Shock compaction heating and collisional processes in the production of type 3 ordinary chondrites: Lessons from the (nearly) unique L3 chondrite melt breccia Northwest Africa 8709

1Alex M. Ruzicka,2,3Jon M. Friedrich,1Melinda L. Hutson,2Juliette W. Strasser,4Robert J. Macke,5Mark L. Rivers,6Richard C. Greenwood,7Karen Ziegler,1Richard N. Pugh
Meteoritics & Planetary Science (in Press) Link to Article []
1Department of Geology and Cascadia Meteorite Laboratory, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland, Oregon, 97201 USA
2Department of Chemistry, Fordham University, 441 East Fordham Road, Bronx, New York, 10458 USA
3Department of Earth and Planetary Sciences, American Museum of Natural History, 79th Street at Central Park West, New York City, New York, 10024 USA
4Vatican Observatory, Vatican City, V‐00120 Italy
5Center for Advanced Radiation Sources, University of Chicago, Argonne, Illinois, 60439 USA
6Planetary Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
7Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87131 USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 8709 is a rare example of a type 3 ordinary chondrite melt breccia and provides critical information for the shock compaction histories of chondrites. An L3 protolith for NWA 8709 is inferred on the basis of oxygen isotope composition, elemental composition, diverse mineral chemistry, and overall texture. NWA 8709 is among the most strongly shocked type 3 chondrites known, and experienced complete melting of the matrix and partial melting of chondrules. Unmelted phases underwent FeO reduction and partial homogenization, with reduction possibly occurring by reaction of olivine and low‐Ca pyroxene with an S‐bearing gas that was produced by vaporization. Chondrules and metal grains became foliated by uniaxial compaction, and during compression, chondrules and fragments became attached to form larger clumps. This process, and possibly also melt incorporation into chondrules to cause “inflation,” may have contributed to anomalously large chondrule sizes in NWA 8709. The melt breccia character is attributed to strong shock affecting a porous precursor. Data‐model comparisons suggest that a precursor with 23% porosity that was impacted by a 3 km/s projectile could have produced the meteorite. The rarity of other type 3 ordinary chondrite melt breccias implies that the immediate precursors to such chondrites were lower in porosity than the NWA 8709 precursor, or experienced weaker shocks. Altogether, the data imply a predominantly “quiet” dynamical environment to form most type 3 ordinary chondrites, with compaction occurring in a series of relatively weak shock events.


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