MUD POT GEOCHEMISTRY AT SULFUR WORKS IN THE LASSEN VOLCANIC CENTER

The Lassen Volcanic Center (LVC) is located at the southern end of the Cascades Volcanic Arc in Northern California. The hydrothermal system of the LVC is the largest and most active in the Cascades range. Hydrothermal areas in the LVC include hot springs, fumaroles, mud pots, and boiling pools. The hydrothermal area of Sulphur Works (SW) is located at the core of ancestral Mt. Tehama. The two largest and most monitored mud pots at SW have a pH of 2.28-2.89 and 3.50-3.62, respectively and vary in temperature from 85°C-90°C. This work investigates the variation in oxygen isotopes of hydrothermal fluids in mud pots at SW from 2000-2017. Oxygen Isotopes (δ¹⁸O) are used to characterize hydrothermal fluids in terms of the proportions of magmatic and meteoric inputs. Due to their characteristic fractionation with increased temperature, magmatic fluids have heavier oxygen isotope signatures (increased δ¹⁸O) than meteoric water (decreased δ¹⁸O) which have lighter oxygen isotope signatures. From 2000-2017 LVC experienced periods of drought and non-drought. The oxygen isotope ratios during drought is represented by samples collected in 2008, and during non-drought years in samples collected in 2000, and 2017. Of our sample set, the δ¹⁸O composition of SW mud pots is lowest in 2008 (δ¹⁸O -4.29‰ to -5.12‰; Mendes et al., 2008) when peak snowpack was 113.8 in and δ¹⁸O is highest in 2017 (δ¹⁸O -1.42‰ to -3.43‰) when peak snowpack was 266.4 in. In 2000 values for the snowpack (178.4 in) and mud pot compositions (δ¹⁸O -2.5‰ to -4.2‰; Janik and Bergfeld, 2010) were intermediate between 2008 and 2017, indicating the change in hydrothermal fluid composition is not a function of time. Thus, water isotope compositions are inversely correlated with snowpack, which suggests that increased meteoric water did not change the composition of the hydrothermal fluids. Instead, it appears that magmatic contributions to the hydrothermal system overprint meteoric water inputs. Given the timescale for which we have sampled is less than two decades, it may be that the drought periods have not yet been recorded by the hydrothermal system. These data raise questions regarding recharge and residence rates of the Lassen hydrothermal system.