Acknowledgements
We thank Dr. Barry Roger and Dr. Hidetoshi Hara for their critical and constructive reviews. We would like to thank Dr. Koshi Yamamoto for his technical support in XRF measurements. We are also grateful to Profs. Francis Hirsch, Punya Charusiri, Talat Ahmad and Ken-ichiro Hisada and all members of IGCP 516 for their valuable suggestions and for encouraging us to pursue this study.