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Exceptional preservation of reidite in the Rochechouart impact structure, France: New insights into shock deformation and phase transition of zircon

Plan, Anders LU orcid ; Kenny, Gavin G. ; Erickson, Timmons M. ; Lindgren, Paula LU ; Alwmark, Carl LU ; Holm-Alwmark, Sanna LU ; Lambert, Philippe ; Scherstén, Anders LU and Söderlund, Ulf LU (2021) In Meteoritics and Planetary Science 56(10). p.1795-1828
Abstract

Reidite, the high-pressure zircon (ZrSiO4) polymorph, is a diagnostic indicator of impact events. Natural records of reidite are, however, scarce, occurring mainly as micrometer-sized lamellae, granules, and dendrites. Here, we present a unique sequence of shocked zircon grains found within a clast from the Chassenon suevitic breccia (shock stage III) from the ˜200 Ma, 20–50 km wide Rochechouart impact structure in France. Our study comprises detailed characterization with scanning electron microscopy coupled with electron backscatter diffraction with the goal of investigating the stability and response of ZrSiO4 under extreme P–T conditions. The shocked zircon grains have preserved various amounts of reidite... (More)

Reidite, the high-pressure zircon (ZrSiO4) polymorph, is a diagnostic indicator of impact events. Natural records of reidite are, however, scarce, occurring mainly as micrometer-sized lamellae, granules, and dendrites. Here, we present a unique sequence of shocked zircon grains found within a clast from the Chassenon suevitic breccia (shock stage III) from the ˜200 Ma, 20–50 km wide Rochechouart impact structure in France. Our study comprises detailed characterization with scanning electron microscopy coupled with electron backscatter diffraction with the goal of investigating the stability and response of ZrSiO4 under extreme P–T conditions. The shocked zircon grains have preserved various amounts of reidite ranging from 4% up to complete conversion. The grains contain various variants of reidite, including the common habits: lamellae and granular reidite. In addition, three novel variants have been identified: blade, wedge, and massive domains. Several of these crosscut and offset each other, revealing that reidite can form at multiple stages during an impact event. Our data provide evidence that reidite can be preserved in impactites to a much greater extent than previously documented. We have further characterized reversion products of reidite in the form of fully recrystallized granular zircon grains and minute domains of granular zircon in reidite-bearing grains that occur in close relationship to reidite. Neoblasts in these grains have a distinct crystallography that is the result of systematic inheritance of reidite. We interpret that the fully granular grains have formed from prolonged exposure of temperatures in excess of 1200 °C. Reidite-bearing grains with granular domains might signify swift quenching from temperatures close to 1200 °C. Grains subjected to these specific conditions therefore underwent partial zircon-to-reidite reversion, instead of full grain recrystallization. Based on our ZrSiO4 microstructural constraints, we decipher the grains evolution at specific P–T conditions related to different impact stages, offering further understanding of the behavior of ZrSiO4 during shock.

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Contribution to journal
publication status
published
subject
in
Meteoritics and Planetary Science
volume
56
issue
10
pages
1795 - 1828
publisher
Wiley-Blackwell
external identifiers
  • scopus:85112757689
ISSN
1086-9379
DOI
10.1111/maps.13723
language
English
LU publication?
yes
additional info
Funding Information: A. P. and U. S. acknowledge support from Swedish Research Council (grant #2019‐05330). A. P. acknowledges support from the Crafoord Foundation. P. L. acknowledges support from the Swedish Research Council (grant #2015‐04084). S. H.‐A. acknowledges support from the Swedish Research Council (grant #2017‐06388). We also thank the team behind CIRIR for local support and housing during A. P.s fieldwork, and for enhanced sampling of the Rochechouart impact structure. We are grateful to Osinski G. for the editorial handling of this paper, and to the two reviewers Wittmann A. and Xiao L. for their extensive and meticulous reviews which improved the manuscript greatly. Publisher Copyright: © 2021 The Authors. Meteoritics & Planetary Science published by Wiley Periodicals LLC on behalf of The Meteoritical Society (MET) Copyright: Copyright 2021 Elsevier B.V., All rights reserved.
id
80d3af34-6192-46f6-8df0-0f4069d61497
date added to LUP
2021-08-23 10:16:14
date last changed
2022-07-04 16:42:54
@article{80d3af34-6192-46f6-8df0-0f4069d61497,
  abstract     = {{<p>Reidite, the high-pressure zircon (ZrSiO<sub>4</sub>) polymorph, is a diagnostic indicator of impact events. Natural records of reidite are, however, scarce, occurring mainly as micrometer-sized lamellae, granules, and dendrites. Here, we present a unique sequence of shocked zircon grains found within a clast from the Chassenon suevitic breccia (shock stage III) from the ˜200 Ma, 20–50 km wide Rochechouart impact structure in France. Our study comprises detailed characterization with scanning electron microscopy coupled with electron backscatter diffraction with the goal of investigating the stability and response of ZrSiO<sub>4</sub> under extreme P–T conditions. The shocked zircon grains have preserved various amounts of reidite ranging from 4% up to complete conversion. The grains contain various variants of reidite, including the common habits: lamellae and granular reidite. In addition, three novel variants have been identified: blade, wedge, and massive domains. Several of these crosscut and offset each other, revealing that reidite can form at multiple stages during an impact event. Our data provide evidence that reidite can be preserved in impactites to a much greater extent than previously documented. We have further characterized reversion products of reidite in the form of fully recrystallized granular zircon grains and minute domains of granular zircon in reidite-bearing grains that occur in close relationship to reidite. Neoblasts in these grains have a distinct crystallography that is the result of systematic inheritance of reidite. We interpret that the fully granular grains have formed from prolonged exposure of temperatures in excess of 1200 °C. Reidite-bearing grains with granular domains might signify swift quenching from temperatures close to 1200 °C. Grains subjected to these specific conditions therefore underwent partial zircon-to-reidite reversion, instead of full grain recrystallization. Based on our ZrSiO<sub>4</sub> microstructural constraints, we decipher the grains evolution at specific P–T conditions related to different impact stages, offering further understanding of the behavior of ZrSiO<sub>4</sub> during shock.</p>}},
  author       = {{Plan, Anders and Kenny, Gavin G. and Erickson, Timmons M. and Lindgren, Paula and Alwmark, Carl and Holm-Alwmark, Sanna and Lambert, Philippe and Scherstén, Anders and Söderlund, Ulf}},
  issn         = {{1086-9379}},
  language     = {{eng}},
  month        = {{08}},
  number       = {{10}},
  pages        = {{1795--1828}},
  publisher    = {{Wiley-Blackwell}},
  series       = {{Meteoritics and Planetary Science}},
  title        = {{Exceptional preservation of reidite in the Rochechouart impact structure, France: New insights into shock deformation and phase transition of zircon}},
  url          = {{http://dx.doi.org/10.1111/maps.13723}},
  doi          = {{10.1111/maps.13723}},
  volume       = {{56}},
  year         = {{2021}},
}