HYDROTHERMAL SYSTEM RESPONSE TO THE 2004-2008 ERUPTION AT MOUNT ST. HELENS, WA

Low SO₂ emissions in the Mount St. Helens gas plume during the 2004–08 eruption argue against injection of a new, volatile-rich magma. Although volatile scrubbing by groundwater can reduce plume emissions, scrubbing typically declines with time, and SO₂ emissions remained low throughout the eruption. Water and gas collected from hot springs in The Breach allow us to detect magmatic volatiles in the hydrothermal system, assess changes related to scrubbing, and aid in identification of new magmatic components.

Chemical trends in the hot springs since the 1980s suggest declining volatile inputs, increasingly effective scrubbing, and dilution by meteoric water. Early water isotope data showed evidence of water-rock interaction, with a distinct shift to isotope compositions with higher values after 1985. By 1994 it appears that trend reversed, and by 2002 hot spring isotopes plotted close to the world meteoric water line. Bulk chemical trends before 2002 showed increasing Cl to SO₄ ratios and declining Cl and SO₄ concentrations. Between 1994 and 2002 average Cl and SO₄ in the hot springs decreased by about 40 and 20 percent, respectively. These declines were accompanied by a >200% increase in HCO₃ concentrations.

Changes in chemistry associated with the 2004–08 eruption are small. In 2007 the temperature at one spring was 81°C, the highest value recorded since 1987. Temperatures remained high in 2008 but were ≤ 72°C. Minor increases in HCO₃ concentrations occurred in 2007-08, but changes in Cl and SO₄ concentrations are small and unsystematic and do not indicate an overall increase in volatile content. These data, are consistent with gas emissions data and petrologic findings, suggesting that any intrusion associated with the 2004-08 eruption was gas-poor.

Isotope data provide the strongest indication of ongoing changes in volatile contents. Magmatic CO₂ had a δ¹³C value around -10.5‰ in the 1980s, shifting to values around -12‰ by 1994. δ¹³C-values of dissolved inorganic carbon in spring waters collected during 2002-06 ranged between -12.3 to -11.9‰, indicative of a degassed magma source. However, five hot spring water samples collected since 2007 have δ¹³C values between -10.9 and -10.6‰, possibly indicating degassing of fresh magma. These findings correspond to similarly increasing δD and δ¹⁸O values.