QUIESCENT DEGASSING OF MT. BAKER VOLCANO, WASHINGTON, USA

Mt. Baker has been quiescently degassing since the most recent phreatomagmatic eruptions in 1843, 1853, and 1858-9, and during subsequent decades of intermittent phreatic activity.  While historical records reference variable amounts of gas discharge following the 1800's activity, the gas chemistry and emission rates were only first quantified during a period of heightened heat-flow and unrest that began in 1975-6.  Gas chemistry and emission rate measurements since this time suggest continued degassing of magma beneath the volcano.  

Airborne emission rate measurements have been completed four times since 1975.  Between 1998 and 2000 the emission rates decreased from 273 to 187 t d^-1 of CO₂ and from 12 to 5.5 t d^-1 of H₂S. These rates are similar to baseline emissions measured at White Island Volcano in New Zealand, or Mount St. Helens during the present dome-building eruption, and are within the short-term variability of baseline emissions at these volcanoes.  However, considering the emission rates and gas chemistry first measured in 1975-early 1980s (~ 112 t d^-1 H₂S, and CO₂/H₂S=18-22), it can be argued that an annual decrease of ~76 t d^-1 CO₂ and 4.2 t d^-1 H₂S has occurred at Mt. Baker.  This trend implies that emissions should have ceased sometime between 2002 and 2003.  The most recent measurement in 2001 did not detect a plume, suggesting that either the wind speed was too high causing the plume to hug the ground surface and thus be unmeasureable from the air, or that the rate of gas emitted by fumaroles at the summit had fallen below the detection limit of this measurement technique. 

All fumarole gas chemistry samples, including the most recently collected in August, 2006, show that no SO₂ is emitted from Mt. Baker.  This implies that no dry pathways exist, and that original S contents are masked by scrubbing, which converts SO₂ to H₂S and H₂SO₄.   Gas analyses suggest that the C-isotopic signatures are distinctly magmatic, and that the gas chemistry hasn't changed significantly over time.  Finally, gas-geothermometer temperatures suggest that the gases were in equilibrium at 220-240^oC, which is typical of hydrothermal reservoirs at volcanic centers.