Vesta as the HED Parent Body: Implications for the Size of a Core and for Large-Scale Differentiation
If Vesta is the parent body of the howardite-eucrite-diogenite (HED) meteorites, then geochemical and petrologic constraints for the meteorites may be used in conjunction with astronomical constraints for the size and mass of Vesta to (1) determine the size of a possible metal core in Vesta, and (2) model the igneous differentiation and internal structure of Vesta. The density of Vesta and petrologic models for HED meteorites together suggest that the amount of metal in the parent body is <25 mass%, with a best estimate of ~5%, assuming no porosity. For a porosity of up to 5% in the silicate fraction of the asteroid, the permissible metal content is <30%. These results suggest that any metal core in the HED parent body and Vesta is not unusually large. A variety of geochemical and other data for HED meteorites are consistent with the idea that they originated in a magma ocean. It appears that diogenites formed by crystal accumulation in a magma ocean cumulate pile and that most non-cumulate eucrites (excepting such eucrites as Bouvante and Stannern) formed by subsequent crystallization of the residual melts. Modelling results suggest that the HED parent body is enriched in rare-earth-elements by a factor of ~2.5-3.5 relative to CI-chondrites and that it has approximately chondritic Mg/Si and Al/Sc ratios. Stokes settling calculations for a Vesta-wide, non-turbulent magma ocean suggest that early-crystallizing magnesian olivine, orthopyroxene, and pigeonite would have settled relatively quickly, permitting fractional crystallization to occur, but that later-crystallizing phases would have settled (or floated) an order of magnitude more slowly, allowing, instead, a closer approach to equilibrium crystallization for the more evolved (eucritic) melts. This would have inhibited the formation of a plagioclase-flotation crust on Vesta. Plausible models for the interior of Vesta, consistent with the data for HED meteorites and Vesta, includes a metal core (<130 km radius), an olivine-rich mantle (~65-220 km thick), a lower crustal unit (~12-43 km thick) composed of pyroxenite, from which diogenites were derived, and an upper crustal unit (~26-42 km thick), from which eucrites originated. The present shape of Vesta (with ~60 km difference in the maximum and minimum radius) suggests that all of the crustal materials, and possibly some of the underlying olivine from the mantle, could have been locally excavated or exposed by impact cratering.