EL Melt Rock
(EL7 in MetBull DB)
(Primitive Enstatite Achondrite; Pilski et al., 2011)
Found October 1989
21° 38' N., 1° 16' E.
A single, relatively fresh (W0/1) stone weighing 421 g was found in the Algerian Sahara Desert. From initial chemical and petrologic analyses of Ilafegh 009, it was determined that the meteorite was a highly metamorphosed enstatite chondrite consistent with a classification of EL6/7 (Otto, 1992). Subsequent analyses by Bischoff et al. (1992) and McCoy et al. (1995) verified its genetic relationship to the EL chondrite group, and brought to light its unique igneous texture reflecting both cumulate and granoblastic characteristics, as well as its unusually large-sized enstatite crystals (up to 0.75 cm) that impart a lighter color to the whole rock than is typically apparent in E chondrites. Ilafegh 009 is currently considered to be a total shock-melted rock from the EL chondrite parent body.
The mineralogy of Ilafegh 009 is similar to that of EL chondrites; in wt%, the modal mineralogy is as follows (McCoy et al., 1992, 1995): orthoenstatite (52.4), twinned enstatite (1.9–3.2), plagioclase (7.1), FeNi-metal + schreibersite (27.2), Cr–Ti-troilite (8.1) in association with daubreélite and ferroan alabandite, CaS (oldhamite), and trace TiN (osbornite). The plagioclase grains contain tiny fluid or gas inclusions. The FeNi-metal consists only of kamacite that contains ~6 wt% Ni and up to 1.3 wt% Si. Nanoscale exsolution precipitates enriched in Ni, P, and Si are ubiquitous throughout the kamacite host phase and may characterize a combination of schreibersite and perryite.
It has been suggested that Ilafegh 009 crystallized from a superheated impact-induced melt (peak pressures >75 GPa) and then was rapidly cooled from peak temperatures (~5000°C/m.y.). The Ar–Ar age of Ilafegh 009 was calculated to be 4.43 b.y., while an older I–Xe age of ~4.565 b.y. indicates this chronometer was not reset (Bogard et al., 2010). This petrogenetic scenario is consistent with the appearance of unfractionated metal and sulfide, and the occurrence of magmatic inclusions within enstatite crystals. The large size of these crystals indicates that all of the original enstatite nuclei were eradicated in the superheated melt. At the same time, all of the trapped argon and many other volatile elements were degassed during this event. The crystallization sequence of the melt is believed to have been enstatite => plagioclase => silica => FeNi-metal/sulfides. The mineral sinoite has also been identified, which is associated with crystallization of an impact melt.
Very fine-grained (few µm-sized), rounded, magmatic inclusions have been identified within the cores and rims of enstatites (Leroux et al., 1997). Those inclusions that consist of plagioclase + metal + troilite are present in the cores, while those that consist of silica-rich glass (both K-rich and K-poor) are found in the enstatite rims as well as interstitially throughout the enstatite and plagioclase host grains—findings that are consistent with the proposed crystallization sequence. Besides these inclusions, undeveloped orthoenstatite nuclei are present within enstatite host grains.
After solidification, the rock was impact-shocked to stage S4, as indicated by undulatory extinction, moderate mosaicism, and planar fractures in the orthopyroxene grains. In addition, microscopic striations are present in some pyroxene lamellae that were formed by the intergrowth of orthoenstatite and clinoenstatite phases resulting from shock (McCoy et al., 1995). In contrast to Ilafegh 009, the impact-melt derived EL chondrite Happy Canyon did not experience temperatures to the same high degree, as concluded from the presence in Happy Canyon of a fine-grained lithology. This lithology was the result of crystallization sustained by pre-existing enstatite nuclei. Happy Canyon was also cooled more rapidly due to the incorporation of cold clastic material, which eventually solidified to form a breccia. In addition, Happy Canyon experienced less severe post-solidification shock than Ilafegh 009. Still, Pilski et al. (2011) propose that this meteorite may more properly be classified with Zakłodzie, and perhaps Happy Canyon, QUE 94204, and Y-8404, as a primitive enstatite achondrite representing the residue from the rapid partial melting of an enstatite chondrite parent body.
The O-isotopes for Ilafegh 009 plot with the E chondrites and aubrites. Interestingly, I–Xe radiochronometry demonstrates that Ilafegh 009, Happy Canyon, and the anomalous aubrite Shallowater all share similar closure times for radiogenic xenon in the iodine host enstatite phase at ~4.565 b.y. ago. This suggests that each meteorite experienced essentially concurrent episodes of an I–Xe clock-resetting, impact-melting event in a common formation region (Kehm et al., 1993). The younger K–Ar age calculated for Ilafegh 009 of 4.34–4.44 b.y. may represent subsequent thermal resetting by impact, while the K–Ar age calculated for both Happy Canyon and Shallowater of 4.53 b.y. may reflect either impact resetting or a prolonged cooling period.
The specimen shown above is a 1.55 g partial slice of Ilafegh 009, which highlights the metallic component of this meteorite. It was originally part of a 25.5 g partial slice in the collection of JNMC–Zurich (see photo below). The middle portion shown is being utilized for further thin section studies.
