40 Ar- 39 Ar Dating of the Manson Impact Structure: A Cretaceous-Tertiary Boundary Crater Candidate

The mineralogy of shocked mineral and lithic grains in the Cretaceous-Tertiary (K-T) boundary claystone worldwide is most consistent with a bolide impact on a continent. Both the concentrations and sizes of these shocked grains are greatest in the western interior of North America. These data suggest that the Manson impact structure in north-central Iowa is a viable candidate for the K-T boundary impact event. Argon-40–argon-39 age spectrum dating of shocked microcline from the crystalline central uplift of the Manson impact structure indicates that there was severe argon-40 loss at 65.7 ± 1.0 million years ago, an age that is indistinguishable from that of the K-T boundary, within the limits of analytical precision.