MOUNT EGERTON


Aubrite-anomalous
(Metal-rich, light-REE-enriched)
standby for mount egerton photo
Found 1941
24° 53' S., 117° 38' E.

The first fragments of this unusually metal-rich, unbrecciated enstatite achondrite were found by an Australian prospector, with subsequent recoveries bringing the total combined weight to ~22 kg. Mount Egerton is a very weathered meteorite containing minor amounts of Si-bearing metal both as a host phase and as inclusions within a predominant brown-stained enstatite phase. The meteorite is composed primarily of cm-sized enstatite crystals with inhomogeneously distributed FeNi-metal composing ~21 wt%—a large abundance for any aubrite. Minor abundances of diopside and various sulfides are present (Cr-bearing troilite, brezinaite, alabandite); notably, Mount Egerton contains no feldspar (Watters and Prinz, 1980).

The metal in Mount Egerton 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 phase. In contrast to the metal in the main-group aubrites, the metal in Mount Egerton exhibits a mostly chondritic highly siderophile element (HSE) pattern similar to that of enstatite chondrites, particularly that of EL chondrite metal (van Acken et al., 2010; Humayun, 2010; van Acken et al., 2012). Although it has been suggested that Mt. Egerton might represent a sampling of the core–mantle boundary of the aubrite parent body (Watters and Prinz, 1980), the unfractionated composition of the metal nodules are inconsistent with an origin from a differentiated core (Barrat et al., 2016). Conversely, the siderophile element pattern (enriched in compatible siderophiles) of Mount 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 Mount Egerton and the anomalous iron meteorite Horse Creek (as well as the anomalous irons LEW 85369, LEW 88055, and LEW 88631) has been described as being compositionally similar (i.e., having complementary HSE patterns in metal) to metal in the anomalous enstatite achondrite NWA 2526, a partial melt residue (after ~20% partial melt extraction) containing ~10–15% metal (Keil and Bischoff, 2008; Humayun et al., 2009; M. Humayun, 2010). Along with the similar partial melt residue Itqiy, these meteorites might share a common origin on an E chondrite-like parent body unique from the Shallowater, EH, EL, and main-group aubrite parent bodies (Keil and Bischoff, 2008). Horse Creek is composed of Si-bearing (2.5 wt%) metal with a perryite component; this is similar to the Si content in host metal in Mount Egerton of 2.03 wt% which also contains perryite. Mount Egerton and Horse Creek also have similar W-isotopic compositions which is unlike that of other aubrites (Barrat et al., 2016). The close similarities shown by Mount Egerton and Horse Creek (and some other aubrites) in trace element abundances, compatible siderophile element enrichments, and Si content of metal, have led some to conclude that Horse Creek is genetically related to aubrites.

Precise trace element and isotopic analyses of a broad sampling of aubrites was conducted by Barrat et al. (2016). They ascertained that all aubrites in the study have identical O-isotopic compositions (considering a degree of contamination in Cumberland Falls and Larned) with a high level of Δ17O homogeneity, consistent with an early stage of extensive melting on the aubrite parent body(ies), possibly involving a magma ocean. However, both Mount Egerton and Larned exhibit significant light-REE enrichments compared to the other aubrites, which have light-REE depletions. In addition, the negative Eu anomalies in pyroxenes from Mount Egerton and Larned are less extreme than in the other aubrites. A further difference between main-group aubrites and the Mount Egerton and Larned aubrites can be seen in the siderophile element contents of metal; as described above, these two meteorites contain metal that is much less fractionated than that in other aubrites. In consideration of these differences, Barrat et al. (2016) concluded that Mount Egerton, Horse Creek, Larned, and possibly NWA 2526, likely represent a separate parent body from that of the main-group of aubrites.

In their study of iron meteorite exposure histories, Welten et al. (2008) found that Horse Creek exhibits a complex exposure history consisting of 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 that 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 in having an abundance of CaS or oldhamite (Abell et al., 2008).

Mount Egerton has a cosmic-ray exposure age of 28 (±4) m.y., which is 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.