Systematic investigations of high‐pressure polymorphs in shocked ordinary chondrites

1Masaaki Miyahara,2,3Akira Yamaguchi,1Masato Saitoh,1Kanta Fukimoto,4Takeshi Sakai,4Hiroaki Ohfuji,5Naotaka Tomioka,6Yu Kodama,7Eiji Ohtani
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13608]
1Graduate School of Advanced Science and Engineering, Hiroshima University, Higashi‐Hiroshima, 739‐8526 Japan
2National Institute of Polar Research, Tokyo, 190‐8518 Japan
3Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, 190‐8518 Japan
4Geodynamics Research Center, Ehime University, Matsuyama, 790‐8577 Japan
5Kochi Institute for Core Sample Research, Japan Agency for Marine‐Earth Science and Technology (JAMSTEC), Nankoku, Kochi, 783‐8502 Japan
6Marine Works Japan, Nankoku, Kochi, 783‐8502 Japan
7Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai, 980‐8578 Japan
Published by arrangement with John Wiley & Sons

Shock‐induced melting textures and high‐pressure polymorphs in 178 ordinary chondrites of all chemical groups and petrologic types were investigated. The shock‐induced melting modes were classified into three types, namely pocket, line, and network. The type of shock‐induced melting depends on the petrologic type. The width of the shock‐induced melt increased with increasing the petrologic type number. The approximate estimated shock‐pressure ranges recorded in and around the shock‐induced melts of the H‐group ordinary chondrites based on the identified high‐pressure polymorphs were as follows: H3, less than 2 GPa; H4–H6, 2–6 GPa. For ordinary chondrites of the L/LL group, the values were as follows: L/LL3, 2–6 GPa; L/LL4, 2–14 GPa; L5: 14–20 GPa; LL5, 2–14 GPa; L6, 17–23 GPa; and LL6, 14–18 GPa. After adopting the estimated shock pressures into the onion shell‐structured parent body model, the shock pressure on the surface was much lower than in the interior. One possibility is that the apparent lower shock pressure on the surface is due to spallation during the impact. Considering the features of the high‐pressure polymorphs, the major disruption history of the parent bodies is different in each chemical group, although the L/LL chondrite parent bodies may have a similar major disruption history.

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