In situ SIMS Th-Pb dating of bastnaesite: constraint on the mineralization time of the Himalayan Mianning-Dechang rare earth element deposits

Bastnaesite, a major economic rare earth element (REE) mineral, has been considered to be a promising geochronological tool for determining mineralization time by U-Pb dating. However, it has been found that this mineral often favours the incorporation of Th relative to U, with Th/U ratios even larger than 1000. Young samples contain considerable amounts of excess Pb-206 from initial Th-230 that is incorporated during formation, which could have a significant effect on U-Pb dating. In this study, we present analytical and data calibration protocols for in situ Th-Pb analyses of bastnaesite using a Cameca IMS 1280 HR SIMS. Six bastnaesite samples from the Himalayan Mianning-Dechang REE belt were analysed in this study to test our analytical procedures. The zircon from the host syenites are dated by the SIMS U-Pb method at 12.7 +/- 0.2 Ma for the Dalucao deposit, 28.8 +/- 0.3 Ma for the Lizhuang deposit, 28.0 +/- 0.2 Ma for the Muluozhai deposit, and 26.6 +/- 0.3 Ma for the Maoniuping deposit, which are interpreted as the intrusion time of the syenites. The SIMS bastnaesite Th-Pb dating yielded ages of 11.9 +/- 0.2 Ma and 11.8 +/- 0.2 Ma for the Dalucao deposit, 28.4 +/- 0.2 Ma for the Lizhuang deposit, 26.7 +/- 0.2 Ma and 26.9 +/- 0.2 Ma for the Muluozhai, and 25.7 +/- 0.2 Ma for the Maoniuping deposits. These bastnaesite ages are interpreted as the mineralization time. To compare the zircon age and corresponding bastnaesite age, the time span from intrusion of the host magma to REE mineralization is approximately 1 Ma or less. This work demonstrated that the Th-Pb dating method for high Th bastnaesite yields more precise ages than the U-Pb system, as the former can avoid the excess 206Pb problem.