Nanoscale records of ancient shock deformation; reidite (ZrSiO (sub 4) ) in sandstone at the Ordovician Rock Elm impact crater

The terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion, burial, and tectonics. Zircon survives these processes as a shocked mineral, and above 20 GPa transforms to reidite, a high-pressure ZrSiO (sub 4) polymorph diagnostic of impact. However, the utility of reidite has been limited by its occurrence; it has only been reported from three relatively young (<36 Ma) impact craters globally. Here we report a new occurrence of reidite in brecciated sandstone from the Ordovician Rock Elm impact crater in Wisconsin, United States. Electron backscatter diffraction mapping was used to identify reidite and microtwins within shocked zircons smaller than 50 mu m in diameter. Reidite occurs both as 200-500-nm-wide lamellar intergrowths and as nanoparticulate grains, and not only provides the first diagnostic evidence for ultrahigh-pressure shock metamorphism at Rock Elm, but is also the oldest reported occurrence of reidite. Considering its small size, and the ubiquitous presence of detrital zircon in siliciclastic rocks, reidite may be more common in the rock record than has been reported but has potentially gone undetected. The recognition that nanoscale reidite can be preserved over deep time within zircon in shock-metamorphosed sandstone presents new opportunities for investigating Earth's impact record, as it could potentially preserve nanoscopic evidence of impact events much older than the one that formed Rock Elm. Given that shocked zircons have been shown to survive sedimentary cycling, the identification of reidite within zircons in siliciclastic rocks could facilitate investigating the impact chronology over much of the geological time scale, as the oldest terrestrial minerals known are detrital zircons.