OXYGEN ISOTOPE COMPOSITION OF SULPHUR WORKS AND BOILING SPRINGS LAKE, LASSEN VOLCANIC CENTER: CORRELATION BETWEEN METEORIC WATER INPUT AND δ18O

The Lassen Volcanic Center (LVC) is located at the southern end of the Cascades Volcanic Arc in Northern California and has the largest and most active hydrothermal system in the Cascades Range. The hydrothermal system of the LVC consists of hot springs, fumaroles, mud pots, acidic creeks, and boiling pools and lakes. Two hydrothermal areas are Boiling Springs Lake (BSL) and Sulphur Works (SW), both have a high temperature and low pH (85-92°C, < 3.6 pH) but are part of two separate hydrothermal cells that circulate within the LVC (Janik & McLaren, 2009). Previous work investigated the proportions of magmatic and meteoric water inputs into the hydrothermal systems and hypothesized that water samples collected from dry years or seasons should have a stronger magmatic component (high δ¹⁸O) in comparison to wetter years and during snow melt seasons, which should have a more influential meteoric component (low δ¹⁸O). This trend was not seen in the seasonal δ¹⁸O data collected in 2017 and 2018 from SW and BSL. At SW in fall 2017 (dry period), the δ¹⁸O values were lower (δ¹⁸O = -1.42‰) than during the spring runoff of 2018 (δ¹⁸O = -0.88‰). Similarly, no correlation was observed at BSL, which also had lower δ¹⁸O values during the fall of 2017 (δ¹⁸O= -3.97‰) and higher δ¹⁸O values during the spring of 2018 (δ¹⁸O= 1.96‰). Thus, there is no correlation between seasonal precipitation variations and δ¹⁸O data. Additionally, no correlation was found between δ¹⁸O data and annual precipitation in drought years and non-drought years. During drought years for which there is oxygen isotope data at SW (in 2000, 2008, 2018 when maximum snow depth < 81.2 in) the δ¹⁸O data ranged from -0.88‰ to -4.29‰. Similar results were found for δ¹⁸O data in 2017 (a non-drought year, max snow fall 131.5 inches), which ranged from -1.46‰ to -3.42‰. This suggests that the variation in meteoric water inputs do not significantly affect the composition of SW and BSL within the same year. A recharge rate can be calculated using past precipitation data and oxygen isotope data to correlate past annual precipitation data with current δ¹⁸O data.