Three new natural secondary reference materials for in situ andradite U–Pb geochronology†

Andradite-rich garnet is a common mineral found in various rock types, such as alkaline igneous rocks and skarns, and often exhibits high U/Pb ratios, making it suitable for in situ U–Pb dating. However, the limited availability of well-characterised andradite reference materials has impeded their broader application in microanalysis. In this study, three natural andradite samples (MKWB, DGS, and Stanley) have been characterised and assessed as potential secondary reference materials for in situ U–Pb geochronology. U–Pb isotopic analyses were conducted across four different laboratories using isotope dilution – TIMS, SIMS, and laser ablation (SF, Q)-ICP-MS, to examine the homogeneity of the euhedral andradite crystals. We report ID-TIMS ²⁰⁶Pb/²³⁸U ages of 264.9 ± 5.8 Ma (2 s, n = 6, MSWD = 3.4), 139.42 ± 0.36 Ma (2 s, n = 5, MSWD = 1.9), and 23.28 ± 0.38 Ma (2 s, n = 3, MSWD = 1.9) for MKWB, DGS, and Stanley, respectively. These three newly characterised natural secondary reference materials should significantly contribute to the advancement of in situ U–Pb andradite-rich garnet geochronology.