A Paleomagnetic Assessment of the Age of Manson Crator

来自爱荷华州中部曼森陨石坑的岩石测年表明，该陨石坑形成于与K/T边界相同的时间框架内，受影响的地层具有正确的成分，可以产生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。虽然Ar40/AR39测年表明陨石坑的年龄为65.71英里，但一项4个样本的古地磁研究报告称，陨石坑岩石只有正常的极性磁化作用。由于K/T边界在全世界范围内都是在一个反转的极性区间内发现的，所以正极性的结果导致了一些人的推断，即K/T边界没有发生曼森碰撞。然而，这项研究规模非常小，而且样品表现出一些模棱两可的磁化行为。这项研究对曼森陨石坑的岩石进行了两次古地磁研究，以确定磁化的极性和年龄。首先是对冲击角砾岩的磁化进行了详细的研究。无论是基质磁化还是碎屑磁化的“凝聚试验”，都有可能揭示Manson撞击时地磁场的极性，从而揭示陨石坑是否与K/T边界具有相同的极性。此外，将研究撞击后的磁性地层学以及撞击前的沉积序列，以进一步缩小撞击事件的年龄范围。磁性地层学研究还有可能缩小曼森陨石坑放射性测量年龄的不确定度，使之从2英里缩小。至少于0.5英里。重要的是要确定曼森撞击是否表明K/T边界现象是多次撞击的结果。

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.