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Article Dans Une Revue Earth and Planetary Science Letters Année : 2015

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

Résumé

Scanning 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 (<50 nm) enriched in U or Ca or both. We interpret the spatial link between U, Ca and porosity as evidence for U uptake. Alteration of euxenite crystals into pyrochlore in the rock led to liberation of U and Ca. These elements were transported by fluids through cracks and incorporated within zircon thanks to porosity by a diffusion–reaction process. The data on this uncommon sample demonstrate that discordance of the U–Pb system in zircon affected by radiation damage includes a component of U-mobility (U-gain–U-loss) superimposed on Pb-mobility (Pb loss–Pb gain).
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Dates et versions

hal-01172200 , version 1 (07-07-2015)

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Anne-Magali Seydoux-Guillaume, Bernard Bingen, Jean-Louis Paquette, Valérie Bosse. Nanoscale evidence for uranium mobility in zircon and the discordance of U–Pb chronometers. Earth and Planetary Science Letters, 2015, 409, pp.43-48. ⟨10.1016/j.epsl.2014.10.044⟩. ⟨hal-01172200⟩
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