HR: 11:50h
AN: V52B-07    [Abstracts]
TI: Emission Rates, Pre-eruption Gas-Saturation and Ascent Degassing During the 2004-2005 Eruption of Mount St. Helens
AU: * Gerlach, T M
EM: tgerlach@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Ct 100, Vancouver, WA 98683 United States
AU: McGee, K A
EM: kenmcgee@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Ct 100, Vancouver, WA 98683 United States
AU: Doukas, M P
EM: mdoukas@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Ct 100, Vancouver, WA 98683 United States
AB: Intermittent airborne measurements of volcanic gases began on 27 September 2004 during the initial unrest. Target gases included CO₂, SO₂ and H₂S. The average emission rates through the end of July 2005 were approximately 650 metric tons/day (t/d) CO₂ and 100 t/d SO₂; H₂S was always <8 t/d. The cumulative emissions are estimated to be about 190 kt CO₂ and 30 kt SO₂. The emission rates are significantly lower than those of 1980 and the earlier dome-building episodes of the 1980s, which frequently produced several thousand t/d CO₂ and several hundred t/d SO₂. Nevertheless, the average CO₂/SO₂ of 10 (molar) for the present emissions is similar to that of the 1980-81 emissions. Despite the lower emission rates, the cumulative CO₂ released and the estimated amount of melt supplied from depth (46 Mt) indicate the dacite would have been gas-saturated at plausible chamber depths. Data on the S content of melt inclusions and matrix glasses underestimate observed SO₂ emissions by over a factor of five, corroborating pre-eruption gas saturation of the dacite. Using the 1980 magma equilibration depth (8 km) for comparison and assuming a similar water content (5 wt%) for the rhyolitic melt fraction (Rutherford, 1993; Blundy and Cashman, 2001) give a gas fraction constrained by H₂O-CO₂ solubility data of 1-2 vol% for the dacite. This compares with an estimate of 10-15 vol% for the 1980 dacite prior to eruption and of 5-30 vol% for common intermediate to silicic magmas (Wallace, 2003). Closed system ascent degassing calculations show that the volume fraction of gas increases to 8 vol% at 4-5 km and reaches 50 vol% at 1 km, where final solidification begins. The gas fraction can potentially increase to >60 vol% during solidification. Allowing for gas separation during extrusion, these results are consistent with observed dacite vesicle fractions averaging 25 vol% (Pallister et al. this session). Ascent degassing calculations also predict melt water contents similar to values measured on rare glassy dacite fragments last equilibrated at depths of 1.2-1.8 km (Mandeville this session).
DE: 3640 Igneous petrology
DE: 8419 Volcano monitoring (7280)
DE: 8430 Volcanic gases
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005