RADIUM ISOTOPE GEOCHEMISTRY OF THERMAL WATERS, YELLOWSTONE-NATIONAL-PARK, WYOMING, USA

Radium isotope activities (Ra-226, Ra-228, and Ra-224), chemical compositions, and sulfur isotope ratios in sulfate were determined for water samples from thermal areas in Yellowstone National Park, Wyoming. Activities of Ra-226 in these waters range from <0.2 to 37.9 dpm/kg. Activity ratios of Ra-228/Ra-226 range from 0.26 to 14.2, and those of Ra-224 /Ra-228 range from 0.73 to 3. 1. Radium concentrations are inversely correlated with aquifer equilibration temperatures (estimated from dissolved silica concentrations), while [ Ra/Ba]aq and Ra-228 / Ra-226 activity ratios depend upon U / Ba and Th / U ratios in aquifer rocks. Major controls on Ra concentration in Yellowstone thermal waters are inferred to be ( 1 ) barite saturation (at Norris Geyser Basin, Mammoth Hot Springs, and other northern areas) and (2) zeolite-water ion exchange (at Upper Geyser Basin). The data are consistent with a model in which ( 1 ) radium and barium are supplied to water by bulk dissolution of aquifer rock, and (2) chemical equilibration of water with rock is rapid relative to the 1602 year half-life of Ra-226. The Ra-228 / Ra-226 activity ratios of the waters may in some cases reflect surface enrichments of Th-232 and/or may indicate that alpha-recoil input of Ra-228 is rapid relative to water-rock chemical equilibration. Activity ratios of Ra-224 /Ra-228 indicate a nearly ubiqUitoUS Ra-224 excess that generally increases with decreasing pH. Near-surface (less-than-or-equal-to 100 m) thermal water flow velocities at Mammoth Hot Springs are estimated from Ra-224 / Ra-228 variation to be greater-than-or-equal-to 1 m h-1.