Development of U–Pb dating of uraninite using a secondary ion mass spectrometer: Selection of reference material and establishment of calibration method. (23rd July 2019)

• Record Type: Journal Article
• Authors: Horie, Kenji; Hidaka, Hiroshi
• Is Part Of: Island arc. Volume 28:Number 5(2019)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2019-07-23
• Subjects: reference material -- secondary ion mass spectrometry -- U–Pb dating -- uraninite; Plate tectonics -- Periodicals; Island arcs -- Periodicals; Geodynamics -- Periodicals; 551.136
• DOI: 10.1111/iar.12319
• Languages: English
• ISSNs: 1038-4871

Abstract

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. Abstract : 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. Abstract : 二次イオン質量分析計を用いた閃ウラン鉱のU–Pb年代測定法を確立した. カナダのファラデー鉱山産の閃ウラン鉱を標準物質として，高感度高分解能イオンマイクロプローブ(SHRIMP‐II)分析から得られた 251 (UO) + / 235 U + と 206 Pb + / 235 U + 間の相関を用いて，Pb/Uの原子数比への較正を行った. 同手法をフランス Ecarpière 鉱山とカナダ Mistamisk 鉱山から採取した閃ウラン鉱に応用した結果は，過去に行われた分析結果と調和的であった. 一方，酸化的環境で形成した二次鉱物を含むフランス Chardon 鉱山産の閃ウラン鉱の年代は，過去の報告よりも有意に古く，鉱体が不整合で接する花崗岩の貫入年代と調和的であった. 花崗岩の貫入に伴い閃ウラン鉱が結晶化し，その後酸化的環境でPb損失を生じたと考えられる. この結果は，SHRIMPを用いた閃ウラン鉱のU–Pb年代測定の有用性を示している.