In situ U–Pb dating of bastnaesite by LA-ICP-MS

Abstract

Bastnaesite, a common accessory mineral in REE ore deposits, is ideal for U–Pb isotopic dating because of its relatively high U and Th contents. We report an analytical procedure for U–Pb dating of this mineral using a 193 nm ArF excimer laser ablation system coupled to an Agilent 7500a (LA-ICP-MS). Laser induced elemental fractionation and instrumental mass discrimination were externally corrected using an in house bastnaesite standard (K-9). The fluence, spot size and repetition rate of laser were evaluated to assess their effects on age determination in detail. The matrix effect on zircon and bastnaesite was also investigated and compared in detail during laser sampling. The results indicate that a matrix-matched standard reference material is essential. In order to validate and demonstrate the effectiveness and robustness of our developed protocol, we dated several bastnaesite samples from the Himalayan Mianning-Dechang REE belt, South-West China. The U–Pb ages of ∼31 to 34 Ma obtained for bastnaesites from Maoniuping, Diaoloushan, Zhengjialiangzi and Lizhuang are in good agreement within error, but differ from the wide range of age (10–40 Ma) obtained using K–Ar and Ar–Ar methods for biotite and muscovite and using U–Pb dating for zircon. These dating applications demonstrate the reliability and feasibility of our established method. In summary, the LA-ICP-MS dating of bastnaesite can be a complementary dating method to the more established TIMS and SIMS techniques with advantages of rapidity, moderate spatial resolution and relatively low cost.