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

这项研究的目的是重新评估曼森撞击结构的年代，该结构被认为是6500万年前一颗大型小行星或彗星撞击地球的地点之一，造成了这个星球上经历的一次更大规模的物种灭绝。重要的是要尽可能确定曼森结构是否与其他被指定为白垩纪/第三纪（K/T）边界事件负责的撞击地点年龄相同，以及曼森和其他撞击地点是否与岩石记录中古生物学定义的边界年龄相同。到目前为止，曼森构造的唯一日期来自于40Ar/39Ar定年法的创新应用，该方法对部分重置的长石样本进行了测定，这些长石是从受到冲击和加热的基底岩石中提取的。根据目前在曼森遗址的钻探计划，对熔融玻璃进行新的测年可能会提供额外的年龄估计，但对受冲击长石的进一步分析至少提供了一种有希望的方法，可以提高曼森长石年龄分析和解释的精度和准确性，它应该有可能提高曼森结构年龄的精度和准确性。目前，这一年龄估计为65.7+1 Ma。利用现有的长石分析解释技术，应能使其精度达到~0.2 Ma以上；对先前公布的数据的初步“热年代学”分析也表明，最可能的年龄实际上可能至少要低0.7亿年，这使得曼森撞击器的年龄更接近于对K/T边界的估计，而加勒比撞击器的年龄约为64.7亿年。对曼森现场长石进行仔细的热年代学分析的其他好处包括了解这些长石的年龄估计实际上有多可靠，以及对撞击地点的撞击后热历史的约束。

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.