HR: 1340h
AN: V53D-1605    [Abstracts]
TI: Pre and Post-Eruptive Geochemical Investigations at Mount St. Helens, WA
AU: * Bergfeld, D
EM: dbergfel@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 434, Menlo Park, CA 94025 United States
AU: Evans, W C
EM: wcevans@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 434, Menlo Park, CA 94025 United States
AU: McGee, K A
EM: kenmcgee@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Court Building 10 Suite 100, Vancouver, WA 98683-9589 United States
AU: Spicer, K R
EM: krspicer@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Court Building 10 Suite 100, Vancouver, WA 98683-9589 United States
AB: Following the September 2004 onset of volcanic activity at Mount St. Helens, geochemical monitoring of spring-fed streams that drain the crater was initiated in mid-October and continues to date. Samples of the thermal source springs from the breach zone were collected on July 26, 2005. Prior to the unrest, we last collected samples from the springs in Loowit Canyon (LC) in Aug. 2002. Since 2002 the morphology of the canyon walls has changed and the largest volume spring, which was located in the upper part of the canyon, has almost disappeared. During the July survey, the largest spring flowed from a landslide on the west wall of the canyon farther downstream. Water temperatures in LC in 2005 were between 57 and 62°C and were comparable to spring temperatures in 2002. Gas emissions, which were first observed in LC spring waters in 2002, were also observed in both Loowit and Step Canyons in 2005. Preliminary geochemical results show only minor changes in the spring water chemistry since 2002. Differences in LC spring water chemistry include a 12% increase in SO₄ concentrations and concomitant decreases in Br and Cl. Cl to Br ratios however have increased to about 670, roughly 17% higher than in 2002. Prior to this, the ratios in the 2002 spring waters were very similar to values from waters collected since the mid 1980s. δ ¹³C-DIC values from all of the thermal springs and a slightly thermal spring on the Pumice Plain range between -10.9 to -12.1 permil, near the values obtained from LC in past years, and are at least 3.5 permil greater than values from regional cold springs, which seem to be dominated by biogenic carbon. The minimal changes in LC spring water chemistry and the lack of a magmatic carbon signal in cold springs outside the beach zone suggest that the most recent eruptive episode drove only a small pulse of magmatic gas into the groundwater system. This may reflect open-conduit conditions and unrestricted gas escape within the crater area since the 1980 eruption.
DE: 8419 Volcano monitoring (7280)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005