Thorium-uranium disequilibrium in a geothermal discharge zone at yellowstone |
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| Institution: | 1. Nuclear Risk Research Center, Central Research Institute of Electric Power Industry, Chiba 270-1194, Japan;2. Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8567, Japan;3. Yakushima Earth Science Club, Kagoshima 891-4205, Japan;1. Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 90095-1567, USA;2. Faculty of Science & Engineering, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand;3. Department of Geosciences, Universität Tübingen, Wilhelmstr. 56, 72074 Tübingen, Germany;4. Department of Geological Engineering, Süleyman Demirel University, 32260 Isparta, Turkey;5. High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC;1. National and Kapodistrian University of Athens, Department of Geology and Geoenvironment, Panepistimioupolis Zografou, 157 84 Athens, Greece;3. School of Geography, Earth & Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK |
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| Abstract: | Whole rock samples of hydrothermally-altered Biscuit Basin rhyolite from Yellowstone drill cores Y-7 and Y-8 were analyzed for 230Th, 234U, 238U, and 232Th. Extreme disequilibrium was found, with (230Th/ 234U) ranging from 0.30 to 1.27. Values of (230Th/232Th) and (234U/232Th) define a linear correlation with a slope of 0.16 ± 0.01, which corresponds to a (230Th/234U) age of approximately 19 ka. The (230Th/234U) disequilibrium was apparently caused by U redistribution which occurred mostly at about 19 ka, and is not related simply to the relative degree of hydrothermal alteration and self-sealing of the rhyolite. Mass balance of U requires a large flux of U-bearing groundwater through the rhyolite at the time of U redistribution; rough estimates of minimum water/rock ratio range from 102 to 104, for a range of possible groundwater U concentrations. Conservative hydraulic calculations indicate that the required groundwater flux could have occurred within a period of hundreds of years prior to self-sealing. The disequilibrium data are consistent with a model involving U redistribution during the initial stages of development of a geothermal discharge zone that formed in response to the hydrogeologic effects of glacial melting and unloading during the decline of the Pinedale Glaciation. |
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