THE HELIUM AND CARBON-ISOTOPE SYSTEMATICS OF A CONTINENTAL GEOTHERMALSYSTEM - RESULTS FROM MONITORING STUDIES AT LONG VALLEY CALDERA (CALIFORNIA, USA)

This study reports fumarole and hot spring gas chemistry of a 3-year m onitoring programme (1986-1988) at the seismically-active Long Valley caldera (LVC) in the Sierra Nevada, eastern California. The focus is o n helium and carbon dioxide (isotopes and concentrations) and their va riation in response to seismic activity in the region. Within the cald era, both species are predominantly magmatic in origin but their isoto pic and elemental characteristics appear to be established prior to sh allow-level intrusion and/or are influenced by pre-eruptive degassing. In response to intra-caldera and regional seismicity over the monitor ing period, CO2 and helium show markedly different behaviour: the grea test change in various carbon-related parameters (CO2%; delta(13)C(CO2 ); CO2/He-3) occurred in 1986 and were most likely related to regional seismicity in the nearby Chalfont Valley. Helium did not respond to t hese events. The largest change (up to similar to 25%) in He-3/He-4 ra tios was seen in 1987 with the occurrence of both increases and decrea ses relative to the almost constant values observed in 1988. The incre ases 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 He-3/He-4 are related to the regional seismicity and that the hydrothermal syst em exerts a (temporal) control on the release of near-surface He-4. At LVC this is related to the timing of the late spring thaw. Results fr om previous monitoring programmes (when the level of seismicity in the caldera was higher) are evaluated against variations in the present w ork. There appears to be a convincing link between higher He-3/He-4 va lues and the level of seismicity in the caldera although factors relat ed to location, magnitude and frequency of seismic events are difficul t to quantify. Because of the inferred small isotopic contrast between new magma and presently-degassing magma at LVC, it is anticipated tha t large variations in He-3/He-4 within the central caldera are unlikel y to occur until the magmatic volatile signal wanes as a function of d egassing and time. Alternatively, only at those localities situated at significant distances from the region of magma intrusion (i.e. away f rom resurgent dome vicinity) are helium and/or carbon likely to respon d dramatically to intra-caldera seismicity.