Quantifying the effect of late bombardment on terrestrial zircons

1Jesse Reimink,2Carolyn Crow,3Desmond Moser,4Benjamin Jacobsen,5Ann Bauer,6Thomas Chacko
Earth and Planetary Science Letters 604, 118007 Link to Article [https://doi.org/10.1016/j.epsl.2023.118007]
1Department of Geosciences, Penn State University, PA, 16802, USA
2Department of Geological Sciences, CU Boulder, CO, 80309, USA
3Department of Earth Sciences, University of Western Ontario, Ontario, N6A 3K7, Canada
4Nuclear and Chemical Science Division, Lawrence Livermore National Laboratory, 94550, USA
5Department of Geoscience, University of Wisconsin, WI, 53706, USA
6Department of Earth and Atmospheric Science, University of Alberta, Alberta, T6G 2E3, Canada
Copyright Elsevier

The first 500 million years of Earth history is thought to be a period of intense planetary bombardment, but the timing and flux of this meteorite bombardment is poorly understood. In particular, on the basis of an inferred lunar impact history, some workers have hypothesized a ∼3.9 Ga terminal cataclysm (TC) in which there was marked increase in the impact flux affecting the Moon, the Earth and possibly other terrestrial planets. Minerals that survived this enigmatic period offer a way to test early planetary bombardment models as they may contain telltale micro- to nanoscale shock features. Here, we present results from a numerical modeling calculation that assesses the probability that a zircon residing in the crust would escape shock melting or shock deformation during a TC bombardment event. Even with conservative pressure estimates for zircon shock deformation and intermediate bombardment intensities, we find that only ∼6% of ≥4.0 Ga crust would be expected to survive a 3.9 Ga cataclysm without experiencing either complete melting or zircon shock metamorphism. We couple this modeling with a search for shock effects in the oldest zircons from the Acasta Gneiss Complex, which would have been present in the Earth’s crust during a putative 3.9 Ga TC. Spatially correlated electron and NanoSIMS ion microscopy of 4.02 Ga igneous zircons from Acasta reveals no evidence of ancient shock. These data, together with similar results from other Hadean zircon suites, confirm that a post-Hadean TC is unlikely to have occurred. We suggest that the dearth of pre-3.9 Ga terrestrial crust and zircons is instead best explained by endogenic processes related to the mechanisms of early crust formation. Our modeling allows us to evaluate bombardment scenarios from the terrestrial zircon record by applying probabilistic interpretations to zircon shock deformation data. This approach will be valuable for other planetary bodies, allowing broader conclusions to be drawn from geographically limited datasets.


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