12 - Howardite–eucrite–diogenite clan

Summary

Introduction

The howardite, eucrite and diogenite (HED) meteorite clan is a suite of igneous rocks from a differentiated asteroid generally thought to be 4 Vesta. According to the Meteoritical Bulletin, as of June 2014, and not accounting for pairing, there were 1246 members of the HED clan, including 284 howardites (16 falls), 797 eucrites (34 falls) and 372 diogenites (11 falls) (www.lpi.usra.edu/meteor/metbull. php). This more than doubles the number listed in the Catalogue of Meteorites [12.1].

The three groups that form the HED clan are:

• (i) Howardites, regolith breccias, a physical mixture of fragments of eucrites and diogenites;

• (ii) Eucrites, fine-grained basalts with ∼40 vol.% plagioclase and ∼60 vol.% pyroxene (pigeonite); they are subdivided on the basis of chemistry and texture into gabbroic or basaltic, cumulate and non-cumulate;

• (iii) Diogenites, orthopyroxenites (usually with <15 vol.% olivine).

Figure 12.1 is a family tree showing the relationship between the different HED groups [12.2, 12.3]. Modal compositions of eucrites and diogenites are given in Table 12.1.

There is mineralogical, chemical and textural diversity among the HEDs [12.3–12.6]. Some of the earliest descriptions subdivided them into monomict or polymict, depending on whether or not they comprise clasts of just one chemical group (monomict) or several groups (polymict) [12.7]. The monomict subgroup includes diogenites (both olivine-bearing and olivine-free) plus all non-polymict eucrites. It contains both brecciated and unbrecciated members. The polymict subgroup is a compositional and textural continuum of regolith and surface breccias that includes components of cumulate eucrites, basaltic eucrites, diogenites and howardites [12.2, 12.8]. Meteorites of the HED clan are described as polymict when they contain more than 90% by volume of a single component (e.g., polymict eucrites contain >90% eucritic material).Howardites are the HED clan meteorites that contain <90% by volume of any single component (see Figure 12.1). They are part of a continuous sequence of polymict breccias from polymict eucrites to polymict diogenites [12.9, 12.10]. Mineralogy is important for distinguishing between polymict eucrites, polymict diogenites and howardites. The boundary of 10% (by volume) clast content that divides the howardites from the polymict subgroups was based on the amount of orthopyroxene detectable by X-ray powder diffraction [12.11].