SGER: Developing the Cr Isotope Method for Investigating Impact Cratering Events

这是一项关于探索性研究小额拨款(SGER)的建议，旨在将新的专业知识和方法应用于早期撞击陨石坑事件的“既定”研究。具体地说，PI和他的合作者Gunter Lugmair博士计划开发一种常规的野外和实验室程序，使用铬同位素方法来研究早期撞击陨石坑。撞击喷出物的铬同位素系统学已被证明是最有效的工具之一，不仅可以用来核实撞击事件的发生，而且还可以用来确定在这些事件期间撞击的陨石的类别。然而，到目前为止，所有研究铬同位素系统学的样品都是经过精心挑选的，在撞击喷出物中含有相当大比例的外星人成分(ETC)。相比之下，地球上大多数疑似撞击地点的ETC含量都很低。这一点特别体现在低Ir和低铬浓度上。当铬浓度较低时，陆地背景铬的同位素组成掩盖了宇宙背景铬的同位素组成。这严重阻碍了铬同位素方法的使用。为了避免这些困难，需要大量的努力来开发在样品中浓缩陨石铬的物理化学程序，并继续改进用于铬同位素分析的质谱学技术。此外，根据地质环境和对其他亲铁元素浓度的测量对样本进行彻底选择是至关重要的。在这项研究中，将分析来自南非和西澳大利亚两个可疑撞击地点的喷发物质的铬同位素系统学。这些地点的样品具有可变的ETC浓度；这些样品以前从未被分析过铬同位素系统学。拟议中的调查的成功完成将是我们在明确识别撞击地点、限制每个地点的外星人撞击的性质以及更好地了解地球表面陨石坑的地质和环境后果方面的一大飞跃。

ABSTRACTThis is a proposal for small grants for exploratory research (SGER) to apply a new expertise and approach to "established" research in early impact cratering events.Specifically, the PI and his collaborator, Dr. Gunter Lugmair, plan to develop a routine field and laboratory procedure for using the Cr isotope method in investigating early impact cratering. The Cr isotope systematics of impact ejecta has been shown to be one of the most effective tools not only for verifying the occurrence of impact events but also for identifying the class of meteorites that impacted during these events. All samples studied for Cr isotope systematics so far, however, were carefully selected, containing a significant proportion of an extraterrestrial component (ETC) in the impact ejecta. In contrast, most of the suspected impact sites on Earth are characterized by a low ETC content. This is reflected particularly by low Ir and Cr concentrations. When Cr concentration is low, the isotopic composition of the terrestrial background Cr masks that of the cosmic Cr. This severely hampers the use of the Cr isotope method. To avoid these difficulties much effort is needed to develop physico-chemical procedures for concentrating meteoritic Cr in the samples and to continue improving the mass-spectrometric technique for the Cr isotope analysis. In addition, a thorough selection of the samples based on their geological settings and measurements of concentrations of other siderophile elements is crucial. For this study, Cr isotope systematics of the ejecta materials from two suspect impact sites in South Africa and Western Australia will be analyzed. Samples from these sites have variable ETC concentrations; these have never been analyzed for Cr isotope systematics before. A successful completion of the proposed investigation will be a great leap forward in our ability to unequivocally identify impact sites, constrain the nature of the extraterrestrial impactor in each site, and better understand the geological and environmental consequences of cratering on the Earth's surface.