Atomic-scale Th and U segregation into dislocation cores and U-Pb age discordance in xenotime

The distribution of Th and U in a crystal of pegmatitic xenotime-(Y) (henceforth named xenotime) from Ås II feldspar quarry (Evje, S Norway) has been studied with transmission electron microscopy (TEM) to determine submicron- to nanoscale processes that might have affected U-(Th-)Pb age record. Xenotime contains high Th and U unaltered domains, and Th and U depleted altered domains, which contain numerous inclusions of (Th, U)-silicate and uraninite. A focused ion beam foil (FIB-foil) prepared from the unaltered xenotime, examined with transmission electron microscopy (TEM), revealed a network of ca. 15–20 nm-thick and up to several hundred nanometers-long dislocations. The dislocation cores contain U, Th and Si enriched material, and are surrounded on both sides by radiation-damaged xenotime. Rarely present “empty” dislocations are indicative of nanoscale-sized porosity in xenotime. The presence of a network of line defects in xenotime indicates plastic deformation of the xenotime crystal, which resulted in an initial stage of the formation of partially open phase boundaries. The dislocations network has been infiltrated by fluid, which mediated coupled dissolution-reprecipitation reactions. Segregation of U and Th into the dislocation cores, and selective transport of highly fluid-mobile U outside of the xenotime is suggested to be a source of Pb-excess and reverse discordance in the 207Pb/235U vs. 206Pb/238U concordia diagrams. The new finding of migration and finally segregation of U and Th into dislocation cores at the atomic-scale demonstrates the importance of nanoscale investigations for better understanding of the obtained age data and provides crucial implications for applications of xenotime geochronology.