Fluid-Mediated Alteration on 4 Vesta - Evidence from Orthopyroxene Clasts in Howardites

摘要

The howardite, eucrite and diogenite (HED) meteorites represent the products of igneous processes and impact mixing on a differentiated asteroid. Eucrites are basaltic composition rocks that were formed as flows and as shallow and deep intrusive bodies. Some eucrites are cumulate gabbros. Diogenites are cumulate orthopyroxenites widely considered to be of deep crustal origin. Impact processes have excavated material from deep levels of the crust, and mixed it with surface rocks into a suite of polymict breccias. Howardites are one such rock type, being composed mostly of mixtures of clasts and mineral fragments of eucritic and diogenitic parentage (see 1). The consensus view is that 4 Vesta is the parent asteroid of HED meteorites (2). As part of a larger study of the record of impact mixing contained within howardites, we undertook an investigation of the compositions of orthopyroxene clasts in a suite of howardites (3). We discovered a subset of orthopyroxene clasts in some howardites with unusual textural and compositional characteristics that are reminiscent of those previously observed in phenocrysts in a pyroxene-phyric melt clast in howardite EET 92014 (4). The textural and compositional characteristics of the phenocrysts in this clast were interpreted as originating via interaction with an FeO-rich fluid phase.