基金Two anonymous reviewers are thanked for their constructive comments and suggestions that helped improve this manuscript. Dr. Yuanbao Wu is thanked for his helpful discussion in preparing the manuscript. Prof. Yongfei Zheng is thanked for the editorial work. This work was supported by the National Natural Science Foundation of China （Grant Nos. 40821061, 90914007, 40576030 and 90714010）, the State Administration of Foreign Expert Affairs of China （Grant No. B07039） and the Special Fund for Basic Scientific Research of Central Colleges, China University of Geosciences.