Cavosie, and two co-authors, reported in 2015 that 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 ZrSiO4 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. The authors reported herein on 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 µm 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 ultra high 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.
The authors concluded that Given the ubiquitous occurrence of detrital zircon in siliciclastic rocks, these findings highlight the preservation potential of reidite in shocked sandstone, a reservoir not generally recognized for preservation of ultrahigh-pressure shock records. For ancient impact structures that have since eroded, reidite may be preserved in detrital shocked zircons that have been eroded and transported, and now reside in younger sedimentary rocks. Shocked zircons with diagnostic shock microstructures are ubiquitous in modern alluvium eroded from complex craters, such as the Vredefort Dome (Cavosie et al., 2010; Erickson et al., 2013b), Sudbury (Thomson et al., 2014), and Santa Fe impact structures (southwest United States) (Lugo and Cavosie, 2014).
With the recognition that Hadean detrital zircons with ages as old as 4.4 Ga are preserved (Valley et al., 2014), reidite may also provide a record of even earlier impact bombardment. Identification of reidite in ancient detrital zircons could provide ground-truth evidence from the terrestrial record of early impact events on Earth.
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