Petrology and mineralogy of an igneous clast in the Northwest Africa 1685 (LL4) chondrite: Comparison with alkali-rich igneous clasts in LL-chondritic breccias

1,2Takafumi Niihara,3,4Tatsunori Yokoyama,5Tomoko Arai,3,6Keiji Misawa
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13719]
1Department of Systems Innovation, School of Engineering, University of Tokyo, Hongo 7-3-1, Tokyo, 113-8656 Japan
2University Museum, University of Tokyo, Hongo 7-3-1, Tokyo, 113-0033 Japan
3Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), 10-3 Midoricho, Tachikawa, Tokyo, 190-8518 Japan
4Tono Geoscience Center, Japan Atomic Energy Agency, 959-31 Izumicho Jorinji, Toki, Gifu, 509-5102 Japan
5Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba, 275-0016 Japan
6National Institute of Polar Research, 10-3 Midoricho, Tachikawa, Tokyo, 190-8518 Japan
Published by arrangement with John Wiley & Sons

We have conducted petrological and mineralogical studies on an igneous clast in the Northwest Africa (NWA) 1685 (LL4) chondrite. This meteorite was described in the Meteoritical Bulletin as containing clasts similar to alkali-rich clasts in LL chondritic breccias Yamato (Y)-74442 (LL4), Bhola (LL3-6), and Krähenberg (LL5). We carefully compared the textures, as well as mineral and matrix compositions, of the NWA 1685 clast with those of the previously described alkali-rich clasts in the LL chondritic breccias. Olivine grains are embedded in glassy matrix and have no chemical zoning. Shock melt veins and fractures were observed only in olivine grains and did not continue into matrix. Potassium abundance of matrix glasses of the NWA 1685 clast is lower than those of alkali-rich igneous clasts in Y-74442, Bhola, and Krähenberg, indicating that the igneous clasts in NWA 1685 are different from the alkali-rich clasts previously reported in the LL chondritic breccias. The NWA 1685 clast might have formed during an impact melting and quenching event on the LL-chondrite parent body, and then been incorporated into a breccia.

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