EL Melt Rock
(EL7 in MetBull 71; Primitive Enstatite Achondrite [Pilski et al., 2011]) (ELa7 in Weyrauch et al., 2018)
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 considered by some 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 (1.93.2), plagioclase (7.1), FeNi-metal + schreibersite (27.2), CrTi-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 might characterize a combination of schreibersite and perryite.
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 Ilafegh 009 is a member of the ELa subgroup.
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 of ~5000°C/m.y. (McCoy et al., 1992). The ArAr age of Ilafegh 009 was calculated to be 4.43 b.y., while an older IXe 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). The inclusions consisting of plagioclase + metal + troilite are present in the cores, while those consisting 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 grainsfindings that are consistent with the proposed crystallization sequence. Besides these inclusions, undeveloped orthoenstatite nuclei are present within enstatite host grains.
It is considered that following solidification, the Ilafegh 009 lithology 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, considered to result from 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. Nevertheless, Pilski et al. (2011) propose that Ilafegh 009 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.
In their advanced microscopic analyses of Ilafegh 009, Boesenberg et al. (2014) proposed a slightly different two-stage petrogenetic history. The first stage in their scenario involves formation as a melt rock, after which it would resemble the other fine-grained, granular-textured EL chondrites such as Zakłodzie and Happy Canyon. They invoke a subsequent heating event that resulted in the re-orientation and sintering (and/or Ostwald ripening) of these small enstatite grains into a coarse-grained, crystallographically-contiguous texture.
The O-isotopes for Ilafegh 009 plot with the E chondrites and aubrites. Interestingly, IXe 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 IXe clock-resetting, impact-melting event in a common formation region (Kehm et al., 1993). The younger KAr age calculated for Ilafegh 009 of 4.344.44 b.y. may represent subsequent thermal resetting by impact, while the KAr age of 4.53 b.y. calculated for both Happy Canyon and Shallowater may reflect the actual closure of this particular chronometric system during metamorphic cooling following accretion.
The specimen shown above is a 1.55 g partial slice of Ilafegh 009 that highlights the metallic component of this meteorite. It was originally part of a 25.5 g partial slice in the collection of JNMCZurich (see top photo below). The middle portion shown was utilized for further thin section studies; see bottom photo of a petrographic thin section micrograph of Ilafegh 009, shown courtesy of Peter Marmet.
Photo courtesy of JNMCZurich
click on image for a magnified view
Photo courtesy of Peter Marmet