Evidence for excavation of deep crustal material of a Vesta-like body from Ca compositional gradients in pyroxene

Abstract

Recent discovery of basaltic achondrite asteroids in the main belt has provided a link between basaltic achondrites (howardite, eucrite and diogenite) and asteroid 4 Vesta. A spectral study suggests that these asteroids are fragments ejected from Vesta and that Vesta's eucritic crust is of substantial thickness. We computed the cooling rate and burial depth of the Moore County cumulate eucrite on the basis of Ca compositional gradients and the width of augite lamella in pyroxene to obtain information on the early evolution of the parent body. Our calculations show that: (1) Moore County cooled initially at a rate of 0.00016°C/yr, which corresponds to a burial depth of about 8 km, (2) after this cooling, a sudden temperature rise occurred, probably due to an impact, and finally (3) Moore County cooled down again at a rate of 0.35°C/yr. The latter events may correspond to the excavation of crustal material. The burial depth of Moore County is consistent with the result of a recent spectral study in which the largest observed eucritic fragment (10 km) sets a lower limit on the thickness of the eucritic crust. Reheating and excavation of a deep crustal material imply impact events which are supported by spectral observations.