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

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. ε(54)Cr values up to 1.54 in irghizites, part of the fallback ejecta, represent the (54)Cr-rich extremity of the Solar System range and suggest a CI-like chondrite impactor. Δ(17)O values as low as −0.22‰ in irghizites, however, are incompatible with a CI-like impactor. We suggest that the observed (17)O depletion in irghizites relative to the terrestrial range is caused by partial isotope exchange with atmospheric oxygen (Δ(17)O = −0.47‰) following material ejection. In contrast, combined Δ(17)O–ε(54)Cr 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.