AFG – a new Cenozoic columbite–tantalite natural reference material for LA-ICP-MS U–Pb geochronology

Abstract

Columbite–tantalite series minerals are widely distributed in granites, pegmatites, carbonatite–alkaline rocks, and peraluminous granites. U–Pb dating of columbite–tantalite minerals can provide robust temporal constraints on petrogenetic and mineralization events. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become an important analytical technique for columbite–tantalite U–Pb dating, providing high spatial resolution geochronological data through relatively straightforward sample preparation. However, matrix effects between reference materials and unknown samples remain the principal limitation for accurate columbite–tantalite U–Pb dating. The matrix-matched reference materials are crucial for primary calibration or as quality monitoring in LA-ICP-MS columbite–tantalite U–Pb analysis. Although some columbite–tantalite U–Pb dating reference materials have been reported in previous studies, samples containing low amounts of common lead are extremely scarce. Moreover, reference materials covering diverse age ranges are critical for external calibration and quality control, particularly for young materials, given the analytical difficulties in measuring low radiogenic Pb content in young samples by laser ablation. In this study, a Cenozoic columbite–tantalite AFG was investigated as new reference material for LA-ICP-MS columbite–tantalite U–Pb dating. The average U and Pb concentrations in AFG are 1213 ± 112 µg g⁻¹ (2 s) and 3.51 ± 0.45 µg g⁻¹ (2 s), respectively. The isotope dilution thermal ionization mass-spectrometry (ID-TIMS) analysis of AFG yielded a weighted mean ²⁰⁶Pb/²³⁸U ratio of 0.00289 ± 0.000017 (2 s, MSWD = 2.7, n = 5) and a weighted mean ²⁰⁶Pb/²³⁸U age of 18.59 ± 0.06 Ma (2 s, MSWD = 3, n = 5). The results from four independent LA laboratories exhibit excellent homogeneity and yield concordant U–Pb ages. All LA-ICP-MS U–Pb analyses yield a weighted mean ²⁰⁶Pb/²³⁸U age of 18.62 ± 0.05/0.38 Ma (2 s, MSWD = 0.75, n = 288), which is consistent with the ID-TIMS result within analytical uncertainty. We propose AFG columbite–tantalite as a promising new reference material in Cenozoic U–Pb geochronological studies.