Boltysh, another end-Cretaceous impact

The Chixculub impact occurred at the Cretaceous/Tertiary (K/T) boundary, and although several other Late Cretaceous and Paleogene impact craters have, at times, been linked with the K/T boundary, isotope geochronology has demonstrated that all have significantly different ages. The currently accepted age of the 24 km diameter Boltysh crater, a K-Ar whole-rock age, places it in the Coniacian at 88 +/- 3 Ma. However, comprehensive Ar-Ar dating of a range of melt samples yields a mean age of 65.17 +/- 0.64 Ma, within errors of the K/T boundary. Several of the fresh samples exhibit signs of excess argon but this seems to be concentrated in rapidly crystallized glass-rich samples. The Ar-Ar age confirms an earlier fission track measurement and thus two dating techniques have yielded an age within errors of the K/T boundary for this crater. Crucially, although the ages of Boltysh and Chixculub are within errors, they may not have formed synchronously. Craters of 24 kin diameter occur much more commonly than impacts of Chixculub dimensions, but their proximity does raise the important question of how many impacts there might have been close to the K/T boundary.