Radium isotope geochemistry of thermal waters, Yellowstone National Park, Wyoming, USA

Radium isotope activities (²²⁶Ra, ²²⁸Ra, and ²²⁴Ra), chemical compositions, and sulfur isotope ratios in sulfate were determined for water samples from thermal areas in Yellowstone National Park, Wyoming. Activities of ²²⁶Ra in these waters range from <0.2 to 37.9 dpm/kg. Activity ratios of 228Ra226Ra">228Ra226Ra range from 0.26 to 14.2, and those of 224Ra228Ra">224Ra228Ra range from 0.73 to 3.1. Radium concentrations are inversely correlated with aquifer equilibration temperatures (estimated from dissolved silica concentrations), while[RaBa">RaBa]_aq and 228Ra226Ra">228Ra226Ra activity ratios depend upon UBa">UBa and ThU">ThU ratios in aquifer rocks. Major controls on Ra concentration in Yellowstone thermal waters are inferred to be

1.

(1) barite saturation (at Morris Geyser Basin, Mammoth Hot Springs, and other northern areas) and

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(2) zeolitewater ion exchange (at Upper Geyser Basin). The data are consistent with a model in which

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(1) radium and barium are supplied to water by bulk dissolution of aquifer rock, and

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(2) chemical equilibration of water with rock is rapid relative to the 1602 year half-life of ²²⁶Ra. The 228Ra226Ra">228Ra226Ra activity ratios of the waters may in some cases reflect surface enrichments of ²³²Th and/or may indicate that α-recoil input of ²²⁸Ra is rapid relative to water-rock chemical equilibration. Activity ratios of 224Ra228Ra">224Ra228Ra indicate a nearly ubiquitous ²²⁴Ra excess that generally increases with decreasing pH. Near-surface (≤100 m) thermal water flow velocities at Mammoth Hot Springs are estimated from 224Ra228Ra">224Ra228Ra variation to be ≥ 1 m h⁻¹.