Hydrothermal Activity in the Southwest Yellowstone Plateau Volcanic Field

In the past two decades, the U.S. Geological Survey and the National Park Service have studied hydrothermal activity across the Yellowstone Plateau Volcanic Field (YPVF) to improve the understanding of the magmatic‐hydrothermal system and to provide a baseline for detecting future anomalous activity. In 2017 and 2018 we sampled water and gas over a large area in the southwest YPVF and used Landsat 8 thermal infrared data to estimate radiative heat flow. Most of the thermal activity in this region is in close proximity to the Yellowstone Caldera boundary. Springs and fumaroles discharge from a variety of lithologies, including some of the youngest rhyolites in the YPVF. Gas compositions and helium isotope ratios of most samples resemble those in other parts of the YPVF. The waters have meteoric origins, and tritium was detected in several samples. Thermal waters from some areas have compositions that plot along a line connecting thermal and nonthermal water endmember compositions. The thermal water endmember equilibrated at 160°C–170°C, lower than waters in Yellowstone's geyser basins. Heat discharged by springs and fumaroles originates from within the Yellowstone Caldera and is transported laterally by advection, mainly along the base of rhyolite flows that cover the inferred caldera boundaries. Plain Language Summary The Yellowstone Plateau Volcanic Field has had three cataclysmic volcanic eruptions over the past ~2.1 million years. The youngest of these eruptions at 0.631 Ma resulted in the formation of the Yellowstone Caldera and was followed by several smaller eruptive episodes of large volume rhyolite flows up to ~70,000 years ago. The diversity of Yellowstone's hydrothermal system is unparalleled with more than 10,000 thermal features that have diverse chemical compositions. To improve the understanding of Yellowstone's magmatic‐hydrothermal system and to provide a baseline for detecting future anomalous activity, we sampled water and gas in southwest Yellowstone and analyzed satellite‐derived thermal infrared images. Most of the hot springs and fumaroles in the study area are near the Yellowstone Caldera boundary. The area is characterized by different rock types including some of the youngest rhyolites in Yellowstone. Whereas gas compositions mostly resemble those in other parts of Yellowstone, the chemical compositions of waters differ from those within the Yellowstone Caldera. The heat and gas discharged by springs and fumaroles originate from wi