EMISSION RATES OF CO2, SO2, AND H2S DURING THE 2004-2005 ERUPTION OF MOUNT ST. HELENS

Volcanic gas observations during the 2004 unrest at Mount St. Helens began on September 27 and have employed both helicopter and fixed-wing aircraft measurements. Target gases included CO₂, SO₂, and H₂S measured by remote (COSPEC, FLYSPEC) and extraction (LI-COR, Interscan) techniques. These measurements defined periods of negligible, wet, and dry volcanic degassing. Early measurements during September 27-30 showed little or no CO₂ above atmospheric levels; SO₂ and H₂S were absent. The absence of these gases implies fairly complete gas scrubbing at high water to gas mass ratios or confinement of the gases by post-1986 sealing of gas transport channels. Measurements during a period of wet volcanic degassing began on October 1, after the first steam and ash explosion, and showed an increase in the number and size of CO₂ peaks together with the increasingly common detection of H₂S. The few available emission rates for CO₂ during this period were <150 metric tons/day (t/d). Measurements during the period of dry volcanic degassing, which began on October 5 or 6, were made as temperatures rose and steaming increased on the south side of the 1980-86 lava dome and as rock adjacent to the invading magma progressively dried out. These conditions enabled a buoyant plume to rise above the crater and enter higher elevation winds, facilitating gas emission rate measurements. The gas emission rates have been notably low and declined gradually throughout the eruption. Emission rates of CO₂ fell from about 1000 t/d in October 2004 to 100-200 t/d in June 2005; corresponding emission rates of SO₂ declined from 240 t/d to 30-50 t/d. Emission rates of H₂S were variable but always <8 t/d. The emission rates of SO₂ are significantly smaller than those during the dome-building eruptions of the 1980's, which frequently emitted 500-1000 t/d of SO₂. However, the mean CO₂/SO₂ of 11 (molar) for the present emissions is similar to that of 9 for the 1980-81 emissions. Despite the lower emission rates, the SO₂ emission rates and S content of melt inclusions suggest that the source magma of the eruption is nevertheless gas-saturated at depth. As of July 2005, gas data have provided no evidence of recharge of new magma to reservoirs at depth.