HYDROTHERMAL GAS COLLECTION AND CO2 ANALYSIS AT LASSEN VOLCANIC CENTER, CALIFORNIA, OVER A TEN-YEAR PERIOD

The Lassen Volcanic Center (LVC), located at the southernmost end of the Cascade Volcanic Arc in Northern California, hosts a well-developed hydrothermal system. Hydrothermal activity releases gases through mud pots, fumaroles, and hot springs as subsurface water is heated by a magmatic source and rises to the surface. The heat source of the hydrothermal system is from ancient Mt. Tehama and includes eleven hydrothermal areas, two of which are Sulphur Works (SW), situated directly above the magmatic core, and Boiling Springs Lake (BSL) located eleven km to the east. Carbon dioxide generally forms up to 99% of hydrothermal gases discharged and is a significant constituent of gases routinely analyzed (Giggenbach, 1988). CO₂ measurements help characterize geothermal systems and are used to determine temporal and spatial variations. Previous gas, steam, and water sampling were conducted by the USGS from 1975 to 2002 (Janik and Bergfeld, 2010) and 2008-2009 (Mendes et al., 2009). Gas samples from SW and BSL have CO₂ of 97.9 mg and 41.7 mg, respectively (Mendes et al., 2009). Higher CO₂ at SW than BSL is consistent with SW location above the core of ancient Mt. Tehama, which is closer to the magmatic source. Recent analyses in 2019 indicate CO₂ at BSL is 51.61 mg, which is greater than measurements in 2009. Increased CO₂ may be related to lower meteoric water input associated with the recent drought in California, which increases the relative proportion of the magmatic contribution of hydrothermal CO₂. This idea can be tested with additional sampling, including at SW.