The Long Valley Caldera in eastern California formed about 760,000 years ago following the eruption of ~600 km3 of high-silica rhyolite. The Long Valley volcanic-hydrothermal system contains sufficient heat to support the Casa Diablo binary geothermal power plant (CD, Fig. 1) sited on the margin of the resurgent dome. Present day volcanic activity is evidenced by periods of seismicity and deformation and the presence of magmatic volatiles in thermal fluids. The U.S. Geological Survey (USGS) uses a variety of techniques to monitor the volcanic system that underlies the Long Valley Caldera. Monitoring data include measurements of seismicity and deformation and chemical analyses of thermal waters and gases that are emitted at hot springs, fumaroles and other degassing features. The geophysical measurements allow monitoring of fluid movement in the crust while analyses of gases and waters provide a means to identify the presence of volatiles (carbon, helium, sulfur, and chloride) scavenged from the magmatic system. By establishing a record of baseline fluid compositions, we have a way to assess future changes in the underlying magmatic system.
This report presents compositional data on bulk gases and isotope values of carbon dioxide and helium for samples across Long Valley Caldera collected between February 1994 and October 2020 (Fig. 1). Many of the samples are from designated monitoring sites where gas was routinely collected over many years. Some data are from locations that were sampled infrequently and are included here for completeness. Most of the analyses were performed at USGS laboratories in Menlo Park, California, Denver, Colorado, and Reston, Virginia. Some helium isotope analyses were performed at Lawrence Berkeley National Laboratory, Berkeley, California.