COMBINING GEOMORPHOLOGY AND GEOCHEMISTRY TO EXPLAIN HOT SPRING COMPOSITIONS IN YELLOWSTONE

Integration of geomorphological analysis with geochemical timeseries implies that observed variability of hot spring compositions reflects interactions between shallow meteoric-derived waters and the deep, subsurface hydrothermal system. This study focuses on a portion of the Yellowstone hydrothermal system south of Mud Volcano that we call the Greater Obsidian Pool Area. Geochemical compositions from a twenty-year dataset are used to distinguish deeply sourced fluids from shallowly sourced, precipitation-derived waters and to quantify the extent of mixing between the two sources. Chloride concentration is used to infer the extent of subsurface water-rock interaction and subsequent mixing with shallow meteoric fluids. The local topography drives groundwater flow towards the hot springs from the southeast. Spatial variations in hot spring geochemistry map to a mixing gradient throughout the region from hot springs dominated by deeply sourced fluids to hot springs dominated by shallow, meteoric fluids. Temporal variations between deeply sourced and shallowly sourced hot springs reveal differing responses to changing yearly surface conditions. The geomorphology of the surface environment of the Greater Obsidian Pool Area helps to explain the driving forces for fluid mixing. LiDAR imagery of the hot spring elevations reveals that the deeply sourced hot springs are above the regional water table, while some of the meteorically influenced hot springs lie closer to the water table indicated by a local lake. This suggests that mixing of fluid sources is influenced by hydrologic barriers and permeability differences in the subsurface. Specifically, the hot spring compositional gradient can be explained as a response to the shielding of the northwestern hot springs by a hydrothermally altered package of glacial debris, while hot springs to the southeast are diluted by shallow meteoric water via groundwater infiltration and, probably, occasional overland flow.