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Exceptional preservation of reidite in the Rochechouart impact structure, France: New insights into shock deformation and phase transition of zircon
Department of Geology Lund University Sölvegatan 12 Lund 223 62 Sweden.ORCID iD: 0000-0002-2869-5874
Swedish Museum of Natural History, Department of Geology.ORCID iD: 0000-0001-8683-3860
Jacobs—JETS Astromaterials Research and Exploration Science Division NASA Johnson Space Center 2101 NASA Parkway Houston Texas 77058 USA.
Department of Geology Lund University Sölvegatan 12 Lund 223 62 Sweden.ORCID iD: 0000-0002-4517-7972
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2021 (English)In: Meteoritics and Planetary Science, ISSN 1086-9379, E-ISSN 1945-5100, Vol. 56, no 10, p. 1795-1828Article in journal (Refereed) Published
Abstract [en]

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 stageIII) 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.

Place, publisher, year, edition, pages
2021. Vol. 56, no 10, p. 1795-1828
Keywords [en]
Space and Planetary Science, Geophysics
National Category
Geology Geochemistry
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-4405DOI: 10.1111/maps.13723OAI: oai:DiVA.org:nrm-4405DiVA, id: diva2:1617151
Funder
Swedish Research Council, 2019- 05330The Crafoord FoundationSwedish Research Council, 2015-04084Swedish Research Council, 2017-06388Available from: 2021-12-06 Created: 2021-12-06 Last updated: 2021-12-14Bibliographically approved

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