Compositional data on gases collected at fumaroles, hot springs, and gas vents across the Long Valley Caldera, California, 1994 ? 2020

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 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. 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.

加利福尼亚州东部的长谷破火山口（Long Valley Caldera）约于76万年前形成，其形成伴随有约600立方千米的高硅流纹岩（high-silica rhyolite）喷发。长谷火山-热液系统（volcanic-hydrothermal system）蕴含充足热量，可为坐落于复活穹丘（resurgent dome）边缘的卡萨迪亚诺二元地热发电厂（Casa Diablo binary geothermal power plant）提供能源支撑。当前的火山活动可通过地震活动性（seismicity）周期、地壳形变（deformation）以及热流体中岩浆挥发分（magmatic volatiles）的存在得到佐证。美国地质调查局（U.S. Geological Survey, USGS）采用多种技术手段对长谷破火山口下方的火山系统开展监测。监测数据涵盖地震活动性与地壳形变的测量结果，以及针对温泉、喷气孔及其他脱气区域（degassing features）排放的热液与气体进行的化学分析。地球物理测量（geophysical measurements）可用于监测地壳（crust）内部的流体运动，而气体与水体分析则可用于识别从岩浆系统中析出的挥发分（碳、氦、硫与氯化物）。通过建立基线流体成分（baseline fluid compositions）记录，我们能够评估下方岩浆系统未来的变化情况。本报告呈现了1994年2月至2020年10月期间采集的长谷破火山口全域样本的整体气体成分（bulk gases）数据，以及二氧化碳与氦的同位素值（isotope values）。其中多数样本来自多年来定期采集气体的指定监测站点，部分数据来自采样频率较低的点位，纳入报告以确保数据集的完整性。大部分分析工作在美国地质调查局位于加利福尼亚州门洛帕克、科罗拉多州丹佛以及弗吉尼亚州雷斯顿的实验室完成，部分氦同位素分析则由加利福尼亚州伯克利的劳伦斯伯克利国家实验室（Lawrence Berkeley National Laboratory）完成。