K/T界面冲击层的岩石矿物特征及其对论证地外撞击事件的意义

LITHOLOGICAL AND MINERALOGICAL CHARACTERISTICS OF IMPACT LAYERS AT THE K/T BOUNDARY AND THEIR IMPLICATIONS FOR DETERMINATION OF EXTRATERRESTRIAL IMPACT EVENTS

分布在世界各地的K／T界面剖面，尽管形成环境各异，但彼此之间的岩石矿物特征却有许多共同之处。对于大多数剖面来说，界面粘土层的底都普遍存在着一个结构特别的冲击层。冲击层的厚度在全球的分布并不均匀，北美和加勒比海地区可达2～50cm，而其它地区则只有几厘米。冲击层的厚度可能反映了距离撞击坑的远近。详细的岩石与矿物学研究结果表明，在北美和加勒比海地区冲击层实际上具有双层结构特征。上面的一层称为火球层，除了富集Ir和其它地外指示元素外，还含有大量的烟灰和富Ni镁铁尖晶石。下面的溅射层则含有大量的冲击玻璃球粒（玻璃陨石）和冲击变质矿物。K／T界面冲击层的发现为Ir异常的地外撞击成因解释提供了更为直接的物理证据。

The K/T boundary layers in aLl parts of the wOrld, even though formed in various different environments, have many common features in lithology and mineralogy. For most sections, there is an impact layer at the bottom of boundary clay layers. The thickness of the impact layer varies from one place to an0ther. It can be as thick as up to 2 - 50 cm in North AInerica and Caribbean area; in otherareaS, such as in Europe and Australia, it is only 1 - 2 mm. Detailed studies on the minerals andpetrologic structure of the impact layer indicate that the impact layer in North AInerica and Caribbeanarea exhibits a double-layer structure. The upper layer is commonly called the fire-ball layer, in whichnot onLy iridium and other siderophile elements are anomalously enriched, but there are large amounts of ash and Ni-rich spinels as well. The Lower layer, called the ejecta layer, contains a great number Of impact glass spherules (tektites) and shocked minerals. The discovery of impact layer and shockedminerals at the K/T boundary in all parts of the wOrld provides more direct evidence for extraterrestrialimpact events at the end of Cretaceous.