Chlorine degassing during the lava dome-building eruption of Mount St Helens, USA, 2004-2005

Edmonds, M. and McGee, K. A. and Doukas, M. (2008) Chlorine degassing during the lava dome-building eruption of Mount St Helens, USA, 2004-2005. In: Volcano Rekindled: the first year of renewed eruption at Mount St Helens, 2004-2005. USGS Professional Papers (1750). United States Geological Survey, Colorado, pp. 573-592.

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Remote measurements of volcanic gases from the Mount St. Helens lava dome were carried out using Open-Path 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 H2O, 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 H2O, 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 H2O/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) H2O/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 H2O/Cl—the gases emitted from the active part of the lava dome have much higher H2O/Cl than expected. These higher H2O/Cl levels result from a combination of two factors (1) the addition of substantial amounts of ground water or glacier-derived H2O to the gases at shallow depths, such that only ~10 mol percent of the measured H2O 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 subduction-zone 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

Item Type: Book Section
Uncontrolled Keywords: 08AREP; IA57;
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Page Range: pp. 573-592
Depositing User: Sarah Humbert
Date Deposited: 16 Feb 2009 13:01
Last Modified: 23 Jul 2013 10:01

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