Shocked chromites in fossil L chondrites: A Raman spectroscopy and transmission electron microscopy study

1,2Surya S. Rout,1,2,3Philipp R. Heck,4Nestor J. Zaluzec,5Takayuki Ishii,6Jianguo Wen,6Dean J. Miller,1,7Birger Schmitz
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12887]
1Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, Illinois, USA
2Chicago Center for Cosmochemistry, The University of Chicago, Chicago, Illinois, USA
3Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
4Photon Science Division, Argonne National Laboratory, Argonne, Illinois, USA
5Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, Germany
6Electron and X-ray Microscopy, Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, USA
7Astrogeobiology Laboratory, Department of Physics, Lund University, SE, Lund, Sweden
Published by arrangement with John Wiley & Sons

Chromites from Middle Ordovician fossil L chondrites and from matrix and shock-melt veins in Catherwood, Tenham, and Coorara L chondrites were studied using Raman spectroscopy and TEM. Raman spectra of chromites from fossil L chondrites showed similarities with chromites from matrix and shock-melt veins in the studied L chondrite falls and finds. Chromites from shock-melt veins of L chondrites show polycrystallinity, while the chromite grains in fossil L chondrites are single crystals. In addition, chromites from shock-melt veins in the studied L chondrites have high densities of planar fractures within the subgrains and many subgrains show intergrowths of chromite and xieite. Matrix chromite of Tenham has similar dislocation densities and planar fractures as a chromite from the fossil meteorite Golvsten 001 and higher dislocation densities than in chromite from the fossil meteorite Sextummen 003. Using this observation and knowing that the matrix of Tenham experienced 20–22 GPa and <1000° C, an upper limit for the P,T conditions of chromite from Golvsten 001 and Sextummen 003 can be estimated to be 20–22 GPa and 1000° C (shock stage S3–S6) and 20 GPa and 1000° C (S3–S5), respectively, and we conclude that the studied fossil meteorite chromites are from matrix.

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