Elucidation of impact event recorded in the lherzolitic shergottite NWA 7397

1Masashi Yoshida,1Masaaki Miyahara,1,2,3Hiroki Suga,4,5Akira Yamaguchi,6Naotaka Tomioka,7Takeshi Sakai,7,8Hiroaki Ohfuji,8Fumiya Maeda,7,8,9Itaru Ohira,8Eiji Ohtani,10Seiji Kamada,11Takuji Ohigashi,11Yuichi Inagaki,12Yu Kodama,3Naohisa Hirao
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13735]
1Graduate School of Advanced Science and Engineering, Hiroshima University, Hiroshima, Higashi, 739-8526 Japan
2Department of Earth and Planetary, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo, Bunkyo-ku, 113-0033 Japan
3Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto Sayo, Hyogo, 679-5198 Japan
4National Institute of Polar Research, Tokyo, 190-8518 Japan
5Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, 190-8518 Japan
6Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kochi, Nankoku, 783-8502 Japan
7Geodynamics Research Center, Ehime University, Matsuyama, 790-8577 Japan
8Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai, 980-8578 Japan
9Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Tokyo, Toshima-ku, 171-8588 Japan
10Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, 980-8578 Japan
11UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki, Aichi, 444-8585 Japan
12Marine Works Japan, Kochi, Nankoku, 783-8502 Japan
Published by arrangement with John Wiley & Sons

The (plagioclase) lherzolitic shergottite Northwest Africa (NWA) 7397 consists of poikilitic and non-poikilitic lithologies. Coarse-grained low-Ca pyroxene oikocrysts enclose olivine and chromite grains in the poikilitic lithology. The major constituents of the non-poikilitic lithology are olivine, Ca-pyroxene, and plagioclase. Minor amounts of chromite, ilmenite, alkali feldspar, Ca-phosphate, and iron-sulfide are included in the non-poikilitic lithology. Most plagioclase grains in the non-poikilitic lithology have become maskelynite. A melt pocket occurs in the non-poikilitic lithology. Plagioclase in contact with the melt pocket has dissociated into zagamiite + stishovite. Apatite and merrillite entrained in the melt pocket have transformed into tuite. Olivine in contact with the melt pocket has dissociated into bridgmanite (almost vitrified) + ferroan-periclase. Alteration products, iron oxides and hydroxides, also occur in the dissociated olivine although it is not clear when the aqueous alteration occurred. The dissociation reactions of olivine and plagioclase into the high-pressure polymorphs (bridgmanite, ferroan-periclase, zagamiite, and stishovite) are found from lherzolitic shergottites for the first time. The estimated peak shock-pressure and -temperature conditions recorded in melt pockets of NWA 7397 are ˜23 GPa and 2,000 °C at least, respectively, based on the high-pressure mineral assemblages.

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