Determining post-core formation accretion in asteroid 4-Vesta and implications for inner Solar System late accretion

Highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, Pd) abundances are reported for eucrites, diogenites and howardites from Vesta. The four orders of magnitude variation (1 × 10 –3 to 1 × 10 –6 × CI chondrite) and measured 187 Os/ 188 Os (0.103 to 3.07) ranges in eucrites primarily reflect post-crystallization impactor additions and constrain meteorites previously identified as pristine (<1 μg/g Ni) as having limited (<0.1%) chondritic HSE additions. Pristine eucrites have fractionated inter-element HSE ratios with respect to chondrites, with a wide range of 187 Os/ 188 Os, making them distinct and more HSE-rich than lunar mare basalts. These eucrites define the crustal Vesta HSE composition as broadly similar to modern terrestrial basaltic oceanic crust. Models assuming either residual metal in the mantle, or its complete exhaustion, during eucrite melt extraction predict an HSE enriched bulk silicate Vesta compared to bulk silicate Earth or bulk silicate Mars, requiring between 0.4 to 1.9% post-core formation accretion (PCFA). A preferred alternative model using regression of the HSE to MgO contents of eucrites and diogenites derives a composition between bulk silicate Earth and bulk silicate Moon, requiring ∼0.12% PCFA. Such a composition is like the HSE contents measured for peridotites considered to come from Vesta. The estimates of PCFA from this large dataset of vestan meteorites are all higher than previously determined estimates, requiring a proportional increase in mass from leftover planetesimal and asteroidal sources to the inner Solar System planets.