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

摘要这是一项探索性研究小额赠款（SGER）提案，旨在将新的专业知识和方法应用于早期撞击坑事件的“既定”研究。具体来说，PI和他的合作者Gunter Lugmair博士计划开发常规现场和实验室程序使用铬同位素方法调查早期撞击坑。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，不仅用于验证撞击事件的发生，而且用于识别在这些事件中撞击的陨石的类别。然而，到目前为止，所有的铬同位素系统学研究的样本都是经过精心挑选的，在撞击喷出物中含有相当大比例的地外成分（ETC）。相比之下，地球上大多数疑似撞击地点的特征是ETC含量低。这特别反映在低Ir和Cr浓度上。当Cr浓度较低时，地球背景Cr的同位素组成掩盖了宇宙Cr的同位素组成。这严重妨碍了铬同位素法的使用。为了避免这些困难，需要作出很大的努力，以制定物理化学程序，集中陨石中的铬的样品，并继续改善质谱技术的铬同位素分析。此外，根据其地质背景和其他亲铁元素浓度的测量结果对样品进行彻底选择至关重要。在这项研究中，将分析南非和西澳大利亚两个疑似撞击地点喷出物的铬同位素系统学。从这些网站的样本有可变的ETC浓度，这些从来没有进行过分析铬同位素系统学之前。成功完成拟议的调查将是我们明确确定撞击地点的能力的一个巨大飞跃，限制每个地点的外星撞击物的性质，并更好地了解地球表面陨石坑的地质和环境后果。

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.