Nanoscale evidence for uranium mobility in zircon and the discordance of U–Pb chronometers

International audienceScanning Transmission Electron Microscopy (STEM) images coupled with U–Pb isotopic analysis give new insight into mobility of U and Pb in zircon. Zircon crystals included in xenotime and in plagioclase of a pegmatitic assemblage comprising euxenite correspond to irradiation related alteration of zircon with amorphization, microporosity, swelling, radial cracking, and uptake of non-formula cations like Ca, leading to increased discordance of the U–Pb system. Fractures and pores represent the vectors for fluid mediated open system behavior. STEM images demonstrate the presence of U-rich clusters (<70 nm) with uraninite structure within the porosity. At high magnification, pores are surrounded by a low density halo (<100 nm) and are coated by a high density wall (