Shock history of the fossil ungrouped achondrite Österplana 065: Raman spectroscopy and TEM of relict chrome‐spinel grains

Abstract

AbstractChrome‐spinel grains from the fossil ungrouped achondrite Österplana 065 (Öst 065) recovered from Middle Ordovician limestone in Sweden were studied using Raman spectroscopy and TEM. All the studied chrome‐spinel grains have a high density of planar fractures and planar features, not seen in chromites from the other L chondritic Ordovician fossil meteorites. Raman spectra of the host chrome‐spinel grain and its planar features are similar and no signatures of high‐pressure phases of chromite were found. The planar features occur along planar fractures, are enriched in ZnO, and are most probably produced due to enhanced leaching during terrestrial weathering in the marine sediment. Dislocation densities within two FIB sections prepared from two chrome‐spinel grains from Öst 065 are similar to the dislocation densities found within chromite grains from the matrix of Tenham L6 chondrite. Using this observation and taking into account the presence of significant fracturing in all the grains, we conclude that the Öst 065 chrome‐spinel grains were subjected to moderate to very strong shock corresponding to shock stages of S4–S6. This makes Öst 065 fossil achondrite the highest shocked fossil meteorite studied so far. This is consistent with the hypothesis that Öst 065 is a piece of the impactor that led to the L chondrite parent body breakup.