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

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.