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- Holm-Alwmark, Sanna, et al.
(författare)
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Empirical constraints on progressive shock metamorphism of magnetite from the Siljan impact structure, Sweden
- 2022
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Ingår i: Geology. - 0091-7613. ; 50:3, s. 377-382
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Tidskriftsartikel (refereegranskat)abstract
- Little is known about the microstructural behavior of magnetite during hypervelocity impact events, even though it is a widespread accessory mineral and an important magnetic carrier in terrestrial and extraterrestrial rocks. We report systematic electron backscatter diffraction crystallographic analysis of shock features in magnetite from a transect across the 52-km-diameter ca. 380 Ma Siljan impact structure in Sweden. Magnetite grains in granitoid samples contain brittle fracturing, crystal-plasticity, and lamellar twins. Deformation twins along {111} with shear direction of <112> are consistent with spinel-law twins. Inferred bulk shock pressures for the investigated samples, as constrained by planar deformation features (PDFs) in quartz and shock twins in zircon, range from 0 to 20 GPa; onset of shock-induced twinning in magnetite is observed at >5 GPa. These results highlight the utility of magnetite to record shock deformation in rocks that experience shock pressures >5 GPa, which may be useful in quartz-poor samples. Despite significant hydrothermal alteration and the variable transformation of host magnetite to hematite, shock effects are preserved, which demonstrates that magnetite is a reliable mineral for preserving shock deformation over geologic time.
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| 3. |
- Plan, Anders, et al.
(författare)
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Exceptional preservation of reidite in the Rochechouart impact structure, France: New insights into shock deformation and phase transition of zircon
- 2021
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379 .- 1945-5100. ; 56:10, s. 1795-1828
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Tidskriftsartikel (refereegranskat)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 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.
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