USING LONG-TERM TEMPORAL CHANGES IN GEOCHEMICAL DATA TO PROBE THE PLUMBING OF A HYDROTHERMAL SYSTEM AT YELLOWSTONE

A twenty-year data set of geochemical measurements on samples from Yellowstone hot springs provides an opportunity to examine temporal changes, and to develop hypotheses about geologic drivers of continental hydrothermal systems. This study focuses on a dynamic portion of the Yellowstone system south of Mud Volcano that we call the Greater Obsidian Pool Area. Geochemical data are used to distinguish deeply circulating hydrothermal fluids from precipitation-derived shallow waters, and to quantify compositional changes during mixing of these solutions. Temporal variations allow us to differentiate the responses of deeply sourced hot springs from those more shallowly sourced. Examining the spatial and temporal distribution of the extent of mixing documented from surface hot springs allows us to produce a conceptual model of the underground workings of the hydrothermal system. Comparing concentrations of sulfate and chloride in Yellowstone hot spring systems helps constrain deep hydrothermal fluid compositions. Sulfate concentration is used as an indicator of volcanic gas input and chloride concentration is used to infer extents of subsurface water rock interaction, decompressional boiling, and mixing with meteoric fluids. Stable hydrogen and oxygen isotopes of water also track the origins and transport of water to the surface. Water-rock interaction preferentially enriches oxygen isotopes while negligibly fractionating hydrogen isotopes. Evaporation and boiling fractionate both isotopes at known ratios. Total boron and chloride concentrations were used to evaluate dilution effects on deep hydrothermal fluids. Hot spring silica concentrations are used via geothermometry to estimate maximum subsurface fluid temperatures. Combining these geochemical methods reveals hydrothermal waters in the Greater Obsidian Pool Area have varying proportions of surface to deeply derived input, permitting a subsurface model of permeability and fluid flow.