A slowly cooled deep crust on asteroid 4 Vesta and the recent impact history of rubble pile vestoids recorded by diogenites

In this study, we investigate the Ar-40/Ar-39 systematics of nineteen diogenites thought to come from deep crustal levels of asteroid 4 Vesta. We applied both Electron Backscattered Diffraction (EBSD) and Ar-40/Ar-39 and methods to the unbrecciated diogenite LAP 031381. We obtained three plateau ages resulting in a combined weighted mean age of 4441 +/- 15 Ma (P = 0.16). The EBSD analyses suggest that LAP 031381 displays minimal evidence of shock and, when combined with petrography observations, diffusion modelling and Ar-40/Ar-39 data, these results suggest that the crustal volume that initially contained this diogenite, reached a temperature of ca. 630 degrees C at similar to 4.44 Ga. This corresponds to a linear cooling rate of similar to 5 degrees C / Ma for a crystallization age of 4550 Ma. Independent thermal models suggest that these conditions were present at a depth of 60 to 65 km at 4.44 Ga.The other eighteen diogenites yielded Ar-40/Ar-39 results that indicate that they have been variously shocked by impact events and seven of them yielded plateau ages ranging from 2413 +/- 189 Ma to 84 +/- 162 Ma. We combined these results with Ar-40/Ar-39 ages from eucrites and howardites and propose that the HED (Howardite, Eucrite, Diogenite) meteorites recorded impact events at the surface of Vesta until similar to 3.4 Ga when they were then ejected during a large collision. The eucrites, diogenites and howardites were then recombined into small rubble pile asteroids which probably make up a large part of the Vestoid family. After ejection, the K/Ar system in plagioclase crystals ceased in most cases to be fully reset by impact events as the temperature spikes reached during small impacts lack enough energy to trigger significant Ar-40* diffusion. On the other hand, ultra-transient and high-temperature - sensitive pyroxene crystals kept a more systematic record of small impacts until recent time. (38)Arc cosmochron cosmogenic exposure ages on diogenites mostly range from 51 +/- 7 Ma to 0 +/- 1 Ma and when combined with other HED cosmochron ages, suggest that almost all the HED meteorites were continuously ejected from secondary rubble pile asteroids mostly between 50 Ma and present.