Timmons M. Erickson

A terrestrial perspective on using ex situ shocked zircons to date lunar impacts

Deformed lunar zircons yielding U-Pb ages from 4333 Ma to 1407 Ma have been interpreted as dating discrete impacts on the Moon. However, the cause of age resetting in lunar zircons is equivocal; as ex situ grains in breccias, they lack lithologic context and most do not contain microstructures diagnostic of shock that are found in terrestrial zircons. Detrital shocked zircons provide a terrestrial analog to ex situ lunar grains, for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons. Electron backscatter diffraction and sensitive high-resolution ion microprobe U-Pb analysis of zircons eroded from the ca. 2020 Ma Vredefort impact structure (South Africa) show that complete impact-age resetting did not occur in microstructural domains characterized by microtwins, planar fractures, and low-angle boundaries, which record ages from 2890 Ma to 2645 Ma. An impact age of 1975 ± 39 Ma was detected in neoblasts within a granular zircon that also contains shock microtwins, which link neoblast formation to the impact. However, we show that granular texture can form during regional metamorphism, and thus is not unique to impact environments. These results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance, and then targeting specific age-reset microstructures. With the recognition that zircon can deform plastically in both impact and magmatic environments, age-resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts. Identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon-based impact chronologies for the Moon, Earth, or other planetary bodies.

Nanoscale records of ancient shock deformation: Reidite (ZrSiO4) 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 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 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 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 Earth9s 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.

Transformations to granular zircon revealed: Twinning, reidite, and ZrO2 in shocked zircon from Meteor Crater (Arizona, USA)

Granular zircon in impact environments has long been recognized but remains poorly understood due to lack of experimental data to identify mechanisms involved in its genesis. Meteor Crater in Arizona (USA) contains abundant evidence of shock metamorphism, including shocked quartz, the high-pressure polymorphs coesite and stishovite, diaplectic SiO2 glass, and lechatelierite (fused SiO2). Here we report the presence of granular zircon, a new shocked-mineral discovery at Meteor Crater, that preserve critical orientation evidence of specific transformations that occurred during formation at extreme impact conditions. The zircon grains occur as aggregates of sub-micrometer neoblasts in highly shocked Coconino Sandstone (CS) comprised of lechatelierite. Electron backscatter diffraction shows that each grain consists of multiple domains, some with boundaries disoriented by 65° around , a known {112} shock-twin orientation. Other domains have {001} in alignment with {110} of neighboring domains, consistent with the former presence of the high-pressure ZrSiO4 polymorph reidite. Additionally, nearly all zircon preserve ZrO2 + SiO2, providing evidence of partial dissociation. The genesis of CS granular zircon started with detrital zircon that experienced shock twinning and reidite formation at pressures from 20 to 30 GPa, ultimately yielding a phase that retained crystallographic memory; this phase subsequently recrystallized to systematically oriented zircon neoblasts, and in some areas partially dissociated to ZrO2. The lechatelierite matrix, experimentally constrained to form at >2000 °C, provided the ultrahigh-temperature environment for zircon dissociation (∼1670 °C) and neoblast formation. The capacity of granular zircon to preserve a cumulative pressure-temperature record has not been recognized previously, and provides a new method for investigating histories of impact-related mineral transformations in the crust at conditions far beyond those at which most rocks melt.

Empirical constraints on shock features in monazite using shocked zircon inclusions

Shock deformation microstructures in monazite have been systematically characterized for the first time in grains from the Vredefort impact structure in South Africa. Electron backscatter diffraction mapping has identified 12 unique orientations of monazite deformation twins, including 7 orientations that have not previously been described in experiments or nature. Other shock features include planar deformation bands and strain-free neoblasts, which have been shown to date deformation. Shock-twinned zircon inclusions within the deformed monazite require pressures of 20 GPa, thus providing critical empirical constraints on formation conditions, confirming a hypervelocity impact origin of the monazite microstructures. The Vredefort monazite grains described here represent the first case of using shocked mineral inclusions to empirically calibrate shock microstructures formed in the host mineral. These results conclusively establish monazite as a recorder of shock deformation, and highlight its use in identifying and dating impact structures.

Fluvial transport of impact evidence from cratonic interior to passive margin: Vredefort-derived shocked zircon on the Atlantic coast of South Africak

Abstract Meteorite impacts produce shocked minerals in target rocks that record diagnostic high-pressure deformation microstructures unique to hypervelocity processes. When impact craters erode, detrital shocked minerals can be transported by fluvial processes, as has been demonstrated through studies of modern alluvium at some of the largest known impact structures. However, the ultimate fate of distally transported detrital shocked minerals in fluvial systems is not well understood and is an important parameter for constraining the location of a source crater. In South Africa, detrital shocked minerals from the 2020 Ma Vredefort impact structure have been documented in the Vaal River basin, downriver from the structure. Here, we report results of an extensive microstructural survey of detrital zircon from the Orange River basin and the Atlantic coast of South Africa to search for the presence of far-traveled Vredefort-derived detrital shocked zircon grains in different modern sedimentary environments. Three shocked grains were found out of 11 168 grains surveyed (0.03%) by scanning electron microscopy, including two in beach sand on the Atlantic coast and one from a sandbar 15 km upstream from the mouth of the Orange River. Shock-produced {112} twins documented by electron backscatter diffraction in each of the three grains confirm their impact provenance, and U-Pb ages from 3130 to 3040 Ma are consistent with derivation from bedrock at the Vredefort impact structure. These results demonstrate the transport of Vredefort-derived shocked zircon to the coast via the Vaal-Orange river system, which requires 1940 km of fluvial transport from their point source on the Kaapvaal craton to the Atlantic coast passive margin. These results further demonstrate that shocked zircon grains can be detected in detrital populations at abundances <1%, and can ultimately be transported outside their basin of origin when they arrive at continental margins. Detrital shocked zircon thus constitutes long-lived evidence of former impacts, as they retain microstructural evidence of shock deformation, as well as geochemical (U-Th-Pb) fingerprints of their source terrain. The study of detrital shocked minerals uniquely merges impact cratering with sedimentology, as identification of detrital grains with diagnostic shock microstructures in siliciclastic sediments can be applied to search the sedimentary record for evidence of eroded impact structures of any age, from the Phanerozoic to the Hadean, which can aid in reconstructing the impact record of Earth.