Back‐transformation mechanisms of ringwoodite and majorite in an ordinary chondrite

1Kanta Fukimoto,1Masaaki Miyahara,2Takeshi Sakai,2Hiroaki Ohfuji,3Naotaka Tomioka,4Yu Kodama,5Eiji Ohtani,6,7Akira Yamaguchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13543]
1Graduate School of Advanced Science and Engineering, Hiroshima University, Higashi‐Hiroshima, 739‐8526 Japan
2Geodynamics Research Center, Ehime University, Matsuyama, 790‐8577 Japan
3Kochi Institute for Core Sample Research, Japan Agency for Marine‐Earth Science and Technology (JAMSTEC), Nankoku, Kochi, 783‐8502 Japan
4Marine Works Japan, Nankoku, Kochi, 783‐8502 Japan
5Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai, 980‐8578 Japan
6National Institute of Polar Research, Tokyo, 190‐8518 Japan
7Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, 190‐8518 Japan
Published by arrangement with John Wiley & Son

We investigated the back‐transformation mechanisms of ringwoodite and majorite occurring in a shock‐melt vein (SMV) of the Yamato 75267 H6 ordinary chondrite during atmospheric entry heating. Ringwoodite and majorite in the shock melt near the fusion crust have back‐transformed into olivine and enstatite, respectively. Ringwoodite (Fa~18) occurs in the SMV as a fine‐grained polycrystalline assemblage. Approaching the fusion crust, fine‐grained polycrystalline olivine becomes dominant instead of ringwoodite. The back‐transformation from ringwoodite to olivine proceeds by incoherent nucleation and by an interface‐controlled growth mechanism: nucleation occurs on the grain boundaries of ringwoodite, and subsequently olivine grains grow. Majorite (Fs16–17En82–83Wo1) occurs in the SMV as a fine‐grained polycrystalline assemblage. Approaching the fusion crust, the majorite grains become vitrified. Approaching the fusion crust even more, clino/orthoenstatite grains occur in the vitrified majorite. The back‐transformation from majorite to enstatite is initiated by the vitrification, and growth continues by the subsequent nucleation in the vitrified majorite.

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