| Abstract: | Abstract— The complete (or near complete) differentiation of a chondritic parent body is believed to result in an object with an Fe-Ni core, a thick olivine-dominated mantle and a thin plagioclase/pyroxene crust. Compositional groupings of iron meteorites give direct evidence that at least 60 chondritic parent bodies have been differentiated and subsequently destroyed. A long standing problem has been that our meteorite collections, and apparently our asteroid observations as well, show a great absence of olivine-dominated metal-free mantle material. While the basaltic achondrites (HED meteorites) represent metal-free pyroxene-dominated crustal samples, the isotopic and geochemical evidence implies that this class is derived from only one parent body (perhaps Vesta). Thus the meteoritic (and perhaps astronomical) evidence also suggests a great absence of crustal material resulting from the collisional disruption of numerous parent bodies. One explanation for the rarity of olivine-dominated metal-free and basaltic asteroids that fits all the available evidence is that all differentiated parent bodies, with the exception of Vesta, were either disrupted or had their crusts and mantles stripped very early in the age of the solar system. The resulting basaltic and olivine-dominated metal-free fragments were continually broken down until their sizes dropped at least below our current astronomical measurement limit (～5–10 km for inner-belt objects) and perhaps completely comminuted such that meteorite samples are no longer delivered. Because of their greater strengths and longer survival time in interplanetary space, only the iron and the stony-iron meteorites remain as the final tracers of this differentiation and collisional history. However, other scenarios remain plausible such as those which invoke “space weathering” processes that effectively disguise the distinctive basaltic and olivine spectra of possible remnant crustal and mantle material within the main asteroid belt. |
