Development of U–Pb dating of uraninite using a secondary ion mass spectrometer: Selection of reference material and establishment of calibration method

A method for U–Pb isotopic dating using secondary ion mass spectrometer (SIMS) was developed for uraninite. Correlation between ²⁵¹(UO)⁺/²³⁵U⁺ and ²⁰⁶Pb⁺/²³⁵U⁺ obtained by a sensitive high‐resolution ion microprobe (SHRIMP) was adopted for a calibration from secondary ion ratios (Pb⁺/U⁺) to the atomic abundance ratios (Pb/U). In this study, a uraninite sample (²⁰⁶Pb/²³⁸U = 0.1647) collected from Faraday mine, Bancroft, Canada, is used as a reference material for the U–Pb calibration. The established method was applied to three uraninite samples collected from the Chardon, Ecarpière, and Mistamisk mines. The calibrated ²⁰⁶Pb^*/²³⁸U ratios of the three uraninites show correlation with Pb/U elemental ratios obtained using an electron probe microanalyzer (EPMA) (correlation coefficients: 0.98, 0.99, and 0.97, respectively), which indicates the reliability of the SHRIMP calibration method used in this study. The analysis of Ecarpière uraninite provides concordant U–Pb data, and a weighted average of the ²⁰⁶Pb^*/²³⁸U age is 287 Ma ±8 Ma (95 % conf.) which is consistent with the previous chronological results by SIMS. Mistamisk uraninite provides discordant U–Pb data with the upper and lower intercept ages of 1 729 and 421 Ma, which correspond to uraninite formation in association with the Hudsonian orogeny and the remobilization of uranium as pitchblende, respectively. The U–Pb age of Chardon uraninite (315 Ma) is consistent with the igneous activity of Mortagne granite, but is older than the previously reported age (264 Ma). Marcasite in the Chardon uraninite altered to goethite under the oxidizing condition, which indicates that U–Pb system in the uraninite crystallized at 315 Ma was disturbed under the oxidizing condition. The established calibration method using Faraday uraninite is useful for U–Pb isotopic dating on the scale of a few micrometers to tens of micrometers, which make it possible to obtain the accurate age of uraninite.