Angrites, a small but diverse suite of ancient, silica-undersaturated volcanic-plutonic mafic meteorites, and the history of their parent asteroid

Abstract

Angrites are medium- to coarse-grained (up to 2–3mm), unbrecciated and substantially unshocked igneous rocks of roughly basaltic composition. They are the most alkali-depleted basalts in the Solar System, but are not very depleted in moderately volatile elements such as Br, Se, S, Zn, In and Cd, and are relatively enriched in refractory elements such as Ca and Ti. They have uniform oxygen isotopic compositions and form a mass fractionation line on a Δ17O vs. δ18O diagram of Δ17O=−0.072±0.007 that sets them apart from other extraterrestrial basalts such as the HEDs. They consist of major Al-Ti-bearing diopside-hedenbergite (formerly called fassaite), calcic olivine, and anorthite. Minor and accessory phases include kirschsteinite, spinel, ulvöspinel, metallic Fe,Ni, troilite, titanomagnetite, whitlockite, ilmenite, and very rare carbonates, celsian, rhönite, and baddeleyite. On the basis of texture and mineralogical composition, they are divided into plutonic (5 members, +8 paired specimens) and volcanic (quenched) (6 members) angrites. Plutonic angrites have hypidiomorphic to granular and cumulate textures and contain nearly equilibrated, homogeneous minerals, whereas volcanic angrites have quench textures and consist of highly zoned minerals. These differences are the result of their different cooling rates: plutonic angrites cooled at model dependent rates of ∼303–323K/a or ∼274.6–275.4K/a (burial depths 14–17 or 68–75m, respectively), whereas volcanic angrites cooled at ∼280–286K/h in the temperature range of 1573–1273K, implying a burial depth of <0.5m. The angrites are amongst the oldest basaltic rocks in the Solar System, and Pb-Pb ages show that the slowly cooled plutonic angrites have younger ages ranging from ∼4558.86±0.30 to 4557.65±0.13Ma, and the more quickly cooled volcanic angrites have older ages ranging from 4564.18±0.14 to 4563.8±0.4Ma. While it has been suggested that the planet Mercury may be the source of the angrites, overwhelming evidence has been accumulated that they are fragments of a differentiated asteroid, probably >100km in radius and with a metal core which, based on 182Hf-182W systematics, formed within ∼2Ma of CAI formation. The origin and source lithologies of these fascinating rocks have been the topics of intense debates, and no consensus has yet been reached. However, the angrites are clearly igneous rocks and not impact melt-rocks or nebular condensates.