Hydrothermal Activity in Southwestern Yellowstone National Park

The Yellowstone Plateau Volcanic Field (YPVF) covers an area of ~6,500 km² at an average elevation of ~2,400 m above sea level. Since 2.1 Ma, three cataclysmic eruptions emplaced voluminous tuffs across parts of the YPVF. Eruption of the 0.631 Ma Lava Creek Tuff formed the ~80 x 50 km Yellowstone Caldera, which is filled with 600-1000 km³ of rhyolite lava flows. More than 10,000 hydrothermal features are found within the YPVF, including geysers, fumaroles, mud pots, and hot springs. In the past nearly two decades, the U.S. Geological Survey and the National Park Service have worked to characterize the water and gas chemistry for thermal features throughout the YPVF to improve our understanding of the magma-hydrothermal system and provide a baseline for detecting future anomalous activity. Campaigns in September 2017 and in July and September 2018 continued that effort by sampling over a 1,000-km² area in the southwest corner of the YPVF. There, hydrothermal activity is focused along the Yellowstone Caldera boundary in an area that includes several of the youngest flows in the YPVF, thick tuff deposits, and Paleozoic sedimentary rocks. We collected 66 water samples from thermal and non-thermal sources that were analyzed for major and trace elements. Select samples were also analyzed for oxygen and hydrogen stable isotopes, tritium and radiogenic strontium isotopes. At 13 locations gas samples were collected and analyzed for chemistry and isotopic analyses of carbon and helium. River and stream discharge data at 30 locations were obtained to determine the flux of major solutes from the hydrothermal areas. Landsat 8 satellite thermal infrared data were processed to detect hydrothermal areas and quantify the radiative heat flow. The maximum water temperature of springs was 91.7 °C, slightly below the local boiling temperature, and the maximum specific conductance was 2468 μS/cm. Water pH ranged from 6.2 to 9.0 and ⁸⁷Sr/⁸⁶Sr values ranged between 0.70985 and 0.71031. Several hot springs had detectable tritium (<3.6 TU). Gas temperatures ranged between 68.3 °C and 94.0 °C with CO₂ being the dominant gas followed by H₂S. The highest helium isotope ratio (Rc/Ra) in the area was 8.27.