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

Abstract

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 ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U and ²⁰⁷Pb/²⁰⁶Pb Pb_c-corrected weighted mean ratios of 0.06008 ± 0.00018 (2s, MSWD = 4.5), 0.4495 ± 0.0013 (2s, MSWD = 1.6) and 0.0542 ± 0.0006 (2s, MSWD = 0.001), respectively. The weighted mean ²⁰⁶Pb/²³⁸U age is 376.50 ± 0.9 Ma (MSWD = 1.3), the ²⁰⁷Pb/²³⁵U age is 376.5 ± 0.62 Ma (MSWD = 0.83) and the ²⁰⁷Pb/²⁰⁶Pb 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 ²⁰⁶Pb/²³⁸U age determinations using various LA-ICP-MS systems. LA-ICP-MS measurement results are characterised by precisions fit for purpose and yield consistent ²⁰⁶Pb/²³⁸U 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.