Hypervelocity impact processes are uniquely capable of generating shock metamorphism, which causes mineralogical transformations and deformation that register pressure (P) and temperature (T) conditions far beyond even the most extreme conditions created by terrestrial tectonics. The mineral zircon (ZrSiO 4) responds to 26 shock deformation is various ways, including crystal-plasticity, twinning, 27 polymorphism (e.g., transformation to the isochemical mineral reidite), formation of 28 granular texture, and dissociation to ZrO 2 + SiO 2 , which provide robust 29 thermobarometers that record different extreme conditions. The importance of 30 understanding these material processes is twofold. First, these processes can mobilize 31 and redistribute trace elements, and thus be accompanied by variable degrees of 32 resetting of the U-Pb system, which is significant for the use of zircon as a 33 geochronometer. Second, some features described herein form exclusively during 34 shock events and are diagnostic criteria that can be used to confirm the hypervelocity 35 origin of suspected impact structures. We present new P-T diagrams showing the 36 phase relations of ZrSiO 4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints. We present case studies to illustrate zircon microstructures formed in extreme environments, and present electron backscatter diffraction data for grains from three impact structures (Mistastin Lake of Canada, Ries of Germany, and Acraman of Australia) that preserve different minerals and microstructures associated with different shock conditions. For each locality, we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the P-T history. We outline a conceptual framework for a zircon-based approach to 'extreme thermobarometry' that incorporates both direct observation of high-P and high-T phases, as well as inferences for the former existence of phases from orientation relationships in recrystallised products, a concept we refer to here as 'phase heritage'. This new approach can be used to unravel the pressure-temperature history of zircon-bearing samples that have experienced extreme conditions, such as rocks that originated in the Earth's mantle, and those shocked during impact events on Earth and other planetary bodies.

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Deformed lunar zircons yielding U-Pb ages from 4333 Ma to 1407 Ma have been interpreted as dating discrete impacts on the Moon. However, the cause of age resetting in lunar zircons is equivocal; as ex situ grains in breccias, they lack lithologic context and most do not contain microstructures diagnostic of shock that are found in terrestrial zircons. Detrital shocked zircons provide a terrestrial analog to ex situ lunar grains, for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons. Electron backscatter diffraction and sensitive high-resolution ion microprobe U-Pb analysis of zircons eroded from the ca. 2020 Ma Vredefort impact structure (South Africa) show that complete impact-age resetting did not occur in microstructural domains characterized by microtwins, planar fractures, and low-angle boundaries, which record ages from 2890 Ma to 2645 Ma. An impact age of 1975 ± 39 Ma was detected in neoblasts within a granular zircon that also contains shock microtwins, which link neoblast formation to the impact. However, we show that granular texture can form during regional metamorphism, and thus is not unique to impact environments. These results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance, and then targeting specific age-reset microstructures. With the recognition that zircon can deform plastically in both impact and magmatic environments, age-resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts. Identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon-based impact chronologies for the Moon, Earth, or other planetary bodies.