Diogenites as polymict breccias composed of orthopyroxenite and harzburgite

Abstract– A few relatively unbrecciated olivine‐rich diogenites consist of an equilibrium assemblage of olivine and magnesian orthopyroxene (harzburgite). More common diogenites with smaller amounts of olivine are breccias containing two distinct orthopyroxenes—one magnesian and one ferroan. These diogenites are mixtures of a harzburgite lithology that is more magnesian, with the “normal” orthopyroxenite lithology that is ferroan and may contain small amounts of plagioclase. Both lithologies likely formed by fractional crystallization in multiple plutons emplaced within the crust of asteroid 4 Vesta. Minor element trends in orthopyroxenes indicate that these plutons exhibited a range of compositions. We propose a revised taxonomy for the HED (howardites, eucrites, and diogenites) suite where all ultramafic samples are referred to as diogenites. Within this group, the prefixes dunitic, harzburgitic, and orthopyroxenitic are used to distinguish diogenites consisting of more than or equal to 90% olivine, olivine + orthopyroxene, and more than or equal to 90% orthopyroxene, respectively. The prefix polymict is used to describe brecciated mixtures of any of these rock types. The recognition that olivine is a significant phase in some diogenites is consistent with spectral interpretations of olivine in a deeply excavated crater on Vesta, and has important implications for the bulk composition and petrogenesis of that body.