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

Abstract

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 ²⁰⁶Pb/²³⁸U and ²⁰⁸Pb/²³²Th ratios (i.e., ²⁰⁶Pb/²³⁸U and ²⁰⁸Pb/²³²Th 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 ²⁰⁶Pb/²³⁸U and ²⁰⁸Pb/²³²Th 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.