In-situ U-Th-Pb geochronometry with submicron-scale resolution: Low-voltage electron-beam dating of complexly zoned polygenetic uraninite microcrystals

Abstract

Complexly zoned microcrystals of uraninite were encountered in orthogneiss from the central Tauern Window in Austria (K1 gneiss, Felbertal scheelite mine) and analysed in-situ for U, Th and Pb with state-of-the-art FE-SEM/EDX techniques. A three times finer spatial resolution was achieved using an acceleration voltage of 8 kV, compared to the classic 15-20 kV set-up of U-Th-total Pb electron microprobe dating. The lower voltage allows a spheroid of material with a diameter of only 0.3 μm to be selectively analysed. Careful tests on three uraninite reference materials show that the low-voltage method yields sufficient precision and accuracy for U-Th-total Pb uraninite dating, with errors on individual spot ages in the order of 10-30 Ma. By means of this innovative analysis technique, small-scale age zoning patterns could be resolved and dated in the uraninite microcrystals from the orthogneiss. Based on microstructures observed in backscattered electron images we interpret that an older uraninite generation in the rock, with a late Permian formation age (~260 Ma), was recycled two times through a coupled dissolution-reprecipitation process at ~210 Ma and at ~30 Ma. The younger dissolution-reprecipitation phase at ~30 Ma coincides with the Alpine regional metamorphism (lower amphibolite facies). The two older ages (~210 Ma and ~260 Ma) have been previously recognized in rocks from the Tauern Window by uraninite dating, but it is the first time here that both are recorded in the same rock and even the same uraninite grain. The present study shows that recrystallized accessory uraninite can provide a sensitive geological "hard disk" where several discrete thermal events of an area are stored. In addition, our work attests that the mineral uraninite has an unexpected geochronological robustness, even on the microcrystal scale.