Baddeleyite as a widespread and sensitive indicator of meteorite bombardment in planetary crusts

Constraining the timing and intensity of Solar System bombardment is critical to understanding planetary formation, evolution and habitability. However, the identification and dating of shock-metamorphic events in the mafic igneous lithologies that dominate planetary materials remains highly challenging, particularly at relatively modest shock pressures. The accessory mineral baddeleyite (monoclinic-ZrO2) is abundant in these mafic crustal rocks, though the microstructural response of this mineral to shock-loading is currently unconstrained. Here we show that baddeleyite yields a predictable sequence of microstructures in the shock pressure range of 5 to ~20 GPa, based on analysis of mafic rocks collected along a well-constrained shock-pressure gradient in the crystalline bedrock of the Sudbury impact structure, Canada. High-resolution electron backscatter diffraction mapping of baddeleyite grains exposed to < 20 GPa of shock loading reveals a range of microstructures including complex orthogonally-related crystalline subgrains, amorphous domains and crystal plasic deformation. Evidence that the observed microstructures are controlled by the transition to and subsequent reversion from the high-pressure (experimentally constrained to > 5 GPa) orthorhombic-ZrO2 polymorph is provided by phase-heritage reconstruction using relationships between reverted monoclinic orientations. Our results demonstrate that baddeleyite is a sensitive shock indicator and barometer over low pressure ranges largely inaccessible with other minerals, augmenting ongoing studies with other mafic minerals into the intensity of inner Solar System bombardmen