We explore the formation conditions and inheritance probability of zircon in impact melts and the implications of using zircon geochronology to investigate planetary impact histories. By modeling the occurrence and crystallization temperature spectrum for zircon in simulated impact melts, we predict the presence of such grains within impactites. We also report U-Pb geochronology of sieve-textured, possibly poikilitic, zircon identified in the pseudotachylyte and granophyre units present within the largest known terrestrial impact crater (Vredefort, South Africa) to explore the accuracy of these grains in dating impact events at an impact structure of known age. Zircons with similar textures have been recently interpreted as growing in an impact melt in lunar meteorite SaU 169 and used to determine the age of the Imbrium impact. Modeling in simulated lunar melt compositions predicts crystallization of zircon in merely ~2% of melting events, in this case via impact. The modeled crystallization temperature spectrum is significantly below Ti-in-zircon crystallization temperatures reported from lunar samples. Zircon formation within an impact melt is dictated by saturation of [Zr] and requires a high abundance for lunar melt compositions. This essentially rules out the possibility of zircon growing in equilibrium with lunar meteorites. Poikilitic textures may be inherited from the lunar crust, presumably due to rapid decompression and/or resorption into an undersaturated magma, as previously recognized in plagioclase. Although either scenario could be due to an impact, endogenic processes cannot be ruled out, and thus lunar poikilitic zircons may not be recording impact melting events. Secondary ion mass spectrometry U-Pb analysis of zircon with similar textures from Vredefort clearly shows that these grains are inherited from the Archean target rocks, with varying degrees of Pb loss, and consequently cannot be used to accurately identify the age of the Vredefort impact structure. Further understanding of the growth and isotopic effects on zircon of shock and heating associated with large impacts could provide another tool that can be used to probe planetary impact histories.

The 1.85 Ga Sudbury impact structure is widely accepted to be the erosional remnant of a tectonized 200- to 250-km-diameter multiring basin. The Garson Member of the Onaping Formation in the Whitewater Group immediately overlies the coherent impact melt sheet—the so-called Sudbury igneous complex. It is of particular interest because it is poorly characterized, and its formational history is not well understood. The Garson Member is up to 500 m thick and is restricted locally, occurring along a 25 km strike length over the southeastern lobe of the Sudbury igneous complex. Detailed mapping and sampling of the Garson Member indicate that the dominant clast type is quartzite (>99%), with a few granite and quartz arenite clasts, and with a large range in clast shape and size. Partially annealed, decorated planar deformation features in quartz grains occur within the quartzite clasts, providing evidence for shock metamorphism. This, accompanied by the recrystallized, but equigranular nature of the groundmass, suggests that the Garson Member originated as a clast-rich impact melt rock. Geochemical analysis suggests a close relationship between the Garson Member and the so-called Onaping intrusion. The Garson Member and the Onaping intrusion, which has recently been interpreted as the remnants of the roof rocks of the Sudbury igneous complex, are located within the same stratigraphic context of the impact structure. We suggest, therefore, that the Garson Member and the Onaping intrusion share a similar origin as roof rocks to the Sudbury igneous complex, and they are, therefore, not part of the Whitewater Group but rather the Sudbury igneous complex.

For asteroid or comet impacts, the mass of the projectile or bolide and its velocity control the scale of damage and secondary catastrophes induced, and the impact flux can be used to determine whether such an impact was likely to occur at the time of interest. Impact cratering processes are still orders of magnitude more deadly than volcanism when considering the potential for atmospheric loading of deleterious particulate and gaseous materials, due to the extraordinarily rapid transfer of energy. Based on impact flux, there could have been sufficient large impactors to cause one or more of the “Big Five” mass extinctions in the last 300 m.y. The best contender so far is the Chicxulub event, but this did not trigger massive volcanism in situ, and the Deccan volcanism was not located correctly to be its antipodal pair. The combination of volcanism with impact cratering is a real possibility for the end-Cretaceous extinction, but there is no established connection. This contribution reviews the wider aspects of impact volcanism, including impact fluxes, impact melting, crater thermal anomalies, and secondary impact crises like antipodal volcanism in the context of Phanerozoic mass extinctions.