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: Marie Edmonds, Kenneth A. McGee, and Michael P. Doukas
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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.
Suggested Citation
Edmonds, M., McGee, K.A., and Doukas, M.P., 2008, Chlorine degassing during the lava dome-building eruption of Mount St. Helens, 2004-2005: U.S. Geological Survey Professional Paper 1750-27, 17 p., https://doi.org/10.3133/pp175027.
ISSN: 2330-7102 (online)
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 |