9 - Ureilites

Summary

Introduction

Ureilites are coarse-grained, ultramafic meteorites. They have igneous textures and are depleted in incompatible lithophile elements, indicating that they have been heated, but they also exhibit primitive, nebula-derived signatures, including variable oxygen isotopic compositions. Along with acapulcoites, lodranites, brachinites and winonaites, ureilites are primitive achondrites and form a bridge between undifferentiated chondrites and fully differentiated meteorites. According to the Meteoritical Bulletin (June 2014), and not accounting for pairing, there are 368 ureilites [9.1], making them the second-largest group of stony achondrites. Only six of the ureilites are falls; the type specimen after which the group is named is Novo Urei. The most recent fall (Almahata Sitta, which fell in the Sudan in October 2008) was observed as an incoming asteroid by several telescopes as it approached the Earth, allowing its trajectory to be calculated and its fall location identified [9.2].

Mineralogy

The information in this section is drawn from review papers by Mittlefehldt, Goodrich and colleagues [9.3–9.5], plus references therein. Many ureilites are an assemblage of ∼60 vol.% olivine and ∼30 vol.% pyroxene, with minor (≤10 vol.%) matrix or interstitial material consisting of carbon (graphite and diamond), iron-nickel metal, sulphides (troilite with up to 34.0 wt.% chromium [9.6]), and finegrained silicates (Table 9.1). Olivine (mg# 76–95) and pyroxene (mainly pigeonite, but some ureilites contain orthopyroxene or augite) core compositions vary widely between ureilites but are homogeneous within each ureilite. The core compositions of olivine and pyroxene in unbrecciated ureilites are shown in Figure 9.1. Most ureilites (∼90%) are unbrecciated; the remainder are polymict breccias. (Note, we follow the suggestion of Downes et al. [9.7] and drop the term ‘monomict’ for unbrecciated meteorites.)

Olivine grains in unbrecciated ureilites are euhedral to anhedral, have inclusion-free cores (Fa_5–24) and are generally rich in Cr₂O₃ and CaO [9.3–9.5]. Their high Cr₂O₃ and CaO contents distinguish ureilite olivines from olivine in other achondrites (Figure 9.2). Many olivine grains have an almost FeO-free rim (∼10–100 μm) containing small inclusions of Fe-Ni metal [9.8, 9.9]. Pigeonite grains lack exsolution lamellae, suggesting rapid cooling from high temperatures [9.10, 9.11]. Pigeonite contains fine-grained metallic inclusions (0.46–2.60 vol.%); the absence of reduction rims around these inclusions implies co-genesis with the pigeonite [9.12].