus melt inclusions may significantly underestimate SO 2 emissions and impacts of explosive volcanism on climate and the atmosphere. Measured CO 2 emissions, together with the H 2 O content of melt inclusions and experimental solubility data, indicate the Mount St. Helens dacite 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 SO 2 . A vapor source is consistent with experimental studies on the Mount St. Helens dacite and removes the need for a much debated shallow magma body.