EH Impact-Melt Breccia
(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 fireball 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 meteorite was purchased from the finder, Harry Buryn, for $10/kg by the Geological Survey of Canada.
The Van SchmusWood (1967) scheme for petrographic type was modified for enstatite chondrites, establishing both a textural type reflecting peak metamorphic temperature (37), and a mineralogical type pertaining to the cooling history (αδ) (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 age data, previous studies had suggested that the EL and EH chondrites originate from different layers on the same parent body. However, subsequent studies utilizing very precise isotopic measurements were made of a statistically larger sampling of E chondrites and aubrites. Although the O-isotopic data for the samples were indeed identical, a three-isotope plot distinguished the EH group from the EL and aubrite groups by its slightly steeper slope; the plots of the EL and aubrite groups were colinear with the terrestrial fractionation 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 mineral 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 troilite and the Fe concentration in niningerite and alabandite (endmembers of the [Mn,Mg,Fe] solid solution series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréelite and the content of Ni in kamacite were demonstrated to be consistent factors for the resolution of four distinct E chondrite groups: EHa, EHb, ELa, and ELb (see table below).
ENSTATITE CHONDRITE SUBGROUPS
Weyrauch et al., 2018
Cr <2 wt%
Cr >2 wt%
Cr <2 wt%
Cr >2 wt%
Fe <20 wt%
Fe >20 wt%
Fe <20 wt%
Fe >20 wt%
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 accretion of chondrules that were ~5.6 m.y. younger than carbonaceous chondrites (based on Mn systematics). It is thought that Abee experienced sulfurization of metal within the protoplanetary nebula (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 ppm, likely derived from primary graphite.
Euhedral enstatite grains crystallized from the silicate melt and kamacite-rich rims formed around the clasts and relict EH material. The high-temperature silica polymorphs cristobalite and tridymite were formed from the chondrule melt and preserved through rapid cooling/quenching. Presolar SiC is present in Abee at a concentration of 6 ppb (Huss, 1990). The mineral keilite (Fe2+,Mg)S crystallized from the melt phase of niningerite and troilite (Rubin, 2008). Large kamacite nodules crystallized from C-rich metalsulfide 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 melting 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 metalsulfide 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 matrix)a mechanical form of differentiation 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 forsterite 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 reducing, oxygen-depleted environment.
In the experiment, as the silicate melt reached a temperature range of 10001300°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 core 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 crystallization of oldhamite (CaS). Moreover, a phase was reached at 1500°C in which SiO2 was reduced 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 AU, suggesting that the asteroid belt is where they originated, or more likely, where they were relocated following a collisional/gravitational perturbation. A heliocentric distance of ~2.02.9 AU was calculated for two E chondrites on the basis of their implanted solar noble gas concentrations (Nakashima et al., 2004). By utilizing MnCr isotopic systematics, Shukolyukov and Lugmair (2004) concluded that the E chondrites formed at a location closer to the Sunbetween at least 1 AU outward to 1.4 AUthan 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 ArAr 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 organic 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 clast (bottom center) and a dark inclusion 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.