Element Redistribution and Age Resetting in Shock-Deformed Zircon from the Chicxulub Impact Structure

Abstract

Zircon has been used to chronicle the geological evolution of the Earth and other planetary bodies. In some circumstances the U-Pb radioisotopic system in zircon can be completely reset by shock metamorphism (e.g. high-pressure phase formation and reversion, and recrystallization), erasing the initial crystallization record and instead recording the impact age. These behaviors of element redistribution accompanied with structure variation in shocked zircon provide pivotal evidence to unravel the extreme impact processes. However, the contributions from a variety of shock effects to element redistribution within shocked zircons are not clear due to the complicated and protracted metamorphic processes associated with an impact event. Here we use high-resolution Nano secondary ion mass spectrometry (NanoSIMS) to show that zircon grains from the Chicxulub impact structure that contain microstructural features such as planar/irregular fractures, zircon twins, reidite and zircon granules, record three main types of element redistribution processes related to shock metamorphism and post-impact modification. The first is the preferential yttrium (Y) enrichments at the zircon-reidite boundaries that is closely related to the formation of the high-pressure polymorph reidite, but the primary zoning is preserved in reidite-bearing zircon. The second process involves shock-related heating, resulting in the solid-state transformation from reidite-bearing zircon to granular zircon, and the growth of neo-formed zircon granules. This process facilitates the loss of radiogenic lead (Pb) and allows the retain of primary zoning of uranium (U) in granular zircon due to the different element diffusion properties, thus providing the chance to date the impact event. Thirdly, the studied zircon grains within the Chicxulub impact structure experienced post-impact hydrothermal alteration to varying degrees by localized element incorporation of additional yttrium (Y), titanium (Ti), uranium (U), lead (Pb) and phosphorus (P). The U-Pb systematics altered by post-impact hydrothermal processes reveal a generally discordant line affected by the external input of U and common Pb, which could be an alternative mechanism of localized age resetting happened in shocked zircon grains. Particularly, this study demonstrates the systematic characteristics of element redistribution in shocked zircons that experienced the sequential metamorphic processes from reidite formation to growth of zircon granules, and subsequent hydrothermal alteration within the Chicxulub impact structure. These findings provide the effective constraints for behaviors and mechanisms of element redistribution and age resetting in zircon under extreme shock and post-impact metamorphic conditions in terrestrial impact craters.