Relationships among physical properties as indicators of high temperature deformation or post-shock thermal annealing in ordinary chondrites

1,2Jon M. Friedrich, 3Alex Ruzicka, 5,6Robert J. Macke, 4James O. Thostenson, 4Rebecca A. Rudolph, 5Mark L. Rivers, 2,7,8Denton S. Ebel
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.12.039]
1Department of Chemistry, Fordham University, Bronx, NY 10458, USA
2Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
3Cascadia Meteorite Laboratory, Portland State University, Department of Geology, Portland, OR 97207-0751, USA
4Microscopy and Imaging Facility, American Museum of Natural History, New York, NY 10024, USA
5Center for Advanced Radiation Sources, University of Chicago, Argonne, IL 60439, USA
6Vatican Observatory, V-00120 Vatican City-State
7Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA
8Graduate Center of the City University of New York, New York 10016, USA
Copyright Elsevier

Collisions and attendant shock compaction must have been important for the accretion and lithification of planetesimals, including the parent bodies of chondrites, but the conditions under which these occurred are not well constrained. A simple model for the compaction of chondrites predicts that shock intensity as recorded by shock stage should be related to porosity and grain fabric. To test this model, we studied sixteen ordinary chondrites of different groups (H, L, LL) using X-ray computed microtomography (μCT) to measure porosity and metal fabric, ideal gas pycnometry and 3D laser scanning to determine porosity, and optical microscopy (OM) to determine shock stage. These included a subsample of six chondrites previously studied using transmission electron microscopy (TEM) to characterize microstructures in olivine. Combining with previous data, results support the simple model in general, but not for chondrites with low shock-porosity-foliation (low-SPF chondrites). These include Kernouvé (H6), Portales Valley (H6/7), Butsura (H6), Park (L6), GRO 85209 (L6), Estacado (H6), MIL 99301 (LL6), Spade (H6), and Queen’s Mercy (H6), among others. The data for these meteorites are best explained by high ambient heat during or after shock. Low-SPF chondrites tend to have older 40Ar/39Ar ages (∼4435-4526 Ma) than other, non-low-SPF type 6 chondrites in this study. We conclude that the H, L, and LL asteroids all were shock-compacted at an early stage while warm, with collisions occurring during metamorphic heating of the parent bodies. Results ultimately bear on whether chondrite parent bodies have internal structures more akin to a metamorphosed onion shell or metamorphosed rubble pile, and on the nature of accretion and lithification processes for planetesimals.

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