Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40 Ar/ 39 Ar systematics, and implications for the isotopic dating of impact events

Schmieder, M.; Tohver, Eric; Jourdan, Fred; Denyszyn, Steven W.; Haines, Peter W.

doi:10.1016/j.gca.2015.04.021

Cited by 52 publications

(69 citation statements)

References 161 publications

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“…These textural‐compositional types of zircon and ZrO 2 ‐bearing zircon crystals and aggregates are comparable to the variably shocked, and locally thermally decomposed, zircon grains previously studied in impact melt rock at other terrestrial impact structures (e.g., Schmieder et al. 2015b; Timms et al. ; Kenny et al.…”

Section: Samples and Analytical Techniquessupporting

confidence: 81%

In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure, Québec, Canada

Schmieder

Shaulis

Lapen

et al. 2019

Meteorit & Planetary Scien

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Laser ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U–Pb geochronology of shocked zircon grains in a vesicular‐fluidal impact melt rock from the ≥54 km Charlevoix impact structure, Québec, Canada, suggests an Ordovician to Silurian age of 450 ± 20 Ma for the impact. This age is anchored by concordant U–Pb results of ~450 Ma for a U‐rich, cryptocrystalline zircon grain in the melt rock, interpreted as a recrystallized metamict zircon crystal; the U–Th–Pb system of the metamict grain was seemingly chronometrically reset by the Charlevoix impact, but withstood later tectonometamorphic events. The new zircon age for Charlevoix is in agreement with a stratigraphically constrained Late Ordovician maximum age of ~453 Ma and corroborates earlier suggestions that the impact occurred most likely in the Ordovician, and not ~100 Myr later, as indicated by previous K/Ar and 40Ar/39Ar geochronologic results. The latter may reflect postimpact thermal overprint of impactites during the Salinian (Late Silurian to Early Devonian) and/or Acadian (Late Devonian) orogenies. U–Pb geochronology of zircon crystals in anorthosite exposed in the central uplift of the impact structure yielded a Grenvillian crystallization age of 1062 ± 11 Ma. The preferred Ordovician age for the Charlevoix impact structure, which is partially overthrusted by the Appalachian front, suggests the impact occurred during a phase of Taconian tectonism and an episode of enhanced asteroid bombardment of the Earth. Our results, moreover, demonstrate that (recrystallized) metamict zircon grains may be of particular interest in impact geochronology.

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Section: Samples and Analytical Techniquessupporting

confidence: 81%

In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure, Québec, Canada

Schmieder

Shaulis

Lapen

et al. 2019

Meteorit & Planetary Scien

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“…Unshocked zircons from impact melts generally yield the most reliable ages (e.g., Moser, 1997). Unambiguous shock-wave ages are not consistently extractable from zircon because of either incomplete U-Pb resetting during impact or subsequent Pb loss (e.g., Tohver et al, 2012;Schmieder et al, 2015).…”

Section: Dating Impacts With Zirconmentioning

confidence: 99%

“…Pb loss in planar microstructure domains can be so irregular that discordia regressions need to be "anchored" to a lower concordia intercept based on independent knowledge of impact age (Kamo et al, 1996;Moser et al, 2011;Wielicki et al, 2014). Granular-textured zircons can yield an impact age (Moser, 1997); however, granular zircons are commonly discordant, show post-impact Pb loss, and yield ages younger than impact melt zircons (Krogh et al, 1993;Kamo et al, 1996;Tohver et al, 2012;Schmieder et al, 2015). Partial age resetting, discordance, and non-systematic Pb loss clearly present challenges for dating impacts with ex situ zircons.…”

Section: Challenges Of Dating Impacts With Ex Situ Shocked Zirconsmentioning

confidence: 99%

A terrestrial perspective on usingex situshocked zircons to date lunar impacts

Cavosie

Erickson

Timms

et al. 2015

Geology

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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.

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“…; Schmieder et al. ). In contrast, pseudotachylites (see original definition of “pseudotachylyte” by Shand ; and compare, e.g., Spray ) are friction‐generated melt rocks formed from the immediate source rock within the crater basement.…”

Section: Discussionmentioning

confidence: 98%

“…, ) but can also produce statistically valid but false “alteration‐plateau” ages (e.g., Baldwin et al. ; Jourdan ; Schmieder et al ). Age spectra obtained from a hydrothermally altered melt rock should thus be considered as strict minimum values for the formation of the melt rock.…”

Section: Discussionmentioning

confidence: 99%

A Late Mesoproterozoic ⁴⁰Ar/³⁹Ar age for a melt breccia from the Keurusselkä impact structure, Finland

Schmieder

Jourdan

Moilanen³

et al. 2016

Meteorit & Planetary Scien

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Abstract-Field investigations in the eroded central uplift of the ≤30 km Keurusselk€ a impact structure, Finland, revealed a thin, dark melt vein that intersects the autochthonous shatter cone-bearing target rocks near the homestead of Kirkkoranta, close to the center of the impact structure. The petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding~8-10 GPa. The mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia. 40 Ar/ 39 Ar dating of dark and clast-poor whole-rock chips yielded five concordant Late Mesoproterozoic miniplateau ages and one plateau age of 1151 AE 10 Ma [AE 11 Ma] (2r; MSWD = 0.11; P = 0.98), considered here as the statistically most robust age for the rock. The new 40 Ar/ 39 Ar age is incompatible with~1.88 Ga Svecofennian tectonism and magmatism in south-central Finland and probably reflects the Keurusselk€ a impact, followed by impact-induced hydrothermal chloritization of the crater basement. In keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration, the Late Mesoproterozoic 40 Ar/ 39 Ar age of~1150 Ma should be treated as a minimum age for the impact. The new 40 Ar/ 39 Ar results are consistent with paleomagnetic results that suggested a similar age for Keurusselk€ a, which is shown to be one of the oldest impact structures currently known in Europe and worldwide.

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Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40 Ar/ 39 Ar systematics, and implications for the isotopic dating of impact events

Cited by 52 publications

References 161 publications

In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure, Québec, Canada

In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure, Québec, Canada

A terrestrial perspective on usingex situshocked zircons to date lunar impacts

A Late Mesoproterozoic ⁴⁰Ar/³⁹Ar age for a melt breccia from the Keurusselkä impact structure, Finland

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Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40 Ar/ 39 Ar systematics, and implications for the isotopic dating of impact events

Cited by 52 publications

References 161 publications

In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure, Québec, Canada

In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure, Québec, Canada

A terrestrial perspective on usingex situshocked zircons to date lunar impacts

A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselkä impact structure, Finland

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A Late Mesoproterozoic ⁴⁰Ar/³⁹Ar age for a melt breccia from the Keurusselkä impact structure, Finland