1. Department of Earth Sciences; The University of Hong Kong, Hong Kong Special Administrative Region; Hong Kong
2. State Key Laboratory of Ore Deposit Geochemistry; Institute of Geochemistry, Chinese Academy of Sciences; Guiyang China
3. Department of Earth and Planetary Sciences; University of California at Davis; Davis CA USA
4. Now at Lawrence Livermore National Laboratory; Livermore CA USA
5. Berkeley Geochronology Center; Berkeley CA USA
6. Department of Earth and Planetary Science; University of California at Berkeley; Berkeley CA USA