Bastnaesite K-9: A homogenous natural reference material for in-situ U–Pb and Th–Pb dating

Abstract

Bastnaesite, a major economic rare earth element (REE) mineral, has been considered to serve as a U–Th–Pb geochronometer to record REE mineralization time. As previously noted, most bastnaesite contains more Th than U, with Th/U ratios larger than 1000 in some cases, indicating Th–Pb dating is more suitable than U-Pb geochronometry. In-situ bastnaesite ThPb dating by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) method and Secondary Ion Mass Spectrometry (SIMS) has been performed. However, there is no independently determined, matrix matched Th-Pb bastnaesite reference material available. In this study, the bastnaesite “K-9” from Karasug siderite carbonatites (Tuva Republic) are evaluated as U–Pb and Th–Pb dating reference material by multiple methods. Major elements, including La, Ce, Pr, Nd, and F are homogenous at < 5 wt% level (1 relative standard deviation, RSD) as verified by electron microprobe analyses (EMPA). SIMS U-Th-Pb dating results prove its homogeneity and concordance of both U-Pb and Th-Pb system at < 2% level (1 RSD). Based on six ID-IRMS (isotope dilution isotope ratio mass spectrometry) analyses,206Pb/238U age of 116.80 ±0.13 Ma and208Pb/232Th age of 116.59 ±0.11 Ma are provided as recommended age data. In summary, K-9 bastnaesite is qualified to serve as an in-situ U–Pb and Th– Pb dating reference material and is expected to contribute significantly to the rapidly advancing bastnaesite geochronology in the coming future.