Chemical and isotopic investigation of the new hydrothermal system at Mount Saint Helens, Washington
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The behavior of young (< 10 yrs.) hydrothermal systems is largely unknown and studies at Mt. St. Helens following the 1980 eruptions provide insight into processes affecting the development and stabilization of new hydrothermal systems. The aim of this study is to characterize the developing geothermal system as it stabilizes and matures, determine which processes affect the characteristics of the young system, identify differences in the behavior and processes at Mt. St. Helens when compared to older, established systems, and demonstrate that magmatic volatiles enter the new hydrothermal .system. Unlike older, established geothermal systems, all fluid flow is shallow (<>0m) and driven by post-eruption recharge. Thermal areas occur on and north of the volcano and these areas are not currently related through a lateral How system. Recharge to the geothermal area on the volcano is in the crater, with fluids discharging toward the north. Fluids on the volcano obtain the majority of their heat from the magma conduit, and geothermal waters contain up to LS% magmatic water which contributes up to 70% of the Cl found in the waters. Air and meteoric water have been circulating through the intracrater dome complex since 1980. Samples of fumarole steam condensates collected in this study have been variably mixed with meteoric water and have contained 50 to 70% magmatic water from 1986 to 1988. The compositions and temperatures of geothermal waters have been changing with time and with increasing distances from the dome. The system as a whole has cooled with time and its TDS has been decreasing with time. Although the geothermal waters do not appear to have attained equilibrium with host rocks, they have begun to stabilize.
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