Aubrite (main-group)

standby for nwa 6350 photo
click on photo for a magnefied view

Found 2010
no coordinates recorded

A small, relatively fresh stone weighing 50.5 g was found in the Sahara and designated NWA 6350. Analysis was conducted by the University of Washington in Seattle (A. Irving), and NWA 6350 was initially determined to be a likely pairing with the fusion crusted, 39.1 g aubrite, NWA 5217, found in 2007 in Morocco. Thereafter, it was considered likely that it was a member of a large pairing group, additionally comprising the NWA-series numbers 4537, 4799, 4832, 4871, 5217, 5419, 6193, 6675, and probably the largest and least weathered mass, 7214, all of which together weigh 5,047.6 g.

Northwest Africa 6350 is a rare unbrecciated (common only to the aubrites Mt. Egerton and Shallowater), cumulate-textured aubrite, formed through igneous processes and fractional crystallation. Northwest Africa 6350 consists of a fine- to medium-grained aggregate of mostly pure enstatite along with minor amounts of sodic plagioclase, daubreelite, Si-bearing kamacite, Cr-bearing troilite, oldhamite, alabandite, niningerite, caswellsilverite, graphite, and rare zincian brezinaite (Bunch and Wittke, NAU). The enstatite grains exhibit a preferred orientation.

According to the authoritative source, the Meteoritical Bulletin Database, out of a total of nearly 6,000 meteorites recovered thus far from the desert regions of Northwest Africa, only a small percentage are aubrites. Besides NWA 6350 and its large pairing group, the anomalous aubrite NWA 1235 was determined to be a unique reduced achondrite genetically related to the enstatite meteorite clan. Although NWA 2736 was initially classified as an aubrite, in-depth studies conducted by Bunch et al. (2006) examining numerous paired samples (with various NWA-series designations) revealed the presence of chondrules. Therefore, that pairing group has been reclassified as an EL3 chondrite.

In addition to these classified aubrites, three separate NWA stones have been classified as enstatite achondrites: 1) the 42.9 g NWA 2526 found in 2003 contains 10% metal; 2) the 132.8 g NWA 1840 found in 2003 has many features similar to Shallowater and might be related to that unique enstatite parent body, or it could represent a 5th enstatite parent body; 3) the 483 g NWA 4642 was found in 2007.

A large proportion (~40%) of aubrites are witnessed falls, which is thought to reflect the fact that these highly reduced meteorites are particularly susceptible to terrestrial weathering once they arrive on Earth. Although NWA 6350 is a comparatively fresh meteorite that has preserved its accessory minerals, the original FeNi-metal component in the form of kamacite has been converted to secondary weathering products manifest as limonite veinlets and orange staining along enstatite grain boundaries. Since all of the stones constituting the pairing group were recovered throughout the years 2005–2010, they have experienced a range of terrestrial weathering processes. They now exhibit weathering grades of W0/1–W5, yet mineral phases associated with the aubrite group are still prevalent in them all (Irving and Kuehner, UWS). The surprisingly rapid alteration processes that affect all aubrites in Earth's oxidizing and wet environs is demonstrated visually in the following photos of stones from a common fall. On the left, shown courtesy of Darryl Pitt, is the fresh (W0/1) 2.2 kg NWA 7214 stone that was recovered in 2006, exhibiting a high content of FeNi-metal flakes throughout with virtually no visible oxidation. The two much smaller stones—the 510 g NWA 6675 in the middle and the 50.5 g NWA 6350 stone on the right—had both remained in the terrestrial environment until their recovery in 2010 and have sustained considerable weathering; the 10× larger NWA 6675 stone has experienced significantly less alteration than NWA 6350 probably due to their comparative sizes.

Keil (2010) suggests that the extremely reducing conditions under which aubrites formed is evidence for a location within 1 AU of the Sun, but on a parent body other than any of the known E chondrites or the Shallowater source object. The size of the differentiated aubrite planetesimal(s) is constrained by those processes which caused it to melt. Arguments suggesting that the heat source was the decay of short-lived radionuclides such as 26Al have not been reconciled with the apparent low Al and plagioclase contents in aubrites. In a similar manner, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D'Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting without an additional heat source. It has been suggested that relatively small planetesimals such as the aubrite planetesimal(s) might have been just the required size to allow heating by induction in the plasma environment of the T Tauri Sun.

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 was recently studied by the Rosetta spacecraft and 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); however, the unique texture and mineralogy of NWA 5217/6350 indicate it derives from a significantly larger parent body.

For additional information on the formation of the aubrite group visit the Mayo Belwa page. The specimen of NWA 6350 shown above is a 1.03 g partial slice.