The mineral zircon (ZrSiO4) is widely used in the geosciences to date geological events. Recent work has documented that the plastic deformation of zircon can modify the distribution of trace elements within the zircon lattice, thereby raising the possibility of being able to date deformation of Earth’s crust. However, the processes responsible for element migration during zircon deformation are unclear. Here we combine atom probe tomography (APT), electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD) to address this problem.
The Stac Fada Member of the Stoer Group, NW Scotland represents ejecta from a meterorite impact event about 1.2 billion years ago. EBSD data from this rock unit reveals the presence of the extremely rare high-pressure polymorph of ZrSiO4, reidite, within the host zircon. This provides unambiguous evidence of shock pressures in excess of ~30 GPa. In addition, the host zircon and reidite lamellae both contain low-angle boundaries, which are interpreted to represent recovery and the migration of shock-induced dislocations into lower energy configurations in the latter stages of the impact event. TKD analysis of one of these low-angle boundaries, captured within a FIB-milled atom probe needle, reveals a lattice disorientation of 2° across a zone of ~20 nm width. Atom probe analysis reveals elevated concentrations of Y, Al, Be and Mg within the low-angle boundary, which we interpret to reflect trace element migration within the cores of mobile dislocations during recovery. The prospect of a dynamic dislocation-migration process being responsible for geologically instantaneous trace element modification within shocked zircon has potential implications for the dating of impact events by high-spatial resolution U-Pb geochronology.