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Chlorine degassing during the lava dome-building eruption of Mount St. Helens, 2004-2005: Chapter 27 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006

Professional Paper 1750-27
This report is Chapter 27 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006. For more information, see: Professional Paper 1750
By: , , and 
Edited by: David R. Sherrod, William E. Scott, and Peter H. Stauffer
https://doi.org/10.3133/pp175027

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Abstract

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.

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Chlorine degassing during the lava dome-building eruption of Mount St. Helens, 2004-2005
Series title Professional Paper
Series number 1750-27
DOI 10.3133/pp175027
Year Published 2008
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Volcano Hazards Program
Description 17 p.
Larger Work Type Report
Larger Work Subtype USGS Numbered Series
Larger Work Title A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006 (Professional Paper 1750)
First page 573
Last page 589
Country United States
City Washington
Other Geospatial Mount St. Helens
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