GEOCHEMICAL RECONNAISSANCE OF THE WEST ASTRINGENT CREEK THERMAL AREA YELLOWSTONE CALDERA WY

The West Astringent Creek Thermal Area (WACTA) is near the northeastern margin of the Sour Creek Dome (SCD) in the Yellowstone caldera, and has a thermally emissive area of 905,088m². Goals are to evaluate the WACTA geochemistry and heat flow, and place them within the context of the overall Yellowstone hydrothermal system. 15 samples were collected from the WACTA hot springs. The springs’ pHs were measured during sampling in the field. O and H isotope ratios and cation concentrations (reported elsewhere) were determined for the fluids in the laboratory. A Rayleigh boiling experiment was conducted in conjunction with the geochemical analyses. Most of the geochemical data from the WACTA fluids are similar to those from other acid-sulfate hot springs within the Yellowstone hydrothermal system. The range of WACTA spring pH (1.95 to 5.68) suggests that the fluids are dominantly acid-sulfate. The δ¹⁸O and δD values range from -16.65 to –8.75‰ and -145.51 to -98.38‰, respectively (‰ VSMOW). When plotted on a standard δ¹⁸O vs. δD plot, their slope is 3.79, which is close to the characteristic trend for other acid sulfate hot springs within the Yellowstone hydrothermal system, which usually have a slope of about 3. Two of the WACTA samples were collected from a thermal lake. The two thermal lake samples may be responsible for the deviation from the characteristic acid sulfate slope. The thermal lake measurements increase the slope of the δ¹⁸O-δD trend because the characteristic slope of large thermal lakes within Yellowstone is approximately five. D-excess calculations for these data indicate that a large portion of the hot spring waters is from deep subsurface thermal water. Faults and fractures surrounding the WACTA likely control flow distribution of recharge to the hot springs. The boiling experiment showed that the δ¹⁸O-δD trend of a reservoir vapor phase has a slope of 6.3 during the entire volumetric flux of water in a closed system. Rayleigh analysis of δD and δ¹⁸O values from hot springs were compared to the laboratory boiling experiment as a means to determine geothermal reservoir volumes and provide surficial analysis techniques for alternative energy resource studies.