基金Acknowledgements The authors would like to thank Dr. Zhang Boyou, Guangzhou Institute of Geochemistry, the Chinese Academy of Sciences and Dr. Zhang Ke, Department of Earth Science, Zhongshan University for their careful reviews of this paper. This work was jointly funded by the State Fund for Natural Science of China （Grand No. 40676039）, the Knowledge Innovation Projects of Chinese Academy of Sciences （Grant Nos. KZCX2-SW-117, KZCX3-SW-224-01）, the State Key Basic Research Development and Planning of China （Grant No. G2000046702）, and the State 10th 5-year Public Welfare Study Project of China （Grant No. 2001DIAS0041-01）.