GIANT IMPACTS AND THE ORIGIN AND EVOLUTION OF ARCHEAN CRATONS

The Moon’s surface preserves evidence of the early impact flux that affected the inner rocky planets in our solar system, while some minimum estimate of a later Archean impact flux is constrained by terrestrial spherule beds at Pilbara and Barberton. However, compared to endogenic (mantle plume or plate tectonic) processes, the role of exogenic (impact-driven) processes in the development of Earth's cratonic crust has received less consideration. Numerical models demonstrate that these impact events could cause instantaneous massive partial melting of the lithosphere and upper mantle, and thus could contribute to both crustal production and reworking. To evaluate the potential role of impacts, we use cumulative sum change point models to investigate isotope time series that track crust production and reworking during the Archean in the context of the local stellar environment via a simplified model of mass density for solar system orbit of the Milky Way—a four-arm barred spiral galaxy (Kirkland et al., 2022, Geology). We identify a c. 190 Ma periodicity in step changes of zircon εHf isotope composition, which is similar to the period between galactic spiral arm passage, consistent with enhanced juvenile crust production and reworking during spiral arm transit. Support for this supposition comes from the distribution of post-Archean hypervelocity terrestrial impact craters, which also show an elevated flux during spiral arm transits. Furthermore, prior to 3.3 Ga, zircon oxygen isotopes show a similar c. 190 Ma periodicity, with less normal distribution of δ¹⁸O during spiral arm entry, consistent with both shallow and deep melting during episodes of enhanced bombardment. After 3.3 Ga this pattern breaks down, perhaps due to an increasing role for endogenous subduction-related processes. These correlations imply that some episodes of continental crust production and reworking were initiated by giant impacts. Although each Archean craton has its own tempo of crust production, as expressed by unique sequences in εHf change points, the probability density of change point frequency shows peaks at c. 3.3 Ga and c. 2.8 Ga, similar to periods of impact-related spherule bed deposition in the Pilbara and Barberton successions at 3.470–3.225 Ga (Norma arm entry) and 2.650–2.488 Ga (Norma exit and Scutum-Centaurus entry).