Time scale of hydrothermal water-rock reactions in Yellowstone National Park based on radium isotopes and radon

Abstract

We have measured 224 Ra (3.4 d), 228 Ra (5.7 yr), and 226 Ra (1620 yr) and chloride in hot spring waters from the Norris-Mammoth Corridor, Yellowstone National Park. Two characteristic cold-water components mix with the primary hydrothermal water: one for the travertine-depositing waters related to the Mammoth Hot Springs and the other for the sinter-depositing Norris Geyser Basin springs. The Mammoth Hot Springs water is a mixture of the primary hydrothermal fluid with meteoric waters flowing through the Madison Limestone, as shown by the systematic decrease of the ( 228 Ra 226 Ra ) activity ratio proceeding northward. The Norris Geyser Basin springs are mixtures of primary hydrothermal water with different amounts of cold meteoric water with no modification of the primary hydrothermal ( 228 Ra 226 Ra ) activity ratio. Using a solution and recoil model for radium isotope supply to the primary hydrothermal water, a mean water-rock reaction time prior to expansion at 350°C and supply to the surface is 540 years assuming that 250 g of water are involved in the release of the radium from one gram of rock. The maximum reaction time allowed by our model is 1150 years.