Mineralogical and geochemical studies of impact melt products from the Chesapeake Bay impact structure

美国地质调查局（USGS）和ICDP在弗吉尼亚州切萨皮克湾（Chesapeake Bay）的35 Ma、约85公里大小的潜艇撞击结构中进行钻探项目。该陨石坑是始新世上部沉积物中北美晶岩散布区及相关微晶岩的来源。本提案的重点是碰撞熔融产物（熔融岩石、碰撞角砾岩中的玻璃弹/颗粒、晶岩、微晶岩、微晶岩），它们起源于陨石坑的不同阶段。所有被命名的岩性都应该发生在切萨皮克火山口内、周围或远离火山口的不同地质环境中。这提供了一个独特的机会来研究迄今为止各种熔体起源的不受限制的不同机制，深入了解蒸汽羽流中发生的过程（混合，高温化学，氧化还原条件），推导出冲击玻璃的不同冷却路径，并开发出区分微陨石和火山玻璃球的工具。为了实现这一目标，我们打算通过矿物学和地球化学技术来表征熔体和目标岩性，通过地球化学和同位素指纹识别（Rb-Sr, Sm-Nd， REE与TIMS和LA-ICP-MS技术）来识别撞击玻璃的前体岩石，通过一种新的互补方法，结合直接耦合的演化气体分析系统（DEGAS）和透射电子显微镜（TEM）上的电子能量损失光谱（EELS）来确定挥发性含量和氧化还原状态。

USGS and ICDP perform a drilling project into the 35 Ma old, ~85-km-sized submarine Chesapeake Bay impact structure, Virginia. This crater is source to the North American tektite strewn field and related microtektites in upper Eocene sediments. This proposal focuses on impact melt products (melt rocks, glass bombs/particles within impact breccias, tektites, microtektites, microkrystites) that originate during different stages of cratering. All named lithologies are supposed to occur at different geological settings in, around, or far off the Chesapeake crater. This gives a unique opportunity to ¿ study the so far rather unconstrained different mechanisms for the origin of the various melts, ¿ get insight into processes taking place in the vapor plume (mixing, high-temperature chemistry, redox conditions), ¿ derive the different cooling paths of impact glasses, and ¿ develop tools to distinguish microtektites from volcanic glass spherules. To reach this goals we intend to ¿ characterize melt and target lithologies by mineralogical and geochemical techniques, ¿ identify precursor rocks of impact glasses by geochemical and isotopic fingerprinting (Rb-Sr, Sm-Nd, REE with TIMS and LA-ICP-MS techniques), ¿ determine volatile content and redox state by a novel complementary approach combining a Directly coupled Evolved Gas Analysing System (DEGAS) with Electron Energy Loss Spectroscopy (EELS) on a transmission electron microscope (TEM).