Aubrite-anomalous
(enstatite pyroxenite)
Found 1941
24° 53' S., 117° 38' E.
An Australian prospector found the first fragments of this unusually metal-rich, unbrecciated, enstatite achondrite, totaling 1.7 kg. Mt. Egerton is a very weathered meteorite containing minor amounts of metal within a brown-stained enstatite phase. It is composed of cm-sized enstatite crystals with ~21 wt% FeNi-metal, an large abundance for any aubrite. The metal exhibits a very fine pseudo-octahedrite pattern upon etching due to the presence of the Ni-silicide perryite. Perryite only occurs in highly reduced meteorites in which pure magnesian silicates are incorporated in a low-Ni metal host. Small realms of schreibersite are present in the metal, metal which exhibits a fractionated siderophile element pattern similar to that of enstatite chondrites (van Acken et al., 2010 Humayun, 2010). Although it has been suggested that Mt. Egerton may represent a sampling of the core–mantle boundary of the aubrite parent body, the composition of the metal nodules are inconsistent with an origin from a differentiated core. Conversely, the siderophile element pattern (enriched in compatible siderophiles) of Mt. Egerton metal suggests that it could represent a residue formed from crystallization of a high degree partial melt (van Acken et al., 2010; Humayun, 2010).
The metal in both Mt. Egerton and Horse Creek has been described as being compositionally similar to the enstatite achondrite NWA 2526, which contains 10% metal. The anomalous iron meteorite Horse Creek is composed of Si-bearing metal with a perryite component. The close similarities which exist between Horse Creek and Mt. Egerton and other aubrites in their Si content of metal and their trace element abundances have led some to conclude that Horse Creek is genetically related to the aubrites. In their study of iron meteorite exposure histories, Welten et al. (2008) found that Horse Creek exhibits a complex exposure history comprising two stages. During the first stage of irradiation, which involved high shielding at a depth of ~60 cm in an object >2 m in diameter, cosmogenic noble gas data indicate a CRE age of 100 (+40/–30) m.y., placing it at the high end of the aubrite range. A second stage irradiation lasting ~1 m.y. occurred at a depth of 5–10 cm on a 30–50 cm diameter body. The investigators argue that the cosmogenic radionuclide and noble gas data for Horse Creek are consistent with that of debris ejected by a minor impact on a km-sized near-Earth object (NEO), which was followed by its rapid delivery to Earth. A possible source object for the Horse Creek meteorite is the Earth-crossing asteroid 3103 Eger, itself possibly derived from a larger main belt object.
Current spectral studies link the aubrites to a few near-Earth Apollo asteroids, specifically 3103 Eger and 434 Hungaria (Kelley and Gaffey, 2002). These two high-albedo, iron-free asteroids are composed of an enstatite-like silicate, and are of the appropriate size to make them primary candidates for the aubrite source body. Further evidence has been compiled which is consistent with 3103 Eger being the aubrite source body. For example, the time of day in which aubrites have fallen constrains the orbit to one similar to that of Eger. In addition, the long cosmic-ray exposure age of aubrites is consistent with a stable residence on a near-Earth asteroid that has a long-lived orbit, similar to that of Eger. Moreover, the orbital parameters derived for Norton County match those of Eger better than all other orbits. Asteroid 3103 Eger was probably once a member of the Hungaria family of asteroids, located in the innermost asteroid belt at 1.9 AU. It was subsequently ejected into an Earth-crossing orbit. Notably, the asteroid 2867 Steins has recently been studied by the Rosetta spacecraft. It was found to have an albedo and spectral properties consistent with those of an aubrite (with an abundance of CaS or oldhamite) (Abell et al., 2008).
Mt. Egerton has a cosmic-ray exposure age of 28 (±4) m.y., among the lowest of all the aubrites. Cosmogenic production rates indicate that it also had a small pre-atmospheric diameter of ~60 cm. Continued searches of the area have resulted in the recovery of thousands of additional small fragments of this aubrite for researchers and collectors alike. The specimens shown above are a cm-sized 2.7 g metallic fragment, similar in composition to the iron meteorite Horse Creek, and a 1.2 g fragment of enstatite silicate devoid of all metal.
