AubriteAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8). They contain large white crystals of enstatite as (main-group) Impact-melt brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material
Fell August 3, 1974 8° 58′ N., 12° 5′ E. Following the appearance of a fireballA fireball is another term for a very bright meteor, generally brighter than magnitude -4, which is about the same magnitude of the planet Venus as seen in the morning or evening sky. A bolide is a special type of fireball which explodes in a bright terminal flash at its end, often with visible fragmentation. 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 meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and as found was an ellipsoidal, milky-white stone covered with a thin translucent fusion crustMelted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of 4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below.. It is the first impact-melt breccia among aubritesAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8). They contain large white crystals of enstatite as (A. Rubin, 2010), composed of mostly coarse-grained enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). with some rounded olivines, which are up to 4× larger than olivines found in other aubrites. PlagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite also occurs in higher abundance than in other aubrites, but only trace amounts of nearly-chondritic metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn are present. Small black, glassy fragments within the solidified matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. are disordered enstatite or shock-blackened olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion. Enstatite grains exhibit mosaic extinctionIn astronomy, the dimming of starlight as it passes through the interstellar medium. Dust scatters some of the light, causing the total intensity of the light to diminish. It is important to take this effect into account when measuring the apparent brightness of stars. The dark bands running across portions and planar fractures. Fe-rich end member alabanditeMagnesium sulfide found in aubrites and EL chondrites. Its formula is MnS. [(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 bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today.. 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 evaporationProcess in which atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. 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 chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such 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 forsteritePure* magnesium end-member (Mg2SiO4) of the olivine solid solution series and an important mineral in meteorites. When magnesium (Mg) is completely substituted by iron, it yields the the pure Fe-olivine end member, fayalite (Fe2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) 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 chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such was systematically melted in a very reducingOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants, oxygen-depleted environment (McCoy et al., 1999).
In the experiment, as the silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the melt reached a temperature range of 1000–1300°C and a degree of partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different 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 coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. 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 crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. of oldhamiteMn-Ca sulfide, (Mn,Ca)S, is a pale to dark brown accessory mineral found in minor amounts in highly reduced meteorites such as many enstatite chondrites, and some aubrites and enstatite achondrites. Oldhamite in enstatite chondrites likely formed by solar nebular gas condensation. CaS Oldhamite was also found in the most fresh (CaS). Additionally, a phase was reached at 1500°C in which tectosilicate was reducedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants to Si within the metallic melt, with the subsequent crystallization of forsterite. Moreover, Ti-rich troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. 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 BasaltBasalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition. Basalt is a mafic, extrusive and fine grained igneous rock 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 fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. 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 mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in silicates, and that there was late accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the of a fractionated metal component from chondritic projectiles. Based on the results of their study, it was concluded that the overall igneous differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities. When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including processes on the aubrite parent body were most like those that occurred on the angriteType of evolved achondrite meteorite that represent some of the earliest stages of asteroidal differentiation and magmatism in our solar system. Angrites are named for the Angra dos Reis meteorite, which fell in Rio de Janeiro, Brazil, in early 1869. They are basaltic (mafic) rocks, often containing porous areas, and parent body, but under very different redoxOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants conditions. Moreover, the more constrained range of aubrite HSE ratios compared to those of shergottitesIgneous stony meteorite with a Martian origin consisting mainly of plagioclase (or a shocked glass of plagioclase composition) and pyroxene. They are the most abundant type of SNC meteorites and the type member is the Shergotty meteorite, which fell in India in 1865. Shergottites are igneous rocks of volcanic or 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 chondriteWork in Progress Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018)1. Meteorites & the Early Solar System: page 581 section 6.1 OC of type 5 or 6 with an apparent shock stage of S1, material, which experienced reductionOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants 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 regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles). pre-irradiation history, while some were infuenced by solar windSupersonic flow of high-speed charged particles continuously blowing off a star (mostly e- and p+). When originating from stars other than the Sun, it is sometimes called a "stellar" wind. The solar wind may be viewed as an extension of the corona into interplanetary space. The solar wind emanates radially 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.