Wed 16th September @ 12 pm, Rm 312.222

Abstract

One enduring early Earth enigma is the absence of evidence in the terrestrial record for meteorite impacts during the Hadean. Given the low likelihood of discovering an intact Hadean impact crater, the sedimentary record of detrital shocked minerals created by the erosion of such craters may offer the best opportunity to discover direct evidence of early terrestrial impacts. Zircon is an excellent recorder of shock deformation and is a ubiquitous mineral in siliciclastic sediments of any age, as evidenced by the preservation of detrital zircons up to 4.4 Ga. No shocked Hadean zircons have been identified thus far, but the recent discovery of shocked zircons in Paleozoic and Precambrian sedimentary rocks demonstrates their preservation potential over deep time. However, to construct an impact chronology from ex situ shocked zircons requires demonstrating that crystal domains analyzed for U-Pb age either formed or were age-reset during impact. Characterization of diagnostic shock microstructures is thus crucial for interpretation of geochronological data. In this talk I will first discuss different zircon morphotypes found in impact environments and how they have been applied to dating terrestrial impact structures. I will then show examples of detrital shocked zircons from impacts of known age that highlight the strengths and limitations of conventional in situ geochronology. The two main conclusions are: (1) Dating impacts with zircons that have been separated from their host rocks is challenging, and requires a marriage of geochronology and microstructural analysis at the micro- to nano-scale, and (2) Detrital shocked zircons provide excellent analogs for developing new analytical strategies to overcome challenges for extracting impact ages from ex situ shocked zircons from any environment, including lunar impact breccias, detrital populations, and other planetary materials.