1. Department of Earth ScienceShimane University Matsue 690‐8504 Japan
2. Geological Survey of JapanAIST 1‐1‐1 Higashi, Tsukuba 305‐8567 Japan
3. Kyoto Fission‐Track Co., Ltd. Kita‐ku, Kyoto 603‐8832 Japan
4. Geochemical Research CenterThe University of Tokyo Bunkyo‐ku, Tokyo 113‐0033 Japan