Abee
EH 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 Click on Term to Read More
(EH4 in MetBull 8)
(EHb IMB in Weyrauch et al., 2018)
Fell June 9, 1952
54 ° 13′ N., 113 ° 0′ W. At 11:05 P.M., people in Alberta, Canada witnessed 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. Click on Term to Read More accompanied by detonations. Five days later, a single 107 kg stone was recovered from a hole approximately 1 m in diameter and 2 m deep in a wheat field near the town of Abee, 90 km north of Edmonton. On December 9, 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 Click on Term to Read More was purchased from the finder, Harry Buryn, for $10/kg by the Geological Survey of Canada.
The Van Schmus–Wood (1967) scheme for petrographic type was modified for 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). Click on Term to Read More 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 Click on Term to Read More, establishing both a textural type reflecting peak metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More temperature (3–7), and a mineralogical type pertaining to the cooling history (α–δ) (Zhang and Sears, 1996; Quirico et al., 2011). Under this classification scheme, Abee has high-temperature thermometers consistent with a classification of EH4γ.
Employing multiple lines of evidence including chemical, petrographic, metamorphic, and cosmic-ray exposure ageTime interval that a meteoroid was an independent body in space. In other words, the time between when a meteoroid was broken off its parent body and its arrival on Earth as a meteorite - also known simply as the "exposure age." It can be estimated from the observed effects Click on Term to Read More data, previous studies had suggested that the EL and EH chondrites originate from different layers on the same 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. Click on Term to Read More. However, subsequent studies utilizing very precise isotopic measurements were made of a statistically larger sampling of E chondrites and 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 Click on Term to Read More. Although the O-isotopic data for the samples were indeed identical, a three-isotope plot distinguished the EH group from the EL and 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 Click on Term to Read More groups by its slightly steeper slope; the plots of the EL and aubrite groups were colinear with the terrestrial 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. Click on Term to Read More line. A third grouplet with intermediate mineralogy has recently been identified, represented by the meteorite Y-793225; an EH-an classification has been proposed (Rubin and Wasson, 2011). Studies of Y-793225 have determined that it is not derived from the EH or EL groups through any metamorphic proccesses, and thus could represent a unique enstatite parent body. The Shallowater and Itqiy meteorites are also considered by some to have originated from two additional unique enstatite parent bodies.
Weyrauch et al. (2018) analyzed the mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More and the Fe concentration in niningerite and alabanditeMagnesium sulfide found in aubrites and EL chondrites. Its formula is MnS. Click on Term to Read More (endmembers of the [Mn,Mg,Fe] solid solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréeliteFe-Cr sulfide (FeCr2S4) that crystallizes with cubic symmetry and found as an inclusion in meteoritic iron. Click on Term to Read More and the content of Ni in kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More were demonstrated to be consistent factors for the resolution of four distinct 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 Click on Term to Read More groups: EHa, EHb, ELa, and ELb (see table below).
ENSTATITE CHONDRITEType of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. Their highly reduced nature indicates that they formed in an area of the Click on Term to Read More SUBGROUPS
Weyrauch et al., 2018
| EHa | EHb | ELa | ELb | |
|---|---|---|---|---|
| Troilite | Cr <2 wt% | Cr >2 wt% | Cr <2 wt% | Cr >2 wt% |
| (Mn,Mg,Fe)S | Fe <20 wt% | Fe >20 wt% | Fe <20 wt% | Fe >20 wt% |
| Daubréelite | Abundant | Missing | Abundant | Missing |
| Kamacite | Ni <6.5 wt% | Ni >6.5 wt% | Ni <6.5 wt% | Ni >6.5 wt% |
A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids. The revised E chondrite classification scheme of Weyrauch et al. (2018) including selected examples from their 80-sample study can be found here. It was determined that Abee is a member of the EHb subgroup.
The formation of Abee began with the 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 Click on Term to Read More of chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More that were ~5.6 m.y. younger than carbonaceous chondrites (based on Mn systematics). It is thought that Abee experienced sulfurization of 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 Click on Term to Read More within the protoplanetary nebulaAn immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive Click on Term to Read More (Lavrentjeva et al., 2006), and thereafter, the rock was transformed in a high-temperature impact event in which up to 90% of the chondrules were melted or resorbed. This shock produced diamonds in Abee in a concentration of ~100 ppmParts per million (106). Click on Term to Read More, likely derived from primary graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More.
Euhedral enstatite grains crystallized from 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 and kamacite-rich rims formed around the clasts and relict EH material. The high-temperature silicaSilicon dioxide, SiO2. polymorphs cristobaliteHigh temperature polymorph of silicon dioxide (SiO2). Has the same chemical composition as coesite, stishovite, seifertite and tridymite but possesses a different crystal structure. This silica group mineral occurs in terrestrial volcanic rocks, martian and lunar meteorites, chondrites and impact glasses like Libyan Desert Glass. Cristobalite has a very open Click on Term to Read More and tridymiteSilica group mineral in which the tetrahedra occur in sheets. Tetrahedra alternately point up or down to share oxygen with tetrahedra of other sheets, forming six-sided rings perpendicular the sheets. Tridymite has a fairly open structure and accommodates Na+, K+ and Ca2+; charge balance is achieved by Al3+ ↔ Si4+. were formed from the chondruleRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More melt and preserved through rapid cooling/quenching. Presolar SiC is present in Abee at a concentration of 6 ppbParts per billion (109). Click on Term to Read More (Huss, 1990). The mineral keilite (Fe+2,Mg)S crystallized from the melt phase of niningerite and troilite (Rubin, 2008). Large kamacite nodules crystallized from C-rich metal–sulfide melt regions along with the precipitation of graphite laths, while F was incorporated into fluor-richterite grains. A dark, fine-grained, oldhamite-rich, plagioclase-rich component (~0.2 vol%) was also an igneous product of the partial melt. As envisioned by Rubin and Scott (1997), repeated impacts shattered the homogeneous igneous lithology and produced 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 Click on Term to Read More of metal and silicate. Thereafter, a second major melting event occurred, probably from impact, producing an enstatite melt that flooded and absorbed the smallest clasts and relict chondrules. Intermixing of the larger silicate clasts and relict chondrules with the metal–sulfide component occurred, followed by rapid quenching and annealing.
An investigation of the compositional variation that exists among the components in Abee was conducted by Higgins and Martin (2018). They propose a less complex process for the observed variability (i.e., clasts enriched in metal compared to the matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More)–a mechanical form of 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 Click on Term to Read More which they termed ‘smithing’, from the analogous technique of ancient iron working. Higgins and Martin (2018) contend that this smithing process occurred between the initial partial melting and secondary major melting events envisioned by Rubin and Scott (1997) (see preceding paragraph). Starting from a brecciated but broadly homogeneous igneous lithology, impact-generated shockwaves occurring over an extended period produced fracturing of the brittle phases (silicates, sulfides, etc.) and enabled the migration of these small fragments from the surface of clasts and into the open matrix, thus leaving the clasts enriched in the more malleable kamacite. The source lithology of the Abee meteorite subsequently underwent heating and recrystallization giving rise to its current compact texture. The authors speculate that the formation of aubrites may also be attributed to such a smithing process.
Although E chondrites and aubrites do share a common O-isotopic signature, certain chemical and mineralogical differences exist which had previously cast doubt on their formation on a common parent body. Some of these differences include a 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) Click on Term to Read More in aubritic sulfides than in E chondrites. A scenario reconciling these differences has been presented in light of an experiment in which an E chondrite 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 Click on Term to Read More, oxygen-depleted environment.
In the experiment, as the silicate melt reached a temperature range of 1000–1300 °C, and the degree of partial melting reached 20%, the metal-sulfide component began to migrate out of the silicate. At 1450 °C, a completely separated metal component could 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. Click on Term to Read More on its parent body. Since the sulfides melted at temperatures as low as 1000 °C, it was demonstrated that aubritic sulfides cannot be a product of nebular synthesis as previously speculated. Instead, tranfer 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. Click on Term to Read More 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 Click on Term to Read More (CaS). Moreover, a phase was reached at 1500 °C in which SiO2 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 Click on Term to Read More to Si within the metallic melt, with the subsequent crystallization of forsterite. In addition, Ti-rich troilite crystallized from a combination of an Fe-rich sulfide melt and a mixed sulfide melt. All of the results of the experiment are consistent with a derivation of the aubrites from an E-type chondritic precursor in a strongly reducing, oxygen-depleted environment.
Abee’s iron-rich, oxygen-poor composition, as well as its greater depletion of refractories than that of the Earth, has led to speculation that E chondrites might have once been a part of the pre-differentiated outer layer of Mercury. However, reflectance spectrometry has determined that E-type and M-type asteroids are similar to E chondrites, and they occupy stable orbits between 1.8 and 3.2 AUThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The Click on Term to Read More, suggesting that the asteroid beltBelt located between 2.12 and 3.3 AU from the Sun and located between the orbits of Mars and Jupiter containing the vast majority of asteroids. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such Click on Term to Read More is where they originated, or more likely, where they were relocated following a collisional/gravitational perturbation. A heliocentricCentered around a sun. Our own Solar System is centered around the Sun so that all planets such as Earth orbit around the Sun. Note that 25% of Americans incorrectly believe the Sun revolves around the Earth. Click on Term to Read More distance of ~2.0–2.9 AU was calculated for two E chondrites on the basis of their implanted solar noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More concentrations (Nakashima et al., 2004). By utilizing Mn–Cr isotopic systematics, Shukolyukov and Lugmair (2004) concluded that the E chondrites formed at a location closer to the Sun–between at least 1 AU outward to 1.4 AU–than the location within the asteroid belt which they now occupy.
An anomalous light N component that is found proportionately in carbonaceous and E chondrites but not on Earth, which is almost certainly of nucleosynthetic origin, points to a similar heliocentric location for the formation of these bodies. The Ar–Ar age was determined by Bogard et al. (2010) to be 4.52 ( ±0.02) b.y., or 4.5621 b.y. calculated relative to Shallowater (Hopp et al., 2011).
An unusual D-depleted, highly disordered, insoluble organicPertaining to C-containing compounds. Organic compounds can be formed by both biological and non-biological (abiotic) processes. Click on Term to Read More matter component was recovered in an acid residue of Abee, thought to be hosted by the late-stage accretion of dark inclusions (Remusata et al., 2012). The specimen of Abee shown above is a 4.9 g partial slice showing the brecciated nature of this meteorite, including a metallic-rimmed clastA mineral or rock fragment embedded in another rock. Click on Term to Read More (bottom center) and a dark inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. Click on Term to Read More of unique enstatite chondritic material (upper right). A superb large slab of Abee can be seen on display at the Smithsonian Institution, Washington D.C. A complete slice measuring 374 × 260 × 7 mm and weighing 1,675 grams is in the collection of Edwin Thomson.
