The brecciated texture of polymict eucrites: Petrographic investigations of unequilibrated meteorites from the Antarctic Yamato collection

1Christian Vollmer,1Stella Rombeck,2Julia Roszjar,3Adam R. Sarafian,1Stephan Klemme
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13453]
1Institut für Mineralogie, Westfälische Wilhelms‐Universität Münster, Corrensstrasse 24, D‐48149 Münster, Germany
2Department of Mineralogy and Petrography, Natural History Museum Vienna, Burgring 7, 1010 Vienna, Austria
3Science and Technology Division, Corning Incorporated, 21 Lynn Morse Rd., Painted Post, New York, New York, 14870 USA
Published by arrangement with John Wiley & Sons

We report on the petrography and mineralogy of five Yamato polymict eucrites to better constrain the formation and alteration of crustal material on differentiated asteroids. Each sample consists of different lithic clasts that altogether form four dominant textures and therefore appear to originate from closely related petrological areas within Vesta′s crust. The textures range from subophitic to brecciated, porphyritic, and quench‐textured, that differ from section to section. Comparison with literature data for these samples is therefore difficult, which stresses that polymict eucrites are extremely complex in their petrography and investigation of only one thick section may not be representative for the host rock. We also show that sample Y‐793548 consists of more than one lithic unit and must therefore be classified as polymict instead of monomict. The variety and nature of lithic textures in the investigated Yamato meteorites indicate shock events, intense post‐magmatic thermal annealing, and secondary alteration. These postmagmatic features occur in different intensities, varying from clast to clast or among coexisting mineral fragments on a small, local scale. Several clasts within the eucrites studied have been modified by late‐stage alteration processes that caused deposition of Fe‐rich olivine and Fe enrichment along cracks crosscutting pyroxene crystals. However, formation of these secondary phases seems to be independent of the degree of thermal metamorphism observed within every type of clast, which would support a late‐stage metasomatism model for their formation.

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