Assessment of M6 monazite as a potential reference material for in situ microbeam analyses of U–Th–Pb geochronology and O–Nd isotopes†

Abstract

In situ U–Th–Pb geochronology of monazite is widely used to reveal geological histories. Well characterized matrix-matched reference materials are crucial for achieving accurate results in microbeam U–Th–Pb dating. In this paper, the internal structure, chemical composition and U–Th–Pb dates of M6 monazite are investigated using multiple analytical methods to evaluate its potential as a reference material for in situ monazite U–Th–Pb dating. The results show that M6 monazite has an average ThO₂ content of 10.7 ± 1.1% (2SD; SD = standard deviation) and Th/U ratio of 28.4 ± 3.3 (2SD). In situ U–Th–Pb dates from laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) laboratories are reproducible and homogeneous at the spatial level of 30 μm. Seven ²⁰⁷Pb/²³⁵U dates from isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) yield a mean date of 485.7 ± 2.3 Ma (2SD), which is our recommended date for M6 monazite. Furthermore, SIMS oxygen isotope determinations show good measurement reproducibility (7.70‰ ± 0.41‰, 2SD) and the LA-MC-ICP-MS analyses yield homogeneous Sm–Nd isotopic composition (¹⁴³Nd/¹⁴⁴Nd = 0.511829 ± 0.000045, 2SD; ¹⁴⁷Sm/¹⁴⁴Nd = 0.2302 ± 0.0139, 2SD), demonstrating that M6 monazite has the potential to be a reference material for in situ O and Sm–Nd isotopic analyses.