Nanoscale constraints on the nucleation and evolution of granular zircon from reidite in impactites at the Chicxulub impact structure

Zircon with granular texture from hypervelocity impact structures can be used to estimate the thermodynamic conditions of impact processes, including pressure and temperature, and in some cases the timing of impact events via U-Pb geochronology. However, two disparate formation models have been proposed to explain the occurrence of zircon neoblasts that preserve systematic orientation relations; one involves zircon-reidite phase transformations (FRIGN zircon), whereas the other features melting and thermal dissociation of zircon in the absence of reidite. Distinguishing between these models is hampered by the lack of observational constraints on the intermediate transformation steps at nanoscale, and what processes give rise to observed systematic orientation relations among zircon neoblasts. Here we report new analyses of reidite-bearing and granular zircons from peak ring core samples of the Chicxulub impact structure using nanoscale methods. We describe lamellar and lense-like reidite habits associated with reidite twins in shocked zircon from impact melt-bearing breccia, along with the first observation of nanoscale zircon granules forming locally within preserved reidite lamellae. The crystallographic orientation of the zircon nano-granules matches the orientations predicted by the FRIGN zircon model, confirming they formed directly by solid-state reversion of reidite to zircon, and represent the earliest stages of the formation of granular zircon. Minor occurrences of baddeleyite at the interface of reidite and neoblastic zircon domains suggest that the reversion of reidite to zircon can occur together with local ZrSiO4 dissociation driven either by the loss of SiO2, which creates excess zirconia, or by local thermal dissociation of reidite. Other partially- and fully-granular zircon grains from the same impact melt-bearing breccia also preserve systematic orientation relationships among zircon neoblasts, consistent with having transformed directly from reidite. The observation that zircon neoblasts maintain systematic orientations from nanoscale to microscale in granular zircon supports the idea that neoblast orientations are encoded at the nucleation stage via solid state phase transformation. Observations in this study provide direct evidence to explain the nature of systematic highpressure phase transitions involving zircon and have implications for unraveling the pressure-temperature history of zircon phase transitions in large impacts on Earth or other planetary bodies.