A new calibrated strategy for the in situ U-Th-Pb dating of bastnasite by xenotime

Bastnaesite is an important mineral geochronometer to record the REE mineralization time for REE deposits. As the only reliable primary standard for in situ U-Th-Pb dating, bastnaesite K-9 is inhomogeneous and nearly out of supply. The accurate in situ U-Th-Pb dating of bastnaesite and its wide application are still constrained by the lack of reliable reference materials. Therefore, a potential primary standard was proposed and assessed in thirty-one ablation settings in this paper. The normalized Pb-206/U-238 and Pb-208/Th-232 ratios (i.e., Pb-206/U-238 and Pb-208/Th-232 fractionation) of xenotime XN01 are respectively similar or slightly different from those of bastnaesite K-9 in most ablation settings. The average value (i.e., calibration factor) of Pb-206/U-238 and Pb-208/Th-232 ratios for XN01 is consistent with those for K-9 with the offsets of <2.4% in the same ablation setting when using the ratio-of-the-mean intensity method for data reduction. Thus, the most obtained ages of K-9 calibrated by XN01 are consistent with its reference age of 118 +/- 1 Ma with the offsets of <2%. Moreover, four typical bastnasite samples were analyzed in three normal ablation settings. Their respective U-Pb and Th-Pb ages calibrated by XN01 were consistent with those by K-9 within uncertainty. Because of high Th contents (>4500 ppm) and Th/U ratios (>400), the Th-Pb ages of these young samples are acceptable and agree well with their respective published Th-Pb ages within uncertainty. Therefore, the matrix effect between xenotime XN01 and bastnaesite can be neglected normally. Xenotime XN01 is more uniform than K-9 and can serve as a primary reference material for the in situ U-Th-Pb dating of bastnaesite. Moreover, bastnaesite Zagi has a sufficient reservoir, uniform Th-Pb, corrected U-Pb ages, and can serve as a secondary reference material for quality control.