11 - Aubrites

Summary

Introduction

Aubrites (or enstatite achondrites) are coarse-grained breccias with a reduced mineralogy dominated by almost FeO-free enstatite. They have a variable metal content, and their mineralogy and oxygen isotope composition is very similar to enstatite chondrites (EC) [11.1–11.4], although the relationship between aubrites and EC is unclear [11.5]. According to the Meteoritical Bulletin (www.lpi.usra.edu/meteor/metbull.php), as of June 2014, and not accounting for pairing, there were nine observed aubrite falls and 62 finds, including six meteorites classified as anomalous aubrites. The type meteorite, Aubres, fell in France in 1836.

Mineralogy

Aubrites formed under highly reducing conditions and contain a variety of minerals rare in other extraterrestrial rocks (with the exception of EC). The following descriptions are based on the studies of [11.1, 11.2, 11.5, 11.6] and references therein. Enstatite, with very low iron content (FeO <1.0 wt.%), is the most abundant phase (75–98 vol.%); other components present in very variable abundances include forsterite (0.3–10 vol.%), diopside (<3 vol.%), plagioclase (0.3–16 vol.%), kamacite (<4 vol.%) and troilite (<7 vol.%). Figure 11.1 shows the restricted compositional range of pyroxene and olivine in aubrites.

Enstatite grains are millimetres to centimetres in size and are essentially homogeneous in composition and almost FeO-free (En_99–100). Diopside (Wo_40–46) occurs both as isolated grains and as exsolution lamellae in enstatite and is also almost FeO-free (Figure 11.1). The high-calcium pyroxene is characterized by a very low Cr2O3 content (<0.1 wt.%), lower that in other achondrite groups (Figure 11.2). Minor amounts of FeO-bearing orthopyroxene (En_77.8–80.6Wo_0.4–1.0) are found within matrix and in clasts [11.8]. Forsteritic olivine (Fa_0.2) occurs in the matrix as distinct grains up to 5 mm in size, and also within pyroxene grains. Olivine is also Cr₂O₃-poor (≪0.1 wt.%) with CaO ∼0.07 wt.% [11.8]. Plagioclase is generally albitic, with a narrow compositional range, An_1.8–8.2 [11.2, 11.6]. Plagioclase in impact melt clasts from Norton County has a range of compositions (An_0–92.3), implying that the grains crystallized rapidly [11.9]. Fe-Ni metal occurs in a variety of morphologies: as individual cm-sized nodules, as micronsized inclusions within silicates, and as interstitial grains between silicates [11.2, 11.10].