Multiple P-T-t paths revealed by granular zircon open new horizons in impact research

• Impactites from two large terrestrial impact structures – Araguainha and Kara – were studied. • At least five discernible granular zircon types were confirmed using transmission electron microscopy. • Each granular zircon type corresponds to its unique P-T -t path. • Granular zircon is a single-grain geothermobarometer for impact process. • It covers a wide range of pressure, temperature and time. Extreme conditions of hypervelocity impacts are recorded in mineral grains that can be used to reconstruct the peak P-T conditions and the post-impact history of the target. Shock-metamorphosed zircons with granular textures can preserve shock metamorphic conditions and ages of impact events, but the mechanisms creating a variety of granular textures have not been fully explored. We demonstrate through detailed TEM analysis of granular zircon aggregates from two complex peak-ring impact structures (Kara, Russia; Araguainha, Brazil) that there are at least five discernible textural types of granular zircon neoblasts: (I) granular zircon reverted from reidite; (II) incongruent melting/dissociation and new crystallization of zircon; (III) solid-state recrystallization of zircon by grain boundary migration; (IV) shearing of zircon under elevated differential stress; (V) recrystallization of zircon with formation of radiating lamellae. Each type shows a unique P-T -t-cooling path related to shock metamorphic conditions. Therefore, granular zircon is not only a clear indicator of the impact process, but the granule types record a wider variety of extreme conditions than any other mineral phase.