PRECAMBRIAN REIDITE UNEARTHED

The ~1.18 Gyr Stac Fada Member of the Stoer Group, NW Scotland represents a Mesoproterozoic impactite. Zircon from the unit has been characterized by state-of-the-art micro- to sub-nanometer imaging. In two zircon grains (1% of grains analysed) cathodoluminescence reveals variably developed < 2 µm wide lamellae within the host zircon. Electron backscatter diffraction (EBSD) data from these lamellae establish that they are the rare, ZrSiO₄ polymorph reidite. This is the only extant example of reidite in the Precambrian rock record and provides unambiguous evidence of shock pressures in excess of ~30 GPa. EBSD data also confirm the previously reported crystallographic relationship {100}_Z//{112}_R and [001]_Z//<110>_R between the host zircon and reidite. However, focused ion beam milling and transmission Kikuchi diffraction (TKD) analysis of a thin foil, taken perpendicular to the EBSD-mapped sample surface, reveals that the interface of the zircon and reidite lamellae are oblique to these crystallographic planes. This observation is not consistent with reidite formation by the purported {100}[001]_. martensitic transformation of zircon.

In detail, the reidite lamellae are locally deformed and EBSD mapping at 50 nm resolution shows the sites of deformation to comprise baddeleyite (ZrO₂) and an amorphous phase, interpreted to be silica. This observation marks the first natural example of reidite decomposition to ZrO₂. 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, Be and Al 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 trace element modification within shocked zircon has potential implications for the dating of impact events by high-spatial resolution U-Pb geochronology.