HR: 1340h
AN: V53D-1587    [Abstracts]
TI: The composition of volcanic gas from the 2004-5 Mount St. Helens lava dome, measured using Open Path Fourier Transform infra red Spectroscopy (OP FTS)
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: Edmonds, M
EM: medmonds@usgs.gov
AF: U.S. Geological Survey, Hawaiian Volcano Observatory 51 Crater Rim Drive, Hawaii National Park, HI 96718 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: OP FTS measurements were undertaken at Mount St Helens, Washington State, on 31 August 2005. The lava dome was used as a source of infra red radiation, which was received by the spectrometer on the east crater rim, after having passed through the volcanic gas cloud issuing from the lava dome. The gases absorbed characteristic wavelengths of radiation in the IR, allowing their concentration-pathlengths to be constrained by comparing the acquired absorbance spectra with reference spectra. The results show that significant quantities of hydrogen chloride (HCl), hydrogen fluoride (HF), carbon monoxide (CO) and carbonyl sulfide (OCS) are degassing from the St Helens lava dome, as well as sulfur dioxide (SO₂), carbon dioxide (CO₂) and hydrogen sulfide (H₂S), which are routinely monitored using other methods. The HCl/SO₂ mass ratio was 1.0-1.3, the HCl/HF and HCl/CO ratios were approximately 10 and the HCl/OCS ratio approximately 13-15. An SO₂ emission rate of 22 tonnes per day was measured synchronously using a helicopter-mounted UV spectrometer downwind of the volcano, which allows estimation of the emission rates of these gas species to be made. HCl emission rates are similar to SO₂ emission rates during this stage of the eruption and were approximately 22-29 t/d on 31 August 2005. The composition of the volcanic gas phase is compared to those collected at silicic volcanoes elsewhere and from St Helens during the 1980's lava dome building eruptions. The high relative proportion of HCl in the 1980's gas samples and in the 2005 OP FTS measurements is noteworthy; HCl is the most abundant gas species after H₂O and CO₂. The emission rate of halogen species can be sustained by degassing of rhyolite melt at low pressures (near or at the surface).
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