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

USGS和ICDP在弗吉尼亚州切萨皮克湾撞击结构的35 Ma老旧、约85公里大小的潜艇上进行钻探项目。该陨石坑是北美玻璃陨石散布区域和始新世晚期沉积物中相关的微玻璃陨石的来源。该提案侧重于陨石坑形成的不同阶段产生的撞击熔融产品(熔岩、撞击角砾岩、玻璃陨石、微玻璃陨石、微玻璃陨石中的颗粒)。所有命名的岩性都应该发生在切萨皮克陨石坑内、附近或远处的不同地质环境中。这提供了一个独特的机会来研究到目前为止各种熔体起源的不同的不同机制，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)，得出撞击玻璃的不同冷却路径，并开发工具来区分微玻璃陨石和火山玻璃球体。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(Rb-SR、Sm-ND、REE与TIMS和LA-ICPMS技术)鉴定撞击玻璃的前体岩石，通过一种新的补充方法来确定挥发分含量和氧化还原状态，这种新的方法结合了直接耦合的演化气体分析系统(DEGA)和电子能量损失谱(EELS)在透射电子显微镜(TEM)上的应用。

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).