The formation of large neoblasts in shocked zircon and their utility in dating impacts

Uranium-lead (U-Pb) geochronology of individual shocked zircon grains has unique potential for dating bolide impact events. Neoblasts in granular-textured zircon have been recognized as the shock-related feature most effective at recording the impact age. Here we report the discovery of large neoblasts (5-100 mu m in dimension) in shocked zircon at the Sudbury impact structure, Canada-the first report of in situ coarsely granular zircon from a terrestrial impact site other than the Vredefort structure, South Africa. The neoblast-bearing sample was taken from a heterogeneous, lithic clast-rich igneous unit associated with the roof rocks of the impact melt sheet, making this the first time a crater has been dated using neoblastic zircon from the upper part of its stratigraphy. Previous in situ discoveries of coarsely granular zircon at Vredefort were all in impact-generated mafic melt emplaced beneath the impact melt sheet. Electron backscatter diffraction analysis of the impact-aged neoblasts indicates that the high-pressure conditions inferred in the formation of many small neoblasts were not necessarily involved in the formation of these large ones. Their large size, internal zonation, and occurrence in a slowly cooling environment collectively suggest that large neoblasts at Sudbury formed by relatively protracted, post-impact growth in shocked zircon incorporated into impact-related melt. Based on insight from large neoblast growth in terrestrial settings, we suggest that the ca. 4.33 Ga neoblasts recently reported in lunar zircon may imply a major basin-forming event on the Moon at that time. New knowledge of the cratering environments in which large neoblasts form also raises the prospect of possibly linking ex situ granular zircon in lunar breccias with specific impact structures-and thus better calibrating the lunar cratering record with radiometric ages.