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

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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.