OXYGEN ISOTOPE VARIATIONS AT SULPHUR WORKS AND BOILING SPRINGS LAKE 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, boiling pools and lakes. Two areas, Sulphur Works (SW) and Boiling Springs Lake (BSL) have high temperature and low pH (85-92°C, <3.6 pH) and are part of two separate hydrothermal cells that circulate within the LVC (Janik & McLaren, 2009). This work characterizes the proportions of magmatic and meteoric inputs to the hydrothermal system, using oxygen isotopes at SW and BSL. There is no seasonal variation suggested for LVC mud pots when dry periods should have a stronger magmatic component (high δ¹⁸O values) compared to spring snow melt months when a stronger influence of meteoric inputs will have lower δ¹⁸O values. In contrast, at SW in fall 2017 (dry), δ¹⁸O is lower (-1.42‰) than during spring runoff in 2018 (δ¹⁸O= -0.88‰). Similarly, no correlation is observed for BSL mud pots, which also have lower δ¹⁸O during the dry period of fall 2017 (δ¹⁸O= -3.97‰) than during the wetter spring runoff of 2018 (δ¹⁸O= 1.96‰). Additionally, δ¹⁸O variations do not correlate with drought conditions. During drought years for which we have oxygen isotopes at SW (in 2000, 2008, 2018 when maximum snow depth < 81.2 in), δ¹⁸O ranges from -0.88‰ to -4.29‰, which overlaps with 2017 (a non-drought year with maximum snow depth = 131.5 in) δ¹⁸O of -1.46 to -3.42‰. This suggests that decreased meteoric water inputs during drought years has not changed the composition of the hydrothermal fluids within the timescale of the snowpack year. Instead, it appears that either, magmatic contributions to the hydrothermal system overprint variations in meteoric water inputs, or that the δ¹⁸O is not impacted within the same water year. Given that the timescale for which we have sampled is less than two decades, it may be that the drought periods occur on longer timescales that we have not yet detected. These data raise questions regarding recharge and residence rates of fluids in the Lassen hydrothermal system.