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

摘要：这是一项探索性研究小额拨款 (SGER) 的提案，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 博士计划开发一个常规现场和实验室程序，以使用铬同位素方法来研究早期撞击坑事件。撞击喷射物的铬同位素系统学已被证明是最有效的工具之一，不仅可以验证撞击事件的发生，而且可以识别这些事件期间撞击的陨石的类别。然而，迄今为止用于铬同位素系统学研究的所有样本都是经过精心挑选的，在撞击喷射物中含有很大比例的外星成分（ETC）。相比之下，地球上大多数疑似撞击地点的特点是 ETC 含量较低。低铱和铬浓度尤其反映了这一点。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。这严重阻碍了Cr同位素方法的使用。为了避免这些困难，需要付出很大的努力来开发用于浓缩样品中陨石铬的物理化学程序，并继续改进铬同位素分析的质谱技术。此外，根据样品的地质环境和其他亲铁元素浓度的测量来彻底选择样品至关重要。在这项研究中，将对来自南非和西澳大利亚两个可疑撞击地点的喷射物的铬同位素系统进行分析。来自这些地点的样品具有不同的 ETC 浓度；以前从未对这些进行过 Cr 同位素系统学分析。成功完成拟议的调查将是我们明确识别撞击地点、限制每个地点的外星撞击物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的能力的巨大飞跃。

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.