The helium and carbon isotope systematics of a continental geothermal system: results from monitoring studies at Long Valley caldera (California, U.S.A.)

This study reports fumarole and hot spring gas chemistry of a 3-year monitoring programme (1986–1988) at the seismically-active Long Valley caldera (LVC) in the Sierra Nevada, eastern California. The focus is on helium and carbon dioxide (isotopes and concentrations) and their variation in response to seismic activity in the region. Within the caldera, both species are predominantly magmatic in origin but their isotopic and elemental characteristics appear to be established prior to shallow-level intrusion and/or are influenced by pre-eruptive degassing. In response to intra-caldera and regional seismicity over the monitoring period, CO2 and helium show markedly different behaviour: the greatest change in various carbon-related parameters (CO2%; δ13C(CO2; CO23He) occurred in 1986 and were most likely related to regional seismicity in the nearby Chalfont Valley. Helium did not respond to these events. The largest change (up to ∼ 25%) in 3He4He ratios was seen in 1987 with the occurrence of both increases and decreases relative to the almost constant values observed in 1988. The increases are consistent with magma intrusion occurring within the caldera in 1987 whilst the decreases occurred significantly later (> 6 months) than any seismic activity. It is suggested that decreases in 3He4He are related to the regional seismicity and that the hydrothermal system exerts a (temporal) control on the release of near-surface 4He. At LVC this is related to the timing of the late spring thaw. Results from previous monitoring programmes (when the level of seismicity in the caldera was higher) are evaluated against variations in the present work. There appears to be a convincing link between higher 3He4He values and the level of seismicity in the caldera although factors related to location, magnitude and frequency of seismic events are difficult to quantify. Because of the inferred small isotopic contrast between new magma and presently-degassing magma at LVC, it is anticipated that large variations in 3He4He within the central caldera are unlikely to occur until the magmatic volatile signal wanes as a function of degassing and time. Alternatively, only at those localities situated at significant distances from the region of magma intrusion (i.e. away from the resurgent dome vicinity) are helium and/or carbon likely to respond dramatically to intra-caldera seismicity.