Helium Isotope Systematics of Long Valley Caldera, California

Authors: Gene A. SUEMNICHT, B.M. KENNEDY and William C. EVANS
Keywords: Long Valley caldera, geochemistry, helium isotopes,hydrothermal system
Conference: World Geothermal Congress Session: Geochemistry
Year: 2015 Language: English
Abstract: Long Valley Caldera supports an active hydrothermal system that has varied through time. Age dates indicate that mineralization occurred in two separate phases related to an intense hydrothermal system in the central caldera 300,000 to 130,000 years ago and the current hydrothermal system that has probably been active in the western part of the caldera for only the last 40,000 years. Prominent surface manifestations occur in many of the older system’s established outflow zones at comparatively low elevations in the south-central portion of the caldera. As much as 80% of the current hydrothermal outflow occurs at Hot Creek on the southeastern edge of the Resurgent Dome. Geochemical estimates of source reservoir temperatures range from 200°C – 280°C. The highest temperatures of 218°C were encountered in a well in the western caldera moat. Geothermal plants at Casa Diablo currently generate 40 MWe (gross) from moderate temperature (170°C) shallow (200m) hydrothermal outflow injecting spent fluid into deeper (750m) permeable zones in the underlying Bishop Tuff. Elevated 3He/4He ratios demonstrate magmatic input to the Long Valley hydrothermal fluids. The most significant change in surface manifestations was a dramatic increase in 3He/4He in a fumarole on the north side of Mammoth Mountain four months after the onset of seismic swarm activity beneath the mountain in 1989. There are no volcanic-type fumaroles with temperatures far above the boiling point or with high-temperature magmatic volatiles (CO, SO2, halogen gases etc.). Areas of dead vegetation developed around Mammoth Mountain about the same time as a period of potential dike intrusion; however, other areas of distressed vegetation kill across the resurgent dome have developed in response to changes in the shallow geothermal system rather than new input of magma or magmatic fluids supposedly centered beneath the resurgent dome. The Long Valley hydrothermal system is heated by magmatic sources and, based on strontium isotopic data, is most likely hosted in fractured metamorphic rocks that are part of Sierran roof pendants that comprise the western basement of the caldera and the roof of the Long Valley magma chamber. The shallow hydrothermal system that supplies geothermal production and surface manifestations is more than 7 km from the apparent source region of the hydrothermal system and significantly removed from eruptive centers in the western caldera moat that represent the largest volume of magmatic input into the caldera over the last 200,000 years. Sharp temperature increases, fluid chemistry changes, or particularly dramatic increases in 3He/4He ratios that might be related to potential intrusive activity have not been detected in more than two decades of sampling of shallow hydrothermal out flow in hot springs, fumaroles, and shallow geothermal production wells during a long period of caldera unrest. Helium isotope compositions have remained relatively stable within the shallow hydrothermal system and the highest 3He/4He levels are detected in relatively low temperature outflow springs in the far eastern part of the caldera. We suggest that the major input of magmatic volatiles is related to the current deep active geothermal system in the western caldera. Intense alteration and circulation within the deep permeable western metamorphic basement complex adds significant 4He to the convecting geothermal fluids and sets a relatively constant 3He/4He level for more distant outflow in the shallow hydrothermal system at lower elevations to the east. Impermeable metamorphic rocks
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