1. Graduate School of Science and Laboratory for Multiple Isotope Research for Astro- and Geochemical Evolution (MIRAGE), Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan, Geological Survey of Japan, AIST, Central 7, Higashi, Tsukuba, Ibaraki 305-8567, Japan, and State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China