Xenotime Xtm-NHBS: a natural reference material for microbeam U–Pb/Lu–Hf geochronology†

Abstract

Xenotime is a common mineral found in various rock types, including magmatic, metamorphic, and sedimentary rocks. It is typically characterised by high uranium (U) and thorium (Th) content, with minimal common lead (Pb), making it an excellent candidate for U–Pb dating. However, the limited availability of well-characterised U–Pb xenotime reference materials (e.g., BS-1 and MG-1) has hindered its wide application in microanalysis. With the rapid advancements in laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS), the demand for xenotime reference materials, particularly for in situ Lu–Hf geochronology, has grown significantly. This study presents the characterisation and assessment of Xtm-NHBS xenotime as a potential primary U–Pb reference material for microanalysis. U–Pb isotopic analyses were performed using isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and laser ablation quadrupole/sector field inductively coupled plasma mass spectrometry (LA-Q/SF-ICP-MS) to evaluate the homogeneity of gem-quality xenotime crystals. The ID-TIMS analysis yielded a ²⁰⁶Pb/²³⁸U age of 498.7 ± 0.4 Ma (2s, n = 5, MSWD = 0.99). The ²⁰⁶Pb/²³⁸U ages obtained via LA-Q/SF-ICP-MS across multiple analytical sessions were consistent with the ID-TIMS results, demonstrating homogeneity with ∼1% precision using in situ techniques. Additionally, the ages of BS-1, MG-1, and XN Datas can be reproduced by LA-SF-ICP-MS when calibrated against the Xtm-NHBS crystal as a primary reference material. Meanwhile, the Lu–Hf age of this crystal by LA-ICP-MS/MS is also presented in this study. The newly characterised natural Xtm-NHBS xenotime offers a significant contribution to advancing in situ U–Pb/Lu–Hf geochronology, enhancing both accuracy and precision in microanalytical applications.