MONITORING THE HYDROTHERMAL SYSTEM AT THE LASSEN VOLCANIC CENTER, CALIFORNIA

This work reports on new monitoring efforts to trace short-term changes within the hydrothermal system of the Lassen Volcanic Center (LVC), located at the southern end of the Cascades Volcanic Arc in northern California. The original main edifice of the LVC was Mount Brokeoff (Mt. Tehama), which has been glaciated as well as hydrothermally altered. Our work is founded on the idea that as magmatic activity of the system changes, the temperature of the hydrothermal systems will also vary, along with CO2 concentration emitted by the magmatic system. Two sites have been targeted at LVC: Sulfur Works in the central part of the complex, and Boiling Springs Lake southeast of the main edifice. Campbell Scientific data loggers are used for continuous water temperature measurements. Water temperatures have been measured at both sites since 2007. Sulfur Works temperatures are typically around 75-90 C with decreased water temperatures during spring and early summer, consistent with snow melt events and is generally lowest in mid-June (76 C). Boiling Springs Lake temperatures are generally between 60 C and 70 C with similar temperature decreases to 40-50 C during the spring. ¹⁸O/¹⁶O of water samples collected from Sulfur Works range from -3.61 to -5.26. Lake water at Boiling Springs Lake ranges only from 1.35 to 1.38 and a single measurement of fumarole water is -6.86. These values are consistent with previous results of Sorey and Ingebritsen (1984) who report that ¹⁸O/¹⁶O becomes lighter further from the volcanic center, suggesting that water from Sulfur Works is more closely connected to the magmatic system than is Boiling Springs Lake. CO2 gas emissions are also monitored to characterize the bubbling fumaroles at Sulfur Works and lake water at Boiling Springs Lake. CO₂ gas samples are collected using 300ml evacuated Giggenbach bottles with 50ml of 4M NaOH. Sulfur Works gas concentrations are 8,150 ppm of CO₂ and Boiling Springs Lake contains 3,475 ppm of CO₂. Each of these sites varies independently from each other in water temperature and composition.