Non-matrix-matched analysis of U-Th-Pb geochronology of bastnasite by laser ablation inductively coupled plasma mass spectrometry

Bastnasite is widespread in alkali granite, carbonatite and REE ore deposits. The U-Th-Pb ages of bastnasite can constrain the mineralization age. Currently, the lack of suitable in situ analysis reference material is the major obstacle of bastnasite U-Th-Pb geochronology. In this study, the matrix effects of U-Pb and Th-Pb age determination in bastnasite using glass NIST 610 as the external calibration reference were evaluated with the 193-nm excimer LA-ICP-MS analysis in normal and additional gases laser ablation modes. The obtained U-Th-Pb ages of bastnasite were approximately 7-11% lower than their reference values in the normal analytical mode (without additional gas). Fortunately, the determined systematic age biases in U-Th-Pb dating were significantly reduced (less than 1-2%) with the addition of water vapor within the ablation chamber. This reduction may be attributed to the observed significant suppression of Pb-206/U-238 and Pb-208/Th-232 downhole fractionation in both NIST 610 glass and bastnasite by introducing a small quantity of water vapor in the ablation chamber. In addition, the obtained Pb-206/U-238 and Pb-208/Th-232 ages of bastnasite K-9, LZ1384, and MAD809 showed great consistency with their corresponding recommended values. The obtained results show that accurate and precise U-Th-Pb ages of bastnasite can be simultaneously obtained by using glass NIST 610 as the primary calibration standard in 193-nm LA-ICP-MS with the water vapor-assisted method. This simple and effective water vapor-assisted non-matrix-matched laser ablation method helps to promote the geological application of bastnasite U-Th-Pb geochronology.