Kovdor‐ GRT – A New Natural Reference Material for Garnet U‐Pb Dating

Calcic garnet is a common component of skarns, alkaline igneous rocks and carbonatites. Recent studies report garnet laser ablation‐ICP‐MS U‐Pb ages ranging from ca . 20 to 3100 Ma demonstrating the advantages of garnet for U‐Pb geochronological studies. However, calibration using well‐characterised matrix‐matched reference materials is a critical issue for accurate age determinations. In this study we present the major and trace element compositions, and U‐Pb systematics obtained from isotope dilution‐TIMS and LA‐ICP‐MS measurements for Kovdor (Kovdor‐GRT) andradite‐shorlomite‐morimotite garnet (Kola Alkaline Province, Kola Peninsula) as a potential reference material for LA‐ICP‐MS U‐Pb dating. ID‐TIMS gave 206 Pb/ 238 U, 207 Pb/ 235 U and 207 Pb/ 206 Pb Pb c ‐corrected weighted mean ratios of 0.06008 ± 0.00018 (2 s , MSWD = 4.5), 0.4495 ± 0.0013 (2 s , MSWD = 1.6) and 0.0542 ± 0.0006 (2 s , MSWD = 0.001), respectively. The weighted mean 206 Pb/ 238 U age is 376.50 ± 0.9 Ma (MSWD = 1.3), the 207 Pb/ 235 U age is 376.5 ± 0.62 Ma (MSWD = 0.83) and the 207 Pb/ 206 Pb age is 380.6 ± 1.7 (MSWD = 0.27). The Concordia age of 376.6 ± 0.86 Ma (MSWD = 0.79) for Kovdor‐GRT, determined by ID‐TIMS, is interpreted as the best estimate for its crystallisation age. The low common Pb (Pb c ) content (Pb c /Pb t varies from 0.05 to 0.08), moderate U mass fraction (12.4–33.0 μg g ‐1 ) and isotopically homogeneous composition allowed meaningful 206 Pb/ 238 U age determinations using various LA‐ICP‐MS systems. LA‐ICP‐MS measurement results are characterised by precisions fit for purpose and yield consistent 206 Pb/ 238 U ages indistinguishable from those obtained by ID‐TIMS. The results highlight the potential of Kovdor garnet as a reference material for in situ U‐Pb dating.