Empirical constraints on shock features in monazite using shocked zircon inclusions

Abstract

Shock deformation microstructures in monazite have been systematically characterized for the first time in grains from the Vredefort impact structure in South Africa. Electron backscatter diffraction mapping has identified 12 unique orientations of monazite deformation twins, including 7 orientations that have not previously been described in experiments or nature. Other shock features include planar deformation bands and strain-free neoblasts, which have been shown to date deformation. Shock-twinned zircon inclusions within the deformed monazite require pressures of 20 GPa, thus providing critical empirical constraints on formation conditions, confirming a hypervelocity impact origin of the monazite microstructures. The Vredefort monazite grains described here represent the first case of using shocked mineral inclusions to empirically calibrate shock microstructures formed in the host mineral. These results conclusively establish monazite as a recorder of shock deformation, and highlight its use in identifying and dating impact structures.