Evidence for a hypogene paleohydrogeological event at the prospective nuclear waste disposal site Yucca Mountain,Nevada, USA,revealed by the isotope composition of fluid-inclusion water |
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| Authors: | Yuri V Dublyansky Christoph Spötl |
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| Institution: | 1. Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, Harbin Engineering University, Harbin 150001, China;2. Institute of Nuclear and New Energy Technology(INET), Tsinghua University, Beijing 100084, China;3. Nuclear Power Insititude of China, Chengdu 610005, China;1. Fast Reactor Cycle System Research and Development Center, Japan Atomic Energy Agency, 4002, Narita-cho, Oarai-machi, Ibaraki 319-1393, Japan;2. NESI Inc., 4002, Narita-cho, Oarai-machi, Ibaraki 319-1313, Japan;3. Nuclear Science and Engineering Center, Japan Atomic Energy Agency, 2-4 Shirakata, Tokai-mura, Ibaraki 319-1195, Japan;4. Research Institute of Nuclear Engineering, University of Fukui, 1-2-4 Kanawa-cho, Tsuruga-shi, Fukui 914-0055, Japan;1. Research Institute of Nuclear Engineering, University of Fukui, Kanawa-cho 1-2-4, Tsuruga-shi, Fukui-ken 914-0055, Japan;2. Egypt Nuclear and Radiological Regulatory Authority, 3 Ahmad El Zomar St., Nasr City, Cairo 11787, Egypt;1. Oak Ridge National Laboratory, Oak Ridge, TN, USA;2. Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA |
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| Abstract: | Secondary calcite residing in open cavities in the unsaturated zone of Yucca Mountain has long been interpreted as the result of downward infiltration of meteoric water through open fractures. In order to obtain information on the isotopic composition (δD and δ18O) of the mineral-forming water we studied fluid inclusions from this calcite. Water was extracted from inclusions by heated crushing and the δD values were measured using a continuous-flow isotope-ratio mass spectrometry method. The δ18O values were calculated from the δ18O values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fluid-inclusion microthermometry.The δD values measured in all samples range between ? 110 and ? 90‰, similar to Holocene meteoric water. Coupled δ18O–δD values plot significantly, 2 to 8‰, to the right of the meteoric water line. Among the various processes operating at the topographic surface and/or in the unsaturated zone only two processes, evaporation and water–rock exchange, could alter the isotope composition of percolating water. Our analysis indicates, however, that none of these processes could produce the observed large positive δ18O-shifts. The latter require isotopic interaction between mineral-forming fluid and host rock at elevated temperature (>100 °C), which is only possible in the deep-seated hydrothermal environment. The stable isotope data are difficult to reconcile with a meteoric origin of the water from which the secondary minerals at Yucca Mountain precipitated; instead they point to the deep-seated provenance of the mineral-forming waters and their introduction into the unsaturated zone from below, i.e. a hypogene origin. |
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