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

Abstract

A method for U-Pb isotopic dating using secondary ion mass spectrometer (SIMS) was developed for uraninite. Correlation between 251 (UO) + / 235 U + and 206 Pb + / 235 U + 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 ( 206 Pb/ 238 U = 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 206 Pb * / 238 U 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 206 Pb * / 238 U 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. K E Y W O R D S reference material, secondary ion mass spectrometry, U-Pb dating, uraninite

Thematic section: Special topics in 4th IGS ‘Precambrian World 2’

“…Horie and Hidaka (2019) report a suitable reference material collected from Faraday Mine, Bancroft, Canada ( 206 Pb/ 238 U = 0.1647) and calibration method for U–Pb uraninite isotopic dating using secondary ion mass spectrometer. Correlation between 251 ( 235 U 16 O) + / 235 U + and 206 Pb + / 235 U + obtained by a sensitive high‐resolution ion microprobe (SHRIMP) was adopted for the calibration from secondary ion ratios (Pb+/U+) to the atomic abundance ratios (Pb/U).…”