Complex nanostructures in shocked, annealed and metamorphosed baddeleyite defined by atom probe tomography

Chemical and isotopic signatures recorded by the accessory phase baddeleyite (ZrO2) yield important insights into the formation and evolution of mafic planetary crusts. However, little work has been conducted regarding the effects of microstructures on the mobilization and diffusion of substitutional and interstitial ions. Coupled nanometer-scale analyses of chemistry and structure in mineral phases is possible using the emerging technique of atom probe tomography (APT). Here we use this technique to describe a range of complex chemical nanostructures within shocked, annealed and metamorphosed baddeleyite grains sampled in crater floor rocks ~550m away from the contact with the Sudbury impact melt sheet. This has revealed a wide range of nanostructural phenomena, including domains of clustered incompatible cations (Fe), separated by high-angle subgrain boundaries or low-angle planar features exhibiting wave-like features decorated with trace amounts of Al, Si and Fe, likely generated by shock metamorphism. In some cases, these nanostructures have facilitated much later, and highly localized, post-impact Pb loss and Si gain ascribed to regional greenschist metamorphism. Characterizing nanoscale heterogeneities within complex, shocked baddeleyite grains using APT may allow for resolution of different deformation pathways and a more confident interpretation of the geologic significance of micron-scale trace element and isotopic analyses.