A Refined Age for the Manson Impact Structure by Means of Detailed 40Ar/39Ar Thermochronology of Shocked Microclines

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The object of this study is to reevaluate the dating of the Manson Impact Structure, which has been suggested to be one of the sites where a large asteroid or comet struck the earth some 65 million years ago, causing one of the larger mass extinctions experienced on this planet. It is important to determine with the best possible resolution if the Manson structure is the same age as other impact sites nominated to be responsible for events at the Cretaceous/Tertiary (K/T) boundary, and if Manson and the other impact sites are the same age as the paleontologically defined boundary seen in the rock record. Up to this point, the only date on the Manson structure has come from innovative application of the 40Ar/39Ar dating method to samples of partially reset feldspars which were taken from the basement rock shocked and heated by the impact itself. New dating of melt glasses recovered by the current drilling program at the Manson site may provide an additional age estimate, but further analysis of shocked feldspars offers at least as promising a means of improving the precision and accuracy of the age of the Manson of feldspar analysis and interpretation, it should be possible to bring the precision and accuracy of the age of the Manson structure. Currently, this age estimate is 65.7+1 Ma. With the application of current techniques of feldspar analysis and interpretation, it should be possible to bring the precision to ~0.2 Ma or better; preliminary "thermochronological" analysis of previously published data also suggests that the most likely age may actually be at least 0.7 m.y. lower, putting the age of the Manson impactor much closer to estimates of the K/T boundary and the Caribbean impactor of some 64.7 Ma. Additional benefits from careful thermochronological analysis of Manson-site feldspars will include an understanding of how robust age estimates from these feldspars actually are, and also, a constraint on the post- impact thermal history of the impact site.