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 impactrelated heating during the LHB.early Earth | impact crater | lunar cataclysm | secondary ion mass spectrometry T he LHB is the period from ca. 3.85-3.95 Ga during which an intense flux of asteroidal and/or cometary bodies is hypothesized to have impacted the Moon (1). A variety of theories have been proposed to explain the LHB (2-5) culminating with the "Nice model" (5-7). This model posits that a fundamental shift in orbital resonance among the Jovian planets at ca. 3.9 Ga destabilized the disk of planetesimals in the outer solar system resulting in the scattering of numerous bodies into the inner solar system. Tera et al. (1) introduced the concept of a late lunar cataclysm to explain isotopic fractionations in rocks returned from the heavily cratered lunar highlands. Specifically, U-Pb, and Rb-Sr isochrons yielded recrystallization ages between 3.85-3.95 Ga. The parent/daughter behavior in these two geochronologic systems are quite different but result in similar system disturbances (i.e., U and Sr are highly refractory whereas Pb and Rb are variably volatile). Thus, a profound thermal event, such as from an impact at the appropriate scale of a "cataclysm," could have caused resetting of both chronometers, albeit for different reasons. Much of the evidence in support of the LHB hypothesis comes from 40 Ar∕ 39 Ar age spectra of lunar highland crust samples (3,8,9), interpreted to yield apparent "plateau" ages between 3.8-4.0 Ga due to the resetting of the K-Ar system via collisional heating (see review in 10).

Rutile U–Pb age depth profiling: A continuous record of lithospheric thermal evolution

“…Using a Pb diffusion law for monazite, the authors inverted the age profiles to show that the hanging wall gneisses underwent a cooling history characterized by initial rapid cooling to 750 • C, followed by cooling at 20 • C Ma −1 until a further phase of rapid cooling. Ion microprobe U-Pb age depth-profiling has also been applied to zircon from high-grade metamorphic rocks to constrain discordance mechanisms (Breeding et al, 2004) and also to date terrestrial impact structures (Abbott et al, 2012;Trail et al, 2007). Given the high temperatures required (>1000 • C) to generate diffusive age gradients over analytically detectable length scales, these studies confirm that zircon age gradients are typically attributable to processes other than volume diffusion, including sub-micron epitaxial overgrowths and deformation.…”

Evolution of the Earth: Plate Tectonics Through Time

“…This includes zircons with metamorphic rims, which might show annealed shock features. Abbott et al (2012) stated that $1/3 of the crust present during the bombardment would have been heated enough by impacts for metamorphic overgrowths to form on zircon grains. They found features in their zircons consistent with this process.…”

Micrometer-scale U–Pb age domains in eucrite zircons, impact re-setting, and the thermal history of the HED parent body

“…Cates and Mojzsis, 2009;Abbott et al, 2012). Optical and electron images of the grains were created prior to depth profile analysis to help determine the target area in which to begin sputtering on the grain as well as to avoid cracks, inclusions, and other imperfections which have the potential to show erroneously high degrees of Pb * loss or contamination.…”

“…Abbott et al, 2012;Abramov et al, 2013). Samples directly returned from the Moon by the Apollo and Luna missions, however, have provided the means to construct size-frequency/age relationships for various well preserved cratered surfaces.…”