T169 Microscopic Impact Records

Monday, 2 November 2015
1:30 PM - 5:30 PM
Room 344

PRECAMBRIAN REIDITE UNEARTHED

Paper number: T169-10
Program: Technical Sessions
Time: 3:45 PM – 4:05 PM

Authors

Abstract

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.