Jon M. Friedrich^1,2, Alan E. Rubin³, Sky P. Beard⁴, Timothy D. Swindle^4,5, Clark E. Isachsen⁴, Mark L. Rivers⁶ and Robert J. Macke⁷

¹Department of Chemistry, Fordham University, Bronx, New York, USA
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
³Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA
⁴Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona, USA
⁵Department of Geosciences, The University of Arizona, Tucson, Arizona, USA
⁶Consortium for Advanced Radiation Sources, University of Chicago, Argonne, Illinois, USA
⁷Vatican Observatory, Vatican City State, Rome

We use a combination of 2D and 3D petrographic examination and ⁴⁰Ar-³⁹Ar analyses to examine the impact histories of a suite of seven ordinary chondrites (Baszkówka, Miller, NWA 2380, Mount Tazerzait, Sahara 98034, Tjerebon, and MIL 99301) that partially preserve their ancient, but postaccretionary, porosity ranging from 10 to 20%. We examine whether materials that seem to be only mildly processed (as their large intergranular pore spaces suggest) may have more complex shock histories. The ages determined for most of the seven OCs studied here indicate closure of the ⁴⁰Ar-³⁹Ar system after primary accretion, but during (Baszkówka) or shortly after (others) thermal metamorphism, with little subsequent heating. Exceptions include Sahara 98034 and MIL 99301, which were heated to some degree at later stages, but retain some evidence for the timing of thermal metamorphism in the ⁴⁰Ar-³⁹Ar system. Although each of these chondrites has olivine grains with sharp optical extinction (signaling an apparent shock stage of S1), normally indicative of an extremely mild impact history, all of the samples contain relict shock indicators. Given the high porosity and relatively low degree of compaction coupled with signs of shock and thermal annealing, it seems plausible that impacts into materials that were already hot may have produced the relict shock indicators. Initial heating could have resulted from prior collisions, the decay of ²⁶Al, or both processes.

Reference
Friedrich JM, Rubin AE, Beard SP, Swindle TD, Isachsen CE, Rivers ML and Macke RJ (in press) Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids. Meteoritics & Planetary Science
[doi:10.1111/maps.12328]
Published by arrangement with John Wiley & Sons

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