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Nat Commun. 2017 Aug 9;8(1):227. doi: 10.1038/s41467-017-00192-5.

Zhamanshin astrobleme provides evidence for carbonaceous chondrite and post-impact exchange between ejecta and Earth's atmosphere.

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Czech Geological Survey, Klárov 3, Prague 1, CZ-118 21, Czech Republic.
Institute of Geology of the Czech Academy of Sciences, v.v.i., Rozvojová 269, Prague 6, CZ-165 00, Czech Republic.
Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Universität Göttingen, Goldschmidtstraße 1, Göttingen, D-37077, Germany.
Institut de Physique du Globe de Paris, Université Paris Diderot, 1 rue Jussieu, Paris, F-75005, France.
Insitut Universitaire de France, Paris, F-75005, France.
Nuclear Physics Institute of the Czech Academy of Sciences, v.v.i., Husinec-Řež, CZ-250 68, Czech Republic.


Chemical fingerprints of impacts are usually compromised by extreme conditions in the impact plume, and the contribution of projectile matter to impactites does not often exceed a fraction of per cent. Here we use chromium and oxygen isotopes to identify the impactor and impact-plume processes for Zhamanshin astrobleme, Kazakhstan. ε54Cr values up to 1.54 in irghizites, part of the fallback ejecta, represent the 54Cr-rich extremity of the Solar System range and suggest a CI-like chondrite impactor. Δ17O values as low as -0.22‰ in irghizites, however, are incompatible with a CI-like impactor. We suggest that the observed 17O depletion in irghizites relative to the terrestrial range is caused by partial isotope exchange with atmospheric oxygen (Δ17O = -0.47‰) following material ejection. In contrast, combined Δ17O-ε54Cr data for central European tektites (distal ejecta) fall into the terrestrial range and neither impactor fingerprint nor oxygen isotope exchange with the atmosphere are indicated.Identifying the original impactor from craters remains challenging. Here, the authors use chromium and oxygen isotopes to indicate that the Zhamanshin astrobleme impactor was a carbonaceous chrondrite by demonstrating that depleted 17O values are due to exchange with atmospheric oxygen.

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