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

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, LH...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2012-08, Vol.109 (34), p.13486-13492
Main Authors: Abbott, Sunshine S, Harrison, T. Mark, Schmitt, Axel K, Mojzsis, Stephen J
Format: Article
Language:English
Subjects:
Archean eon
Atmosphere - chemistry
Crystallization
Earth
Earth (Planet)
Geochronology
Geological time
Grain growth
Hadean eon
heat
Heating
High temperature
History, Ancient
Hot Temperature
lead
Lead - chemistry
Meteoroids
Minerals
Physical Sciences
Signatures
Temperature
Temperature distribution
Temperature measurement
temperature profiles
Thorium - chemistry
Time Factors
Titanium - chemistry
Uranium - chemistry
zircon
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Summary: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.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1208006109