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 (Fs_16–17En_82–83Wo₁) 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.

^1,2Piers Koefoed,^1,3Olga Pravdivtseva,^1,2Heng Chen,⁴Carina Gerritzen,⁵Maxwell M. Thiemens,^1,2Kun Wang
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13545]
¹McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, Missouri, 63130 USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri, 63130 USA
³Department of Physics, Washington University in St. Louis, St. Louis, Missouri, 63130 USA
⁴Institut für Mineralogie und Geologie, Universität zu Köln, Köln, Germany
⁵Laboratoire G‐Time, CP 160/02, Université Libre de Bruxelles, Av. F. Roosevelt 50, Bruxelles, Belgium
Published by arrangement with John Wiley & Sons

Here, we apply recently developed high‐precision K isotope analyses to individual components of the LL4 chondrite Hamlet in order to investigate key processes which occurred during chondrite formation. The K isotopic compositions of all Hamlet chondrules range from −1.36‰ to −0.24‰ δ41K while the matrix and bulk samples show ranges of −0.89‰ to −0.80‰ and −0.86‰ to −1.08‰ δ41K, respectively. This range of δ41K values is significantly less than what was seen by in situ K isotopic analysis of Semarkona and Bishunpur chondrules, a likely effect of the different chondrite petrologic types, analytical artifacts in the SIMS analyses, and chondrule rim effects. Strong evidence for secondary parent‐body alteration effects within Hamlet suggests its K fractionation and distribution are dominantly controlled by these processes. Interestingly, the strong correlation between δ41K and chondrule mass suggests that chondrule size played a significant role in the K isotopic distribution within Hamlet. This trend is likely a result of either inherited initial differences in the chondrule K isotopic ratios which were not completely overprinted or mechanisms involved in the metamorphism processes creating variations. This K isotope correlation with chondrule mass could also be suggestive of chondrule‐forming nebular processes; nevertheless, it is currently unable to definitively favor any specific model. The K isotopic similarities between Hamlet and bulk ordinary chondrites suggest that all LL chondrites, if not all ordinary chondrites, may have formed via the same processes. Nevertheless, analysis of more pristine chondrules from chondrites of lower metamorphic grade is required to further assess any nebular processes of chondrule formation.