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      Dendritic reidite from the Chesapeake Bay impact horizon, Ocean Drilling Program Site 1073 (offshore northeastern USA): A fingerprint of distal ejecta?

      1 , 2 , 2 , 3 , 4 , 5
      Geological Society of America

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          High-pressure minerals provide records of processes not normally preserved in Earth’s crust. Reidite, a quenchable polymorph of zircon, forms at pressures >20 GPa during shock compression. However, there is no broad consensus among empirical, experimental, and theoretical studies on the nature of the polymorphic transformation. Here we decipher a multistage history of reidite growth recorded in a zircon grain in distal impact ejecta (offshore northeastern United States) from the ca. 35 Ma Chesapeake Bay impact event which, remarkably, experienced near-complete conversion (89%) to reidite. The grain displays two distinctive reidite habits: (1) intersecting sets of planar lamellae that are dark in cathodoluminescence (CL); and (2) dendritic epitaxial overgrowths on the lamellae that are luminescent in CL. While the former is similar to that described in literature, the latter has not been previously reported. A two-stage growth model is proposed for reidite formation at >40 GPa in Chesapeake Bay impact ejecta: formation of lamellar reidite by shearing during shock compression, followed by dendrite growth, also at high pressure, via recrystallization. The dendritic reidite is interpreted to nucleate on lamellae and replace damaged zircon adjacent to lamellae, which may be amorphous ZrSiO4 or possibly an intermediate phase, all before quenching. These results provide new insights on the microstructural evolution of the highpressure polymorphic transformation over the microseconds-long interval of reidite stability during meteorite impact. Given the formation conditions, dendritic reidite may be a unique indicator of distal ejecta.

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          Geological Society of America
          October 07 2020
          October 07 2020
          [1 ]Space Science and Technology Centre and The Institute for Geoscience Research, School of Earth and Planetary Science, Curtin University, Perth, Western Australia 6102, Australia
          [2 ]School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA
          [3 ]GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany
          [4 ]U.S. Geological Survey, 926A National Center, Reston, Virginia 20192, USA
          [5 ]Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
          © 2020


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