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