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

Journal of Volcanology and Geothermal Research, 40 (1990) 169-180 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands Time scale of hydrothermal water-rock reactions in Yellowstone National Park based on radium isotopes and radon JORDAN F. CLARK and KARL K. TUREKIAN Department of Geology and Geophysics, Yale University, Box 6666, New Haven, CT 06511, U.S.A. (Received January 15, 1989; revised and accepted August 8, 1989 ) Abstract Clark, J.F. and Turekian, K.K., 1990. Time scale of hydrothermal water-rock reactions in Yellowstone National Park based on radium isotopes and radon. J. Volcanol. Geotherm. Res., 40: 169-180. We have measured 224Ra (3.4 d), 228Ra (5.7 yr), and 226Ra (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 (228Ra/22eRa) 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 (228Ra/226Ra) 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. Introduction Hydrothermal activity, in the form of hot springs and geysers, is the geologic feature which most vividly characterizes Yellowstone Na- tional Park. The greatest diversity in dynamic and depositional features ascribable to this ac- tivity is found in the Norris Geyser Basin and the Norris-Mammoth Corridor (see the recent summary by White et al., 1988). The source of heat is clearly magmatic but the exact process by which the extensive heat loss is maintained is not well understood. Four- nier and Pitt (1985) and Fournier (1989) sug- gest that the hydrothermal system at Yellow- stone is driven by the crystallization of magma and the cooling of local hot rocks; White et al. (1988) suggest that the hot-spot refluxing of basaltic magma beneath a cover of continental rocks and acidic volcanics is the source of the heat. The scale of the reaction time between the percolating meteoric water and the rocks in the region of the magmatic activity is controlled by the flow rate, the path length, and the rate of heating to near the critical temperature at depth (Truesdell and Fournier, 1976). The high-tem- perature waters are subsequently cooled by steam separation, conduction, and mixing with a cold-water reservoir during ascent; the final temperature of the hot spring is generally well below the reaction temperature. © 1990 Elsevier Science Publishers B.V.