1. Department of New Energy Science, Graduate School of Science and EngineeringUniversity of Toyama 3190 Gofuku, Toyama 930‐8555 Japan
2. Institute of Geology and Geoinformation, Geological Survey of JapanAdvanced Industrial Science and Technology (AIST) Central 7, 1‐1‐1 Higashi, Tsukuba Ibaraki 305‐8567 Japan
3. Kyoto Fission‐Track Co., Ltd. 44‐4 Minamitajiri‐cho, Omiya, Kita‐ku, Kyoto 603‐8832 Japan
4. Department of Geology and MineralogyKyoto University Kitashirakawa Oiwakecho, Sakyo‐ku, Kyoto 606‐8502 Japan
5. Geochemical Research CenterThe University of Tokyo 7‐3‐1 Hongo, Bunkyo‐ku, Tokyo 113‐0033 Japan
6. Hiruzen Institute for Geology and Chronology 2‐5 Nakashima, Naka‐ku, Okayama 703‐8252 Japan
7. Department of Earth and Planetary Sciences, Graduate School of Environmental StudiesNagoya University, Furo‐cho, Chikusa‐ku Nagoya Aichi 464‐8601 Japan