A Numerical Model for Twin Nucleation in Shocked Zircon and Comparison With Natural Samples

Abstract

Twins in zircon are increasingly being used as an indicator of shock metamorphism on Earth and other planetary bodies, yet the conditions of their formation have not been quantified. We modeled the shear stress involved with homogeneous nucleation of the most common zircon twin false(112false)false[true1̄true1̄1false]. This stress is between 1.7 to 2.6 GPa for the range of crystal sizes common to the crust, stresses that are restricted to impact deformation regimes as opposed to tectonism. The twin nucleation stress is largely pressure independent, and it is likely that twins can form over a much larger range of shock pressures than is currently appreciated. This prediction is validated by the occurrence of twins in shocked zircon from distal regions of the 2.02 Ga Vredefort impact structure.

Material mixing in shock-induced pseudotachylites, Vredefort impact structure, South Africa

“…This shock deformation must have occurred prior to fragmentation and granite clast incorporation into the dolerite. Extreme differential stress, required to form shock microtwins in zircon (Jones et al, 2018), most likely corresponds to the interaction of the rarefaction wave with the shock wave (Cox et al, 2018;Huber and Kovaleva, 2019). Because the host PT vein could not have formed before the passage of the shock wave, the granite must have been shocked before its fragments were incorporated into the PT vein.…”