The KNW rutile—a natural reference material for microbeam U–Pb age and trace element determination

Abstract

Rutile is an accessory mineral that is widely distributed in magmatic, metamorphic, and sedimentary rocks. Thus, rutile U–Pb geochronology and geochemistry (e.g., Zr-in-rutile thermometry) can provide important information on the geological evolution of a region. Accurate and precise in situ U–Pb dating and trace element content measurement require well-characterized reference materials to correct matrix-dependent elemental fractionation. We present a natural rutile reference material (the KNW rutile) for microbeam U–Pb age and trace element determination. The KNW rutile was collected from a pegmatite in Kragerø, Norway, which is ∼20 km east of the location where the R10 rutile was sampled. It is ∼21 mm × 10 mm × 8 mm in size, with a total mass of ∼30 g. The KNW rutile has a homogeneous U–Pb age, as shown by numerous LA-ICP-MS spot analyses (weighted mean ²⁰⁶Pb/²³⁸U age = 1089.5 ± 3.3 Ma; MSWD = 2.7; n = 229). KNW rutile contains very little common Pb with only 12 of 241 spot analyses showing an amount of common Pb (f₂₀₆: 0–20%). Seven ID-TIMS analyses produced a concordia age of 1088.2 ± 1.5 Ma (MSWD = 0.63) and a weighted mean ²⁰⁶Pb/²³⁸U age of 1088.2 ± 1.9 Ma (MSWD = 0.19), which is our recommended U–Pb age. The degree of homogeneous distributions of eleven trace elements was evaluated using LA-ICP-MS and EPMA. V, Cr, Nb, Sc, Zr, and Hf are sufficiently homogeneously distributed, whereas Ta, U, Pb, Fe, and W are heterogeneous. The determination of reference values for Zr and other trace elements as well as their uncertainties at the 95% confidence level followed International Organization for Standardization (ISO) guidelines and the certification protocol of the International Association of Geoanalysts (IAG) closely. The KNW rutile has a Zr content of 1183 ± 198 μg g⁻¹ (95% confidence level). The KNW rutile is a useful addition to the reference materials previously distributed for microbeam U–Pb age and trace element determination.