The Rietputs Formation in South Africa

Cavosie, A. J.; Erickson, Timmons M.; Montalvo, Pedro; Prado, Diana C.; Cintron, Nadja O.; Gibbon, Ryan J.

doi:10.1002/9781119227250.ch9

Cited by 12 publications

(12 citation statements)

References 77 publications

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“…SPE550-20 1st pgs page 13 domains measured by SIMS. In this regard, the U-Pb system in shock-twinned xenotime is thus similar to that in shock-twinned zircon, where Pb loss does not record the age of impact (e.g., Erickson et al, 2013b;Cavosie et al, 2015Cavosie et al, , 2018Montalvo et al, 2017;Montalvo et al, 2019). Second, radioisotope ages of ca.…”

Section: Significance Of Pb Loss In Deformed Xenotimementioning

confidence: 94%

Extreme plastic deformation and subsequent Pb loss in shocked xenotime from the Vredefort Dome, South Africa

Cavosie

Kirkland

Reddy

et al. 2021

Large Meteorite Impacts and Planetary Evolution VI

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Accessory mineral U-Pb geochronometers are crucial tools for constraining the timing of deformation in a wide range of geological settings. Despite the growing recognition that intragrain age variations within deformed minerals can spatially correlate to zones of microstructural damage, the causal mechanisms of Pb loss are not always evident. Here, we report the first U-Pb data for shock-deformed xenotime, from a detrital grain collected at the Vredefort impact structure in South Africa. Orientation mapping revealed multiple shock features, including pervasive planar deformation bands (PDBs) that accommodate up to 40° of lattice misorientation by <100>{010} slip, and also an ~50-µm-wide intragrain shear zone that contains {112} deformation twin lamellae in two orientations. Twenty-nine in situ secondary ion mass spectrometry (SIMS) U-Pb analyses from all microstructural domains yielded a well-defined discordia with upper-intercept age of 2953 ± 15 Ma (mean square of weighted deviates [MSWD] = 0.57, n = 29, 2σ), consistent with derivation from Kaapvaal craton bedrock. However, the 1754 ± 150 Ma lower concordia intercept age falls between the 2020 Ma Vredefort impact and ca. 1100 Ma Kibaran orogenesis and is not well explained by multiple Pb-loss episodes. The pattern and degree of Pb loss (discordance) correlate with increased [U] but do not correlate to microstructure (twin, PDB) or to crystallinity (band contrast) at the scale of SIMS analysis. Numerical modeling of the Pb-loss history using a concordia-discordia-comparison (CDC) test indicated that the lower concordia age is instead best explained by an alteration episode at ca. 1750 Ma, rather than a multiple Pb-loss history. In this example, the U-Pb system in deformed xenotime does not record a clear signature of impact age resetting; rather, the implied high dislocation density recorded by planar deformation bands and the presence of deformation twins facilitated subsequent Pb loss during a younger event that affected the Witwatersrand basin. Microstructural characterization of xenotime targeted for geochronology provides a new tool for recognizing evidence of deformation and can provide insight into complex age data from highly strained grains, and, as is the case in this study, elucidate previously unrecognized alteration events.

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Section: Significance Of Pb Loss In Deformed Xenotimementioning

confidence: 94%

Extreme plastic deformation and subsequent Pb loss in shocked xenotime from the Vredefort Dome, South Africa

Cavosie

Kirkland

Reddy

et al. 2021

Large Meteorite Impacts and Planetary Evolution VI

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“…Detrital shocked quartz, monazite, xenotime, and zircon eroded from the Vredefort Dome have been reported within the structure (Cavosie et al, 2010(Cavosie et al, , 2021, and at distances up to nearly ~2000 km downstream from the impact structure in modern alluvium from the Vaal and Orange rivers (Erickson et al, 2013;Montalvo et al, 2017). Detrital shocked minerals have also been reported in mid-Pleistocene (~1.7-1.3 Ma) fluvial terraces of the Vaal River up to ~750 km downstream from the Vredefort Dome (Cavosie et al, 2018a), and in Permian glacial deposits (diamictite and tillite) of the ca. 300 Ma Dwyka Group in a southwest (down-ice) direction from the Vredefort structure (Pincus et al, 2015).…”

Section: Detrital Shocked Mineralsmentioning

confidence: 96%

“…ZrSiO 4 (Cavosie et al, 2018a) Stishovite High pressure polymorph of SiO 2, forming at shock pressures ≥10 GPa.…”

Section: Frign Zirconmentioning

confidence: 99%

Impact Earth: A review of the terrestrial impact record

Osinski

Grieve

Ferrière

et al. 2022

Earth-Science Reviews

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“…The impact structure was buried after the impact and then partially exhumed ca. 1 Ga later (Kovaleva et al., 2019), and then affected by Permian glaciation (Cavosie et al., 2017). More recently, the rocks of the Karoo supergroup buried the structure, and Jurassic dolerite dikes cut through some portions of the crater.…”

Section: Geological Settingmentioning

confidence: 99%

Can Archean Impact Structures Be Discovered? A Case Study From Earth's Largest, Most Deeply Eroded Impact Structure

Huber,

Kovaleva,

Rae

et al. 2023

JGR Planets

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The record of terrestrial impact events is incomplete with no Archean impact structures discovered, despite the expected abundance of collisions that must have occurred. Because no Archean impact structures have been identified, the necessary conditions to preserve an impact structure longer than 2 Byr are unknown. One significant effect of shock metamorphism is that the physical properties of the target rocks change, resulting in distinctive geophysical signatures of impact structures. To evaluate the preservation potential of impact structures, we evaluate the deeply eroded Proterozoic Vredefort impact structure to examine the changes in physical properties and the remnant of the geophysical signature and compare the results with the well‐preserved Chicxulub impact structure. The major structural features of Vredefort are similar to the expected profile of the Chicxulub structure at a depth of 8–10 km. The Vredefort target rocks, while shocked, do not preserve measurable changes in their physical properties. The gravity signature of the impact structure is minor and is controlled by the remnant of the collapsed transient crater rim and the uplifted Moho surface. We anticipate that erosion of the Vredefort structure by an additional 1 km would remove evidence of impact, and regardless of initial size, erosion by >10 km would result in the removal of most of the evidence for any impact structure from the geological record. This study demonstrates that the identification of geologically old (i.e., Archean) impact structures is limited by a lack of geophysical signatures associated with deeply eroded craters.

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The Rietputs Formation in South Africa

Cited by 12 publications

References 77 publications

Extreme plastic deformation and subsequent Pb loss in shocked xenotime from the Vredefort Dome, South Africa

Extreme plastic deformation and subsequent Pb loss in shocked xenotime from the Vredefort Dome, South Africa

Impact Earth: A review of the terrestrial impact record

Can Archean Impact Structures Be Discovered? A Case Study From Earth's Largest, Most Deeply Eroded Impact Structure

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