Petrological, petrofabric, and oxygen isotopic study of five ungrouped meteorites related to brachinites

Hasegawa, Hikari; Mikouchi, Takashi; Yamaguchi, Akira; Yasutake, Masahiro; Greenwood, Richard and Franchi, Ian A. (2019). Petrological, petrofabric, and oxygen isotopic study of five ungrouped meteorites related to brachinites. Meteoritics & Planetary Science, 54(4) pp. 752–767.

DOI: https://doi.org/10.1111/maps.13249

Abstract

Northwest Africa (NWA) 6112, Miller Range (MIL) 090206 (plus its pairs: MIL 090340 and MIL 090405), and Divnoe are olivine-rich ungrouped achondrites. We investigated and compared their petrography, mineralogy, and olivine fabrics. We additionally measured the oxygen isotopic compositions of NWA 6112. They show similar petrography, mineralogy, and oxygen isotopic compositions and we concluded that these five meteorites are brachinite clan meteorites. We found that NWA 6112 and Divnoe had a c axis concentration pattern of olivine fabrics using electron backscattered diffraction (EBSD). NWA 6112 and Divnoe are suggested to have been exposed to magmatic meltflows during their crystallization on their parent body. On the other hand, the three MIL meteorites have b axis concentration patterns of olivine fabrics. This indicates that the three MIL meteorites may be cumulates where compaction of olivine grains was dominant. Alternatively, they formed as residues and were exposed to olivine compaction. The presence of two different olivine fabric patterns implies that the parent body(s) of brachinite clan meteorites experienced diverse igneous processes.

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