A search for thermal excursions from ancient extraterrestrial impacts using Hadean zircon Ti-U-Th-Pb depth profiles

Proc Natl Acad Sci U S A. 2012 Aug 21;109(34):13486-92. doi: 10.1073/pnas.1208006109. Epub 2012 Aug 6.

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.

Publication types

  • Historical Article
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Atmosphere / chemistry*
  • Earth, Planet*
  • History, Ancient
  • Hot Temperature
  • Lead / chemistry
  • Meteoroids
  • Temperature
  • Thorium / chemistry
  • Time Factors
  • Titanium / chemistry
  • Uranium / chemistry

Substances

  • Lead
  • Uranium
  • Thorium
  • Titanium