ABEE


EH Impact-Melt Breccia
(EH4 in MetBull DB)
standby for abee photo
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 Schmus–Wood (1967) scheme for petrographic type was modified for enstatite chondrites, establishing both a textural type reflecting peak metamorphic 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.

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 ~100 µg/g of diamonds in Abee, likely derived from 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 [(Fe,Mg)S] crystallized from the melt phase of niningerite [(Mg,Fe)S] 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.

A subsequent period of meteorite impacts shattered this homogeneous melt, and partial melting of metal and silicate occurred. 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 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).

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 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 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.

Previously, employing multiple lines of evidence including chemical, petrographic, metamorphic, and cosmic-ray exposure age data, studies suggested that the EL and EH chondrites originate from different layers on the same parent body. More recently, very precise isotopic measurements were made of a statistically larger sampling of E chondrites and aubrites. Although their O-isotopic data 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 also been proposed (Rubin and Wasson, 2011). Studies have determined that it was 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 to have originated from two additional unique enstatite parent bodies.

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 ~2.0–2.9 AU was calculated for two E chondrites on the basis of their implanted solar noble gas 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.

Finally, an anomalous light N component found proportionately in carbonaceous and E chondrites but not on Earth, and almost certainly of nucleosynthetic origin, points to a similar heliocentric location for the formation of these bodies. The Ar–Ar age was determined to be 4.52 (±0.02) b.y. (Bogard et al., 2010) or 4.562.1 b.y. calculated relative to Shallowater (Hopp et al., 2011). 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.