Increasingly trace element concentrations on the grain scale are being used in the geosciences to understand and interpret large-scale geological processes. However this requires a fundamental assumption that trace elements can diffuse only negligible distances through a pristine crystal lattice. For example, for the reliable use of the mineral zircon (ZrSiO4) as a U-Th-Pb geochronometer, an assumption of minimal mobility of the radiogenic isotopes in geological timeframes must be made (1). Recent studies have shown that deformation can modify trace element distributions and Pb/U ratios within zircon (2) but it is not yet possible to predict how these may influence the accuracy of dating or our ability to utilise trace element data. Here, using a combination of high-end microscopy techniques, we document the effects of crystal-plastic deformation on atomic-scale elemental distributions in zircon.
Zircons within thin sections from a ~2.5 Ga old gneiss from the Napier Complex, Antarctica, were mapped using integrated EBSD/EDS in the SEM to determine regions of radiation damage (“metamict” areas) and to identify grains displaying evidence of crystal-plastic deformation. A FIB-SEM was then used to prepare atom probe tips from deformed regions in candidate grains; these tips were characterised using transmission Kikuchi diffraction (TKD) in the SEM and then evaporated in the atom probe. The grains were subsequently analysed using an ion microprobe (SHRIMP) in order to determine accurately the U-Th-Pb concentrations and to correlate the atomic scale measurements with dates.
Here we present data from several zircons and show that the movement and distribution of dislocations within the zircon lattice result in significant segregation of trace elements and, in some cases, pronounced diffusion along dislocation arrays. We discuss the significance of these findings on future studies of isotope and trace element concentrations in both zircons and other geologically important minerals.