Kenneth A. McGee Michael P. Doukas David R. Sherrod William E. Scott Peter H. Stauffer Marie Edmonds 2008 Remote measurements of volcanic gases from the Mount St. Helens lava dome were carried out using OpenPath Fourier-Transform Infrared spectroscopy on August 31, 2005. Measurements were performed at a site ~1 km from the lava dome, which was used as a source of IR radiation. On average, during the period of measurement, the volcanic gas contained 99 mol percent H₂ O, 0.78 percent CO₂ , 0.095 percent HCl, 0.085 percent SO₂ , 0.027 percent HF, 4.8×10^-4 percent CO, and 2.5×10^-4 percent COS close to the active vent. The fluxes of these species, constrained by synchronous measurements of SO₂ flux, were 7,200 t/d H₂ O, 140 t/d CO₂ , 22 t/d SO₂ , 14 t/d HCl, 2.0 t/d HF, 54 kg/d CO, and 59 kg/d COS, ±20 percent. Observations of H₂ O/Cl in the vapor and melt are compared to models of closed- and open-system degassing and to models where a closed system dominates to depths as shallow as ~1 km, and gases are then allowed to escape through a permeable bubble network. Although several features are consistent with this model--for example, (1) H₂ O/Cl in the gases emitted from stagnant parts of the lava dome, (2) the concentration of Cl in the matrix glass of erupted dacite, and (3) the glass H₂ O/Cl--the gases emitted from the active part of the lava dome have much higher H₂ O/ Cl than expected. These higher H₂ O/Cl levels result from a combination of two factors (1) the addition of substantial amounts of ground water or glacier-derived H₂ O to the gases at shallow depths, such that only ~10 mol percent of the measured H₂ O is magmatic, and (or) (2) some Cl present as alkali chloride (NaCl and KCl) in the gas phase. The mean molar Cl/S is similar to gases measured at other silicic subductionzone volcanoes during effusive activity; this may be due to the influence of Cl in the vapor on S solubility in the melt, which produces a solubility maximum for S at vapor Cl/S ~1. application/pdf 10.3133/pp175027 en U.S. Geological Survey Chlorine degassing during the lava dome-building eruption of Mount St. Helens, 2004-2005 reports