The petrology, geochemistry and age determination of impact melt from the USGS-ICDP drill core Eyreville-B from the late Eocene Chesapeake Bay impact structure

大型撞击坑在全球分布着物质，它们的喷射层提供了重要的地平线标记，使来自不同地点的沉积物相互关联，例如，在始新世渐新世边界的地球气候转折点之前的晚始新世沉积物。位于意大利马萨塔诺的全球层型剖面和点(GSSP)包括三个富Ir喷射层，时间间隔为2 Ma，其中两个分别归因于切萨皮克湾(85公里直径)和波皮盖(100公里直径)的撞击构造。目前切萨皮克湾陨石坑的辐射年龄测定是基于玻璃陨石(远距离撞击玻璃)的全熔融Ar年龄。在40Ar-39Ar的初步工作中，我已经证明了这些玻璃陨石上的精确Ar年龄不能通过全聚变实验获得，而是需要阶梯加热测量。我建议对切萨皮克湾陨石坑内首次发现的撞击熔融岩石进行系统的岩石学、地球化学和放射性研究，并完善相关玻璃陨石的40Ar-39Ar年龄。这项研究还将得到另外三个晚始新世碰撞构造--波皮盖、密斯塔斯汀和瓦纳皮提的碰撞熔岩的40Ar-39Ar年龄测定的补充。放射性和岩石学结果与撞击喷射物分布的新数值模型相结合，得出了始新世晚期沉积物中全球标志层的年龄、组成和分布：这是研究地球气候临界点之前的事件序列的基本数据集。只有USGS-ICDP Eyreville B钻芯为这项研究的主要焦点提供了样本材料：切萨皮克湾陨石坑的年代测定。

Large impact craters distribute material globally, and their ejecta layers present important horizon markers allowing inter-correlation of sediments from different sites, e.g. for late Eocene sediments pre-dating the tipping point of Earth’s climate at the Eocene Oligocene boundary. The Global Stratotype Section and Point (GSSP) in Massignano (Italy) contains three Iridium rich ejecta layers covering a 2 Ma time interval, with two being attributed to the Chesapeake Bay (85 km Ø) and Popigai (100 km Ø) impact structures, respectively. Current radiometric age determination for the Chesapeake Bay crater is based on total fusion Ar ages of tektites (distal impact glasses). In preliminary 40Ar-39Ar work, I have shown that precise Ar ages on these tektites cannot be acquired by total fusion experiments but require step heating measurements. I propose a systematic petrologic, geochemical and radiometric study of the first found impact melt rocks from within the Chesapeake Bay crater and refine the 40Ar-39Ar ages for the related tektites. The study will be complemented by 40Ar-39Ar age determinations of impact melt rocks from three other late Eocene impact structures, Popigai, Mistastin and Wanapitei. Radiometric and petrological results combined with new numerical models on impact ejecta distribution result in age, composition and distribution of global marker horizons in later Eocene sediments: a fundamental data set for investigations on the succession of events predating the tipping point of Earth’s climate. Only the USGS-ICDP Eyreville B drill core provides the sample material for the main focus of this research: dating the Chesapeake Bay crater.