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William C Evans

Emeritus
Email: wcevans@usgs.gov
Office Phone: 650-329-4514
ORCID: 0000-0001-5942-3102
Geochemistry of Hydrothermal Systems
Reconnaissance and chemical and isotope sampling of thermal springs in the western United States has not generally provided information of sufficient detail to permit the geothermal potential of most individual areas to be determined with any certainty. This is especially true in the Cascade Mountain Range, where the chemical geothermometers indicate much lower temperatures of water-rock equilibrium than the sulfate-isotope geothermometer and the geologic setting seem to require. This discrepancy could be due to simple mixing of thermal and fresh water or rapid equilibration of water with the surrounding country rock as the fluids rise to the surface; alternatively, the sulfate-isotopic composition could be an artifact...
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Mammoth Mountain is a dacitic dome complex located on the southwestern rim of Long Valley Caldera, California. Mammoth Mountain has exhibited unrest over the past ~30 years, characterized by seismicity over a broad range of depths, elevated 3He/4He ratios in fumarolic gas and large-scale diffuse CO2 emissions. Monitoring of this unrest has included collection of fumarole gas samples for geochemical analysis and tree cores for radiocarbon analysis of annual growth rings. This report updates the long-term geochemical record at Mammoth Mountain, compiling the chemical and isotopic (d13C-CO2, 3He/4He) compositions of 59 gas samples collected from Mammoth Mountain fumarole from 1998 to 2016. In addition, we report radiocarbon...
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Gas chemical and isotopic compositions are reported for samples collected from 9 volcanic and geothermal areas in California: Medicine Lake Volcano, Mount Shasta, Lassen Volcanic Center, Clear Lake Volcanic Field, Mono Lake Volcanic Field, Long Valley Caldera, Mammoth Mountain, Coso Volcanic Field, and Salton Sea Geothermal Field. Beginning in 2014, gas samples were collected as part of the USGS California Volcano Observatory's newly established gas geochemical monitoring program for moderate to very-high-threat volcanoes. Data on gas samples collected prior to 2014 by this and previously published studies are also reported to provide longer-term records for some areas. Gas samples in this study were analyzed for...
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