Aubrite (main-group)
Impact-melt breccia
standby for mayo belwa photo
Fell August 3, 1974
8° 58' N., 12° 5' E.

Following the appearance of a fireball with detonations, which were heard 25 km away, a single stone weighing 4.85 kg was recovered in the Adamawa district of northeastern Nigeria and taken to the Geological Survey of Nigeria. Subsequent weighing by the British Museum showed it to weigh 4.272 kg.

The meteorite as found was an ellipsoidal, milky-white stone covered with a thin translucent fusion crust. It is the first impact-melt breccia among aubrites (A. Rubin, 2010), composed of mostly coarse-grained enstatite with some rounded olivines, which are up to 4× larger than olivines found in other aubrites. Plagioclase also occurs in higher abundance than in other aubrites, but only trace amounts of nearly-chondritic metal are present. Small black, glassy fragments within the solidified matrix are disordered enstatite or shock-blackened olivine. Enstatite grains exhibit mosaic extinction and planar fractures. Fe-rich end member alabandite [(Mn,Fe)S] is a unique feature to Mayo Belwa.

Sub-mm- to mm-sized vuggy cavities (~5 vol%) present in the matrix contain a diopside–enstatite–plagioclase lining with amphibole fluor-richterite crystals projecting from the walls. These vugs hint at a complex shock history involving a vapor phase for this aubrite, possibly at some depth on the parent body. The characteristic hardness of Mayo Belwa compared to most other aubrites is consistent with a formation at some depth. In a similar manner, the "fossil" EL3/6 meteorite NWA 2965 and pairings contain vesicles (~6.8 vol%), which A. Rubin (2016) has attributed to impact-induced evaporation of sulfides. He reasoned that the sparsity of metal observed in some parts of the mass, especially in the less weathered bluish-gray portions, is the result of metal–sulfide melt drainage into nearby regions as represented by Al Haggounia 001 with its large component of limonite (32.6 vol%) replacing FeNi-metal (0.29 vol%) along with sulfide (4.0 vol%).

Although aubrites and E chondrites share a common O-isotopic signature, chemical and mineralogical differences exist, as well as CRE age distinctions (no aubrites have CRE ages as young as E chondrites), which cast doubt on their formation on a common parent body. Some of these differences include the higher abundance of Ti and forsterite in aubritic sulfides than in E chondrites. Interestingly, a scenario reconciling these differences has been presented in light of an experiment in which an E chondrite was systematically melted in a very reducing, oxygen-depleted environment (McCoy et al., 1999).

In the experiment, as the silicate melt reached a temperature range of 1000–1300°C and a degree of partial melting of 20%, the metal–sulfide component began to migrate out of the silicate. At 1450°C, a completely separated metal component would have established a metallic core on its parent body. Since the sulfides melted at temperatures as low as 1000°C, it is demonstrated that aubritic sulfides cannot be a product of nebular synthesis as previously speculated. Instead, transfer of S and Ca from the S-rich silicate melts resulted in magmatic crystallization of oldhamite (CaS). Additionally, a phase was reached at 1500°C in which tectosilicate was reduced to Si within the metallic melt, with the subsequent crystallization of forsterite. Moreover, Ti-rich troilite crystallized from a combination of an Fe-rich sulfide melt and a mixed sulfide melt. All of these results from the experiment are consistent with a derivation of the aubrites from an E chondrite-type precursor in a strongly reducing, oxygen-depleted environment.

In a separate experiment (Fogel, 2001), the finding of roedderite in Peña Blanca Spring and Khor Temiki indicates that these aubrite melts were produced in peralkaline conditions, having a 1:1 ratio of K and Na. These are conditions at which a forsterite–enstatite–roedderite cotectic exists at 1155°C. By this scenario, it was demonstrated how the simultaneous crystallization of forsterite and enstatite from such a melt might have been stabilized with roedderite. However, such forsterite is also expected to be Si-rich, which is not the case in the new unbrecciated, olivine-bearing aubrite, LAP 03719 (McCoy et al., 2005). Furthermore, it was discovered that LAP 03719 lacks roedderite. Characteristics of this new aubrite call into question the origin of aubrites from known enstatite chondrites.

In a different study, it was found that two aubrites (Khor Temiki and LEW 87007) and an EH3 chondrite (Parsa) contain inclusions called Aubrite Basalt Vitrophyres, which were crystallized from a partial melt of a parental source having a composition similar to E chondrites (R. Fogel, 2005). These igneous inclusions may be the missing basalt component that provides a link between aubrites and their E chondrite-like precursors. They exhibit metal and sulfide depletions like those present in aubrites, which is consistent with fractionation of a partial melt. In addition, although neither aubrites nor aubrite basalt vitrophyres should contain forsterite, it occurs in both of these basaltic materials, probably due to peralkaline conditions as explained in the paragraph above.

In a broad study of aubrites based on a comprehensive data set for highly siderophile elements (HSE), and on new Os-isotopic composition analyses, van Acken et al. (2012) have constructed a compatible parent body petrogenesis. They found that HSE concentrations in aubrite silicates are significantly higher than would be expected given a completely segregated metal core. To account for this HSE signature, they proposed that core material was remixed with mantle silicates, and that there was late accretion of a fractionated metal component from chondritic projectiles. Based on the results of their study, it was concluded that the overall igneous differentiation processes on the aubrite parent body were most like those that occurred on the angrite parent body, but under very different redox conditions. Moreover, the more constrained range of aubrite HSE ratios compared to those of shergottites and HEDs likely reflects the smaller size and faster cooling of the aubrite parent body. By contrast, studies conducted by Boesenberg et al. (2013) led them to conclude that the precursor of aubrites is more consistent with bulk ordinary chondrite material, which experienced reduction and sulfidation processes over disparate regions of the parent body.

The pre-atmospheric diameter of Mayo Belwa, as calculated from cosmogenic production rates, was ~120 cm. Mayo Belwa and Norton County have the longest cosmic-ray exposure ages among stony meteorites, 117 (±14) m.y. and 115 (±6) m.y., respectively, possibly representing a common ejection event. Although six other aubrites have CRE ages that establish an apparent cluster at 56 (±6) m.y., it has been demonstrated that many of these are breccias which experienced a regolith pre-irradiation history, while some were infuenced by solar wind gases or chondritic inclusions prior to space exposure (Lorenzetti et al., 2003). All of these factors render the probability of a CRE age cluster very low. The anomalous aubrites Mt. Egerton and Shallowater, and the EL impact melt Happy Canyon share CRE ages ranging from 23 to 28 m.y., which may reflect a common ejection event.

The specimen of Mayo Belwa shown above is a 1.3 g cut and polished fragment. A more impressive 68.1 g specimen of Mayo Belwa resides in the Jay Piatek Collection, seen here courtesy of Dr. Piatek.