T270 Shock Processes and Shock Attenuation Associated with Hypervelocity Impact Events

Wednesday, 7 November 2012
1:30 PM - 5:30 PM
213A

POLYSYNTHETIC TWINNING IN SHOCKED ZIRCON

Paper number: T270-9
Program: Technical Sessions
Time: 3:35 PM – 3:50 PM

Authors

Abstract

Deformation twin lamellae in shocked zircon have recently been recognized as an impact microstructure amenable to study using electron backscatter diffraction (EBSD) methods. Moser et al. (2011) first reported {1k2} twins in shocked zircon from bedrock at the Vredefort Dome impact structure using EBSD, and termed the <1 µm wide lamellae ‘microtwins’, with an orientation of 65° about [110]. Timms et al. (2012) used EBSD to document microtwins in shocked zircons from the Moon, and confirmed they occur in {112}, showing two twin orientations in a single grain; a martensitic (shear) mechanism was proposed for their formation. Erickson et al. (in press, Amer. Min.) used EBSD to describe two orientations of {112} microtwins in single detrital shocked zircons eroded from the Vredefort Dome. Erickson et al. used exterior BSE imaging and crystallographic modeling to demonstrate four {112} planar microstructures in shocked zircon, and showed that {112} microtwins in EBSD images are visible on grain exteriors as a planar microstructure. Erickson et al. further described the microtwins as polysynthetic.

We describe a new occurrence of microtwins in detrital shocked zircons, from the Sudbury impact structure, which further supports the interpretation that they are polysynthetic. EBSD analysis reveals up to 3 orientations of microtwins in Sudbury grains, the most documented so far in a single shocked zircon. Up to 23 discrete parallel twins occur in a given orientation, all oriented 65° from the host crystal. Microtwins in Sudbury zircons are in some cases better preserved than those from Vredefort, perhaps due to annealing history; rocks at Vredefort expose deeper levels of the impact structure.

In all cases, microtwins in shocked zircon occur as multiple contact twins with parallel composition surfaces, in this case {112}; this is the standard definition of polysynthetic twinning. Given that this type of twinning has not been reported from non-shocked zircons, we suggest that polysynthetic twinning in zircon may be a diagnostic impact microstructure. The formation conditions of microtwins are not well known; the apparent synchronous formation of microtwins and planar fractures constrains a lower pressure limit of 20 GPa.