Enstatite achondrite meteorites (aubrites) and the histories of their asteroidal parent bodies

Abstract The aubrites are nearly monomineralic enstatite pyroxenites, consisting mostly of nearly FeO-free enstatite, with minor albitic plagioclase, nearly FeO-free diopside and forsterite, metallic Fe,Ni, troilite, and a host of rare accessory minerals, many unknown from Earth, that formed under highly reducing conditions. As a result, many of the normally lithophile elements such as Ti, Cr, Mn, Na, etc. behave partly as chalcophiles (i.e., occur in sulfides), and Si is partly siderophile and occurs in metallic Fe,Ni. Aubrites must therefore have formed in a very unique part of the solar nebula, possibly within 1 AU of the Sun. While of the 27 aubrites, 15 are fragmental breccias, 6 are regolith breccias, and 6 are described as non-brecciated, their ingredients are clearly of igneous origin and formed by melting and fractional crystallization, possibly of a magma ocean. This is indicated by the occurrence of a variety of lithic clasts of igneous origin, and by the REE and other trace element distributions. Their highly reduced nature and their oxygen isotopic compositions suggest close kinship to the enstatite chondrites. However, they did not form from known EH or EL chondrites on their parent bodies. Rather, they formed from enstatite chondrite-like material on at least two separate parent bodies, the Shallowater parent body and, for all other aubrites, on the aubrite parent body. Visible and near-infrared reflectance spetra of asteroids suggest that the aubrite parent bodies may be asteroids of the E-type and perhaps the E(II) sub-class, such as 3103 Eger and 2867 Steins (the target of the Rosetta Mission). If aubrites formed by the melting and fractional crystallization of enstatite chondrite-like parent lithologies, which should have contained ∼10 vol% plagioclase, then meteorites of enstatite-plagioclase basaltic composition should exist, which is not the case. These early basaltic melts may have been removed from the aubrite parent body by explosive pyroclastic volcanism, and these small pyroclasts would have been destroyed in space long ago. Age dates suggest that the aubrites formed very early in the history of the solar system, within a few Ma of CAI formation, and that the heat sources for heating and melting of their parent bodies were, most likely, short-lived radionuclides such as 26Al and, perhaps, 60Fe. Finally, attention has been drawn to the surface composition of Mercury of low bulk FeO and of nearly FeO-free enstatite, perhaps with plagioclase, diopside and sulfide. While known aubrites clearly did not originate from Mercury, recent calculations suggest that several percent of high-speed ejecta from Mercury reach Earth. This is only factors of 2–3 less than typical launches from Mars and, since there are now 53 Martian meteorites in our collections, meteoriticists should be alert to the potential discovery of a genuine meteorite from Mercury which, superficially, should resemble aubrites. However, recent results from the Neutron Spectrometer of the Messenger Flyby of Mercury have been interpreted to suggest that the planet’s surface may, in fact, contain abundant Fe–Ti-oxides and, if true, a meteorite from Mercury should not resemble any currently known meteorite type.