title: A search for thermal excursions from ancient extraterrestrial impacts using Hadean zircon Ti-U-Th-Pb depth profiles.
abstract: Few terrestrial localities preserve more than a trace lithic record prior to ca. 3.8 Ga greatly limiting our understanding of the first 700 Ma of Earth history, a period inferred to have included a spike in the bolide flux to the inner solar system at ca. 3.85-3.95 Ga (the Late Heavy Bombardment, LHB). An accessible record of this era may be found in Hadean detrital zircons from the Jack Hills, Western Australia, in the form of μm-scale epitaxial overgrowths. By comparing crystallization temperatures of pre-3.8 Ga zircon overgrowths to the archive of zircon temperature spectra, it should, in principle, be possible to identify a distinctive impact signature. We have developed Ti-U-Th-Pb ion microprobe depth profiling to obtain age and temperature information within these zircon overgrowths and undertaken a feasibility study of its possible use in identifying impact events. Of eight grains profiled in this fashion, four have overgrowths of LHB-era age. Age vs. temperature profiles reveal a period between ca. 3.85-3.95 Ga (i.e., LHB era) characterized by significantly higher temperatures (approximately 840-875 °C) than do older or younger zircons or zircon domains (approximately 630-750 °C). However, temperatures approaching 900 °C can result in Pb isotopic exchange rendering interpretation of these profiles nonunique. Coupled age-temperature depth profiling shows promise in this role, and the preliminary data we report could represent the first terrestrial evidence for impact-related heating during the LHB.
authors.affiliation: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA.
authors.forename: Sunshine S
authors.initials: SS
authors.lastname: Abbott
authors.forename: T Mark
authors.initials: TM
authors.lastname: Harrison
authors.forename: Axel K
authors.initials: AK
authors.lastname: Schmitt
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Mojzsis
mesh_terms: D001272:Atmosphere / Q000737:chemistry*; D018534:Earth, Planet; D049690:History, Ancient; D006358:Hot Temperature; D007854:Lead / Q000737:chemistry; D019110:Meteoroids; D013696:Temperature; D013910:Thorium / Q000737:chemistry; D013997:Time Factors; D014025:Titanium / Q000737:chemistry; D014501:Uranium / Q000737:chemistry
chemical_list: D007854:Lead; D014501:Uranium; D013910:Thorium; D014025:Titanium