A Paleomagnetic Assessment of the Age of Manson Crator

爱荷华州中部曼森陨石坑岩石的年龄测定表明，该陨石坑与K/T边界形成于同一时间框架，撞击地层的成分正确，产生了世界范围内边界粘土层中发现的冲击矿物，但在北美边界粘土中含量最多，粒度最大。尽管Ar40/Ar39定年法将陨石坑的年龄定为65.7+1 m.y，但一项4个样本的古地磁研究报告称，陨石坑的岩石只有正常的极性磁化。由于K/T边界在世界范围内处于一个相反的极性区间，正常的极性结果导致一些人推断曼森撞击没有发生在K/T边界。然而，这项研究的规模非常小，而且，样品显示出一些模糊的磁化行为。这项研究包括对曼森陨石坑的岩石进行双重古地磁研究，以确定磁化的极性和年龄。首先对冲击角砾岩的磁化特性进行了详细的研究。基质磁化和碎石磁化的“砾岩测试”都有可能揭示曼森撞击时地磁场的极性，从而判断陨石坑是否形成于与K/T边界相同的极性。进一步研究撞击后的磁地层学和撞击前的沉积层序，进一步缩小撞击事件的年龄范围。磁地层学研究也有可能将曼森陨石坑的放射性测定年龄的不确定性从2微米缩小到小于0.5微米。确定曼森撞击是否表明K/T边界现象是多重撞击的结果是很重要的。

Age dating of rocks from manson crater in central Iowa indicates that this crater was formed in the same time frame as the K/T boundary, and the impacted strata are of the correct compositions to have generated the shocked minerals found in the boundary clay layer worldwide, but most abundantly and of the largest grain sizes in the North American boundary clays. Although Ar40/Ar39 dating places the crater age at 65.7+1 m.y., a 4-sample paleomagnetic study reported the crater rocks to have only normal polarity magnetization. Because the K/T boundary is found worldwide in a reversed polarity interval, the normal polarity results have led to the inference by some that the Manson impact did not happen at the K/T boundary. However, the study was exceedingly small, and moreover, the samples displayed some ambiguous magnetization behavior. This study involves a two-fold paleomagnetic study of the rocks in Manson Crater to determine the polarity and age of magnetization. First is a detailed study of the magnetization of the impact breccia. Both the matrix magneti- zation and a "conglomerate test" of the clast magnetizations potentially can reveal the polarity of the geomagnetic field at the time of the Manson impact, hence whether or not the crater was formed in the same polarity as the K/T boundary. Further the magnetostratigraphy of the post-impact, as well as the pre-impact sedimentary sequences will be studied to further narrow the age range of the impact event. The magnetostratigraphy study also has the potential to narrow the uncertainty of the radiometrically determined age of Manson crater from 2 m.y. to less than 0.5 m.y. It is important to determine whether the Manson impact is an indication that the K/T boundary phenomena are the result of multiple impacts.