1. Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba; Ibaraki 305-8567 Japan
2. Graduate School of Science and Technology; Niigata University; Nishi-ku, Niigata 950-2181 Japan
3. Faculty of Science; Shinshu University; Matsumoto, Nagano 390-8621 Japan
4. Kyoto Fission-Track Co., Ltd.; Kita-ku, Kyoto 603-8832 Japan
5. Department of Chemistry; Gakushuin University; Toshimaku, Tokyo 171-8588 Japan
6. Geochemical Research Center; The University of Tokyo; Bunkyo-ku, Tokyo 113-0033 Japan