¹Litasov, K.D.,²Badyukov, D.D.,¹Pokhilenko, N.P.
Doklady Earth Sciences 485, 327-330 Link to Article [DOI: 10.1134/S1028334X19030322]
¹Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090, Russian Federation
²Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, 119991, Russian Federation

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¹M. B. Biren,²J.‐A. Wartho,¹M. C. VAN Soest,¹K. V. Hodges,^3,4H. Cathey,⁵B. P. Glass,^6,7C. Koeberl,⁸J. W. Horton Jr,⁹W. Hale
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13316]
¹Group 18 Laboratories, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, 85287 USA
²GEOMAR Helmholtz Centre for Ocean Research Kiel, D‐24148 Kiel, Germany
³LeRoy Eyring Center for Solid State Science, Arizona State University, Tempe, Arizona, 84287 USA
⁴Central Analytical Research Facility, Queensland University of Technology, Brisbane, Queensland, 4000 Australia
⁵Department of Geological Sciences, University of Delaware, Newark, Delaware, 19716 USA
⁶Department of Lithospheric Research, University of Vienna, A‐1090 Vienna, Austria
⁷Natural History Museum, Burgring 7, A‐1010 Vienna, Austria
⁸U.S. Geological Survey, 926A National Center, Reston, Virginia, 20192 USA
⁹IODP Core Repository, Bremen, D‐28359 Germany
Published by arrangement with John Wiley & Sons

Single crystal (U‐Th)/He dating has been undertaken on 21 detrital zircon grains extracted from a core sample from Ocean Drilling Project (ODP) site 1073, which is located ~390 km northeast of the center of the Chesapeake Bay impact structure. Optical and electron imaging in combination with energy dispersive X‐ray microanalysis (EDS) of zircon grains from this late Eocene sediment shows clear evidence of shock metamorphism in some zircon grains, which suggests that these shocked zircon crystals are distal ejecta from the formation of the ~40 km diameter Chesapeake Bay impact structure. (U‐Th/He) dates for zircon crystals from this sediment range from 33.49 ± 0.94 to 305.1 ± 8.6 Ma (2σ), implying crystal‐to‐crystal variability in the degree of impact‐related resetting of (U‐Th)/He systematics and a range of different possible sources. The two youngest zircon grains yield an inverse‐variance weighted mean (U‐Th)/He age of 33.99 ± 0.71 Ma (2σ uncertainties n = 2; mean square weighted deviation = 2.6; probability [P] = 11%), which is interpreted to be the (U‐Th)/He age of formation of the Chesapeake Bay impact structure. This age is in agreement with K/Ar, ⁴⁰Ar/³⁹Ar, and fission track dates for tektites from the North American strewn field, which have been interpreted as associated with the Chesapeake Bay impact event.

¹Juulia-Gabrielle Moreau,^1,2Tomas Kohout,³Kai Wünnemann,⁴Patricie Halodova,^5,6Jakub Haloda
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.06.004]
¹Department of Geosciences and Geography, University of Helsinki, Finland
²Institute of Geology, The Czech Academy of Sciences, Prague, Czech Republic
³Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
⁴Centrum výzkumu Řež, Husinec-Řež, Czech Republic
⁵Czech Geological Survey, Prague, Czech Republic
⁶Oxford Instruments NanoAnalysis, Bucks, United Kingdom
Copyright Elsevier

Shock-darkening, the melting of metals and iron sulfides into a network of veins within silicate grains, altering reflectance spectra of meteorites, was previously studied using shock physics mesoscale modeling. Melting of iron sulfides embedded in olivine was observed at pressures of 40–50 GPa. This pressure range is at the transition between shock stage 5 (CS5) and 6 (CS6) of the shock metamorphism classification in ordinary and enstatite chondrites. To characterize CS5 and CS6 better with a mesoscale modeling approach and assess post-shock heating and melting, we used multi-phase (i.e. olivine/enstatite, troilite, iron, pores, and plagioclase) meshes with realistic configurations of grains. We carried out a systematic study of shock compression in ordinary and enstatite chondrites at pressures between 30 and 70 GPa. To setup mesoscale sample meshes with realistic silicate, metal, iron sulfide, and open pore shapes, we converted backscattered electron microscope images of three chondrites. The resolved macroporosity in meshes was 3–6%. Transition from shock CS5 to CS6 was observed through (1) the melting of troilite above 40 GPa with melt fractions of ~0.7–0.9 at 70 GPa, (2) the melting of olivine and iron above 50 GPa with melt fraction of ~0.001 and 0.012, respectively, at 70 GPa, and (3) the melting of plagioclase above 30 GPa (melt fraction of 1, at 55 GPa). Post-shock temperatures varied from ~540 K at 30 GPa to ~1300 K at 70 GPa. We also constructed models with increased porosity up to 15% porosity, producing higher post-shock temperatures (~800 K increase) and melt fractions (~0.12 increase) in olivine. Additionally we constructed a pre-heated model to observe post-shock heating and melting during thermal metamorphism. This model presented similar results (melting) at pressures 10–15 GPa lower compared to the room temperature models. Finally, we demonstrated dependence of post-shock heating and melting on the orientation of open cracks relative to the shock wave front. In conclusion, the modeled melting and post-shock heating of individual phases were mostly consistent with the current shock classification scheme (Stöffler et al. 2018, 2019).