1. Department of Natural History Sciences, Graduate School of Sciences; Hokkaido University; Sapporo 060-0810 Japan
2. Kyoto Fission-Track Co., Ltd.; Kita Kyoto 603-8832 Japan
3. Division of Earth and Planetary Sciences; Kyoto University; Sakyo Kyoto 606-8502 Japan
4. Okayama University of Science; 1-1 Ridai-cho, Kita-ku Okayama 700-0005 Japan