The study of radiation damage resulting from alpha-decay from U and Th chains in radiocative U-Th minerals is of tremendous importance. Indeed, this damage critically affects the long term behavior of minerals. It is caused by both the ejection of recoil-nuclei and alpha-particles, leading to fundamental modifications in the physical and chemical properties of minerals. If the recoil-nuclei induced defects have been extensively studied, the consequences of the alpha-particles ejection have been less investigated. In particular, the (Y,REE,U,Th)-(Nb,Ta,Ti) oxides, like euxenite, fergusonite, pyrochlore, zirconolite, are known to contain nanometric spherical voids or bubbles, interpreted to contain radiogenic helium. However no direct evidence of the trapping of helium in these voids has been shown up to now. In this study, in-situ analyses by STEM EELS on individual nano-bubbles from an euxenite crystal, sampled in its host c. 920 Ma old pegmatite in Norway, deliver, for the first time, a positive identification of helium and an estimation of helium pressure in such bubbles. The chemically unaltered euxenite crystal proves amorphous and homogeneously speckled with bubbles ranging from 5 to 68 nm in diameter, around a log-normal distribution centred at 19 nm. The euxenite contains 9.87 wt% UO2 and 3.15 wt% ThO2. It accumulated a theoretical alpha-decay dose of 3.46 x 1020 α/g (i.e. 170 He/nm3), at a dose rate of 11926 α/g/s. This corresponds to production of 0.23 wt% He. The density of He inside the bubbles, estimated from EELS data, ranges from 2 to 45 He/nm3, leading to a pressure of 8 to 500 MPa. The proportion of produced He trapped in bubbles is about 10%. The bubbles, acting as traps, clearly influence He diffusion. They may contribute to the swelling of euxenite during amorphization and to the fracturing of the host rock. These results suggest that both dose and dose rate are key parameters for the nucleation, growth and coalescence of He bubbles. Simulation of the behaviour of high-level nuclear waste glasses by extrapolation from actinide doped glass or externally irradiated materials (very high dose rates) may not be predictive on the macroscopic effects (swelling, fracturing), the change of properties of material in which He accumulates, and the presence/size of He bubbles. Furthermore, because alteration is promoted by amorphization, fluid interaction with euxenite crystals saturated with He bubbles will mobilize and redistribute He through the entire rock. Helium may be infused in other minerals, e.g. zircon or apatite, the most utilized minerals for (U-Th)/He thermochronology, and may completely disturb (U-Th)/He dating.
To cite this abstract:Anne-Magali Seydoux-Guillaume, Marie-Laure David, Kevin Alix, Lucien Datas, Bernard Bingen; Trapping of helium in nano-bubbles from 920 Ma euxenite crystals revealed by STEM-EELS analysis.. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/trapping-of-helium-in-nano-bubbles-from-920-ma-euxenite-crystals-revealed-by-stem-eels-analysis/. Accessed: March 2, 2021
EMC Abstracts - https://emc-proceedings.com/abstract/trapping-of-helium-in-nano-bubbles-from-920-ma-euxenite-crystals-revealed-by-stem-eels-analysis/