OXF: a new natural reference material for use in LA-ICP-MS U–Pb columbite–tantalite geochronology†

Abstract

Columbite–tantalite is predominantly found in rare-metal granites and pegmatites, alkaline and carbonatitic rocks, and columbite–tantalite-bearing peraluminous granites. It is recognized as a potentially valuable mineral for use in U–Pb isotopic dating. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) can be used in the U–Pb dating of columbite–tantalite. LA-ICP-MS requires matrix-matched reference materials for isotopic fractionation/mass bias correction. Some columbite–tantalite reference materials contain common Pb. However, columbite–tantalite reference materials with low contents of common Pb are scarce. Thus, in this study, we proposed a new potential reference material, OXF, with a low common Pb content for use in LA-ICP-MS U–Pb dating of columbite–tantalite. LA-ICP-MS analysis revealed that the average U, Th, and Pb concentrations in OXF are 428 ± 156 ppm (2s), 6.3 ± 2.5 ppm (2s), and 20.0 ± 10.0 ppm (2s), respectively, with a total weight of 336.6 g. Chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) revealed that the weighted mean ²⁰⁶Pb/²³⁸U age of OXF is 262.85 ± 0.64 Ma (MSWD = 1.8, 2s). The LA-ICP-MS U–Pb ages of the four laboratories in the China University of Geosciences (Wuhan) (CUGW), the China University of Geosciences (Beijing) (CUGB), Wuhan Sample Solution Analytical Technology Co. Ltd, China (WHSS), and the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) were determined. The ²⁰⁶Pb/²³⁸U ages of the CUGW, the CUGB, WHSS, and the IGGCAS were 262.71 ± 0.33 Ma (2s, n = 151), 263.3 ± 2.3 Ma (2s, n = 46), 263.5 ± 1.3 Ma (2s, n = 42) and 264.3 ± 2.4 Ma (2s, n = 25), respectively. The weighted mean ²⁰⁶Pb/²³⁸U age of the four laboratories obtained using LA-ICP-MS and by combining the U–Pb data obtained from the laboratories was 262.83 ± 0.29 Ma (2s, n = 358), which aligns with its value obtained using CA-ID-TIMS after allowing for analytical uncertainties. OXF demonstrated homogeneous U–Pb ages with minimal Pb contributions, indicating its potential for serving as a primary reference material in LA-ICP-MS calibrations.