Enstatite achondrite, ungrouped
Partial melt residue
(EH7-anomalous in MetBull 85)
Found July 2000
26° 35.45' N., 12° 57.13' W.
In about 1990, following a detonation with accompanying light, a stone weighing 410 g was found by a nomad in Saguia el Hamra, Western Sahara. In July of 2000, the Labenne family searched for meteorites in the same area near Itqiy, and Luc found a 4,310 g stone. The large, smooth stone was covered with a thin, black fusion crust still exhibiting flow lines. Although this meteorite has a low weathering grade (W12) consistent with a recent fall, 14C results infer a terrestrial age for Itqiy of 5,800 (±500) years. Initial classification of Itqiy was completed at the Lunar & Planetary Laboratory, University of Arizona.
Itqiy consists of ~78 vol% equigranular silicates composed of coarse-grained enstatite with a size range of 0.54 mm. These are chemically similar to silicates in EL chondrites but have a significantly higher CaO content (Patzer et al., 2001). These pyroxene grains form 120° triple junctions which are consistent with an extended annealing process and a high degree of recrystallization. Undulose extinction, irregular fractures, and occasional mosaicism within the grains reflect severe shock exposure consistent with a shock stage of S24, while evidence of deformation and a lack of twinning suggests a shock classification of S3. Based on a Raman spectroscopic analysis of enstatite crystals, Zhang et al. (2018) derived a shock stage for Itqiy of S45.
Kamacite forms 0.22 mm diameter grains and vein networks comprising ~22 vol% of the meteorite, with a compositional range similar to the EH chondrites (Patzer et al., 2001). In contrast, kamacite spherules embedded within sulfide have a composition similar to EL chondrites. No taenite is present and only rare troilite occurs. Patzer et al. (2001) also reported that the MgMnFe-sulfides present in Itqiy are compositionally different from those in both EH or EL chondrites, and the FeCr sulfides are unusual as well. Moreover, the Mg/Si and Fe/Si ratios are significantly higher than those in EH or EL chondrites. Plagioclase and relict chondrules are absent.
An absence of radiogenic gases in Itqiy probably reflects a recent loss through an impact-melting event, likely related to shock heating during its excavation. The signature of trapped noble gases in Itqiy shows a subsolar component similar to that of E chondrites of petrologic grades 46 (as opposed to the sub-Q signature of type-3 E chondrites), which suggests a possible genetic relationship to equilibrated E chondrites. Moreover, from the similar CRE ages between Itqiy (30.1 [±3.0] m.y.) and E chondrites (28.8 m.y.), as well as by their corresponding O-isotopic compositions, it could be concluded that they formed in a similar region of the solar nebula.
While similarities do exist between Itqiy and the EH and EL chondrites, the many inconsistencies make a definitive assignment tenuous. High-precision Cr isotope analyses were conducted by Zhu et al. (2021) to investigate both the origin (54Cr systematics) and the timing (53Mn53Cr chronometry) of the enstatite achondrites. As shown in the diagrams below, these E achondrite samples represent three distinct groups or parent bodies separate from the E chondrite groups:
Diagrams credit: Zhu et al., Goldschmidt Conference, #5519 (2021)
'Tracing the origin and differentiation of the enstatite achondrite parent bodies using Cr isotopes'
Patzer et al. (2001) found the compositional and textural characteristics of Itqiy to be analogous to those observed in the lodranites, i.e., derivation from a residual melt from which an ~20% basaltic partial melt rich in plagioclase and sulfide had been removed. In a similar scenario, Bouvier et al. (2016) found that Itqiy is the most incompatible element-depleted crustal sample known, consistent with a residue after a LREE-rich partial melt extraction. This event occurred under highly reducing conditions on a metal-enriched E chondrite parent body, where subsequent cooling over a long period allowed extensive equilibration to occur. Other mineralogical features of Itqiy, including its shock features, are consistent with a late impact-heating event to temperatures below 900°C, followed by rapid cooling. An ArAr study by Bouvier et al. (2016) indicates a late-stage impact event <1.3 b.y. ago.
Studies of the 42.9 g enstatite achondrite NWA 2526 by Keil and Bischoff (2008) concluded that this meteorite, containing ~1015% metal, shares many textural and mineralogical characteristics with Itqiy (both partial melt residue after ~20% partial melt extraction) and possibly QUE 94204, potentially forming a grouplet of meteorites. Moreover, metal in Mount Egerton and in the anomalous iron meteorite Horse Creek (as well as the anomalous irons LEW 85369, LEW 88055, and LEW 88631) has been described as being compositionally similar (i.e., having complementary HSE patterns in metal) to metal in NWA 2526 (Keil and Bischoff, 2008; Humayun et al., 2009; M. Humayun, 2010). Along with Itqiy, these meteorites might share a common origin on an E chondrite-like parent body unique from the Shallowater, EH, EL, and main-group aubrite parent bodies (Keil and Bischoff, 2008; Izawa et al., 2011).
It is noteworthy that the enstatite achondrite inclusions MS-MU-019 and MS-MU-036 recovered from the Almahata Sitta fall have been compared to Itqiy (Bischoff et al., 2016), and continued studies could help resolve potential genetic links among these anomalous meteorites. The photo above is a 1.3 g interior slice of Itqiy, and the pictures below show the complete Itqiy mass in situ.