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

• Published in: Journal of analytical atomic spectrometry Vol. 37; no. 12; pp. 2599 - 2614
• Main Authors: Tang, Yanwen, Liu, Na, Yang, Jiehua, Gonçalves, Guilherme O, Liu, Liang, Lan, Tingguang, Gao, Jianfeng, Han, Junjie
• Format: Journal Article
• Published: 30.11.2022
• ISSN: 0267-9477; 1364-5544
• DOI: 10.1039/d2ja00233g

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 206 Pb/ 238 U and 208 Pb/ 232 Th ratios ( i.e. , 206 Pb/ 238 U and 208 Pb/ 232 Th 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 206 Pb/ 238 U and 208 Pb/ 232 Th 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. Xenotime XN01 can serve as the primary standard for the accurate in situ U-Th-Pb dating of bastnasite in normal ablation settings.