THE BORON ISOTOPE SYSTEMATICS OF THE YELLOWSTONE-NATIONAL-PARK (WYOMING) HYDROTHERMAL SYSTEM - A RECONNAISSANCE

Boron concentrations and isotope compositions have been measured in fourteen hot spring waters, two drill hole waters, an unaltered rhyolite flow, and hydrothermally altered rhyolite from the geothermal system in Yellowstone National Park, Wyoming. The samples are representative of the major thermal areas within the park and span the range of fluid types. For the fluids, the B concentrations range from 0.043-2.69 mM/kg, and the δ11B values range from -9.3 to +4.4%.. There is no relationship between the dissolved B concentrations or isotope compositions with the concentration of any major element (other than Cl) or physical property. Each basin is characterized by a restricted range in B/Cl ratios and δ11B values. Hot spring waters from the Morris Basin, Upper Geyser Basin, Calcite Springs, and Clearwater have δ11B values close to that of unaltered rhyolite (-5.2%.) and are interpreted to have derived their B from this source. Waters from Mammoth Hot Springs, Sheepeater, and Rainbow Springs have lower δ11B values close to -8%.. These lower values may reflect leaching of B from sedimentary rocks outside the Yellowstone caldera, but they are similar to the δ11B value of hydrothermally altered rhyolite (-9.7%.). Hence, the light boron isotope compositions recorded in these hot spring waters may reflect leaching of previously deposited hydrothermal minerals. Cooler springs along the Yellowstone River just outside the park boundary have lower B concentrations and higher δ11B values that may reflect mixing with shallow meteoric water. © 1990.