ITQIY


Enstatite Achondrite, ungrouped
54Cr = –0.26 [±0.03]; δ53Cr = –0.03 [±0.03] ‰)
Partial melt residue
(EH7-anomalous in MetBull 85)

standby for itqiy photo
Found July 2000
26° 35.45' N., 12° 57.13' W.

Around the year 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 (W1–2) 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 0.5–4 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 S2–4, 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 S4–5.

Kamacite forms 0.2–2 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 Mg–Mn–Fe-sulfides present in Itqiy are compositionally different from those in both EH or EL chondrites, and the Fe–Cr 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 4–6 (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 (53Mn–53Cr chronometry) of the enstatite achondrites. As shown in the two diagrams below, these E achondrite samples represent three distinct groups or parent bodies. Although the high δ53Cr values of aubrites distinguishes them from E chondrites, Zhu et al. (2021) surmise that this is the result of "isotope fractionation of isotopically light Cr-sulphide during core formation, resulting in an isotopically heavy mantle".

  1. Itqiy (ε54Cr = –0.26 [±0.03]; δ53Cr = –0.03 [±0.03] ‰) and possibly MS-MU-019, MS-MU-036, and NWA 2526
  2. Shallowater (ε54Cr = –0.12 [±0.04]; δ53Cr = 0.10 [±0.03] ‰)
  3. Main-group aubrites (ε54Cr = 0.06 [±0.12]; δ53Cr = 0.24 [±0.03] ‰)

ε54Cr vs. Δ17O Diagram for Enstatite Meteorites
standby for e achondrite cr isotope diagram

standby for e achondrite cr isotope diagram
Diagrams credit: Zhu et al., Goldschmidt Conference, #5519 (2021)
'Tracing the origin and differentiation of the enstatite achondrite parent bodies using Cr isotopes'
See also article by Zhu et al. in GCA, vol. 308 (2021)
'Tracing the origin and core formation 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 Ar–Ar 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 ~10–15% metal, shares many textural and mineralogical characteristics with Itqiy (both partial melt residues after ~20% 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).

Bischoff et al., 2016 found that the enstatite achondrite inclusions MS-MU-019 and MS-MU-036 recovered from the Almahata Sitta fall were comparable to Itqiy in both their texture and mineralogy. Following that, Zhu et al. (2022 and references therein) determined that the Δ17O, ε48Ca, ε54Cr, and ε50Ti values (the latter from Trinquier et al., 2009) for each of these samples overlap within uncertainties and support a common parental source body distinct from the EH parent body (see diagrams below). The photo shown above is a 1.3 g interior slice of Itqiy, and the pictures below show the complete Itqiy mass in situ.

Δ17O, ε54Cr, ε50Ti, and ε48Ca Relationships for Itqiy and AhS Clasts
blue triangles = ungrouped NC chondrites; red triangles = ungrouped CC chondrites
standby for ox-cr-ti-ca isotope diagrams
click on image for a magnified view

Diagrams credit: Zhu et al., GCA, vol. 342, pp. 156–168, fig. 2 (2023, open access link)
'Chondrite diversity revealed by chromium, calcium and magnesium isotopes'
(https://doi.org/10.1016/j.gca.2022.12.014)

standby for itqiy discovery photo

standby for itqiy photo
Photos courtesy of Luc Labenne—Labenne Meteorites