Emission rates of CO2, SO2, and H2S, scrubbing, and preeruption excess volatiles at Mount St. Helens, 2004-2005

Abstract

Airborne surveillance of gas emissions began at Mount St. Helens on September 27, 2004. Reconnaissance measurements-SO 2 column abundances and CO 2 , SO 2 , and H 2 S concentrations—showed neither a gas plume downwind of the volcano nor gas sources within the crater. Subsequent measurements taken during the period of unrest before the eruption began on October 1 and for several days after October 1 showed only small point sources of gas within the crater. These sources defined a pattern of scrubbed degassing that evolved from near-zero emissions, to scattered CO 2 -only sources, to growing sources of CO 2 with minor H 2 S and SO 2 , and finally to myriad sources of CO 2 with increasingly SO 2 -dominant sulfur gases. Scrubbing strongly hydrolyzed SO 2 but also affected CO 2 and H 2 S. From October 7 on, a coherent plume spilled over the crater rim, yielding emission rates for CO 2 and SO 2 , but not always for H 2 S. Virtually all SO 2 and most CO 2 outgassed from the growing dome of dacite; some CO 2 came from sources on the 1980-86 lava dome and in the Loowit springs area. The 2004-5 emission rates were notably low and variable. Emission rates for CO 2 peaked early (10/7/2004) at 2,415 metric tons per day (t/d), but the median rate was 655 t/d; only about 20 percent of the rates were greater than 1,000 t/d and about 45 percent were less than 500 t/d. Emission rates of SO 2 never exceeded 240 t/d, and the median rate was only 72 t/d; 70 percent of SO 2 emission rates were 40, and its rhyolitic melt phase contained about 4.4 wt. percent H 2 O and 37 parts per million (ppm) CO 2 . After closed-system ascent to the depth range of groundmass crystallization (~0.5-1 km), X H2O is 0.98, H 2 S/SO 2 is 4-7, and the melt contains 1.1-1.5 wt. percent H 2 O and 1-3 ppm CO 2 . Sulfur dioxide becomes dominant over H 2 S at depths less than 0.2 km. Because of excess-volatile depletion, open-system degassing involves depths shallower than about 2.5 km, weak CO 2 and SO 2 emissions prone to the effects of scrubbing, and-in contrast to early 1980 degassing-no measurable effect on the equilibrium ( 210 Pb/ 226 Ra) values of the 2004-5 dacite. The 2004-5 gas emissions are incompatible with new gas-rich magma introduced into the reservoir in the months.