NORTHWEST AFRICA 5131

A single stone weighing 533 g was found in Northwest Africa and purchased by F. Kuntz. A sample was submitted for analysis and classification (Wittke and Bunch, NAU; Irving, UWS) and NWA 5131 was found to have a mineralogical composition similar to that of LL chondrites. However, following an O-isotopic analysis (Rumble III, Carnegie Institution; #5222), it could be demonstrated that the oxygen three-isotope plot falls within the CR chondrite field.

*Previously, Floss (2000) and Patzer et al. (2003 #1352, 2004) proposed that the acapulcoite/lodranite meteorites should be divided based on metamorphic stage:

1. primitive acapulcoites: near-chondritic (Se >12–13 ppm [degree of sulfide extraction])
2. typical acapulcoites: Fe–Ni–FeS melting and some loss of sulfide (Se ~5–12 ppm)
3. transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
4. lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
5. enriched acapulcoites (addition of feldspar-rich melt component)

A similar distinction could be made among the winonaites in our collections, as well as among members of the newly proposed group ténéréites (Agee et al., 2020). One of the most "primitive" members identified in this new group is NWA 7317, which contains relict chondrules comparable to a petrologic type 6 chondrite. However, most ténéréites have experienced more extensive thermal metamorphism involving incipient melting and now exhibit highly recrystallized textures, characteristics analogous to the "typical" acapulcoites. Metamorphic progression in other ténéréites involved higher degrees of partial melting and even separation of a basaltic fraction (e.g., NWA 011 pairing group). Samples representing such an advanced metamorphic stage are known as lodranites in the acapulcoite/lodranite metamorphic sequence, while the term "evolved" could be used to represent a similar metamorphic stage in the ténéréite group.

The meteorite is a highly metamorphosed assemblage with strong similarities to the ungrouped primitive achondrite Tafassasset, and Wittke et al. (2011) suggest that it could be part of that large strewn field. Fine-grained portions exhibit 120° triple junctions, while other areas consist of mineral phases described as having a poikiloblastic texture, defining possible relict chondrules. These recrystallized chondrule relicts would be consistent with a porphyritic, metal-bearing, olivine–pyroxene chondrule type.

A more advanced stage of metamorphism than that exhibited by the CR6 chondrites NWA 7317 (and pairings) and NWA 3100 has been invoked to explain the recrystallized poikiloblastic texture in NWA 5131, and therefore the term metachondrite may be most appropriate for this meteorite (Wittke et al., 2011). It was also argued that the similarity in O-isotopic compositions that is observed among the non-metamorphosed CR chondrites, the metamorphosed CR6 chondrites, and NWA 5131, when compared to the igneous achondrite NWA 011 (and pairings), is consistent with their derivation from a common large parent body, one which experienced internal partial melting while retaining a chondritic regolith.

It was asserted by Agee et al. (2020) that the similarity in O, Cr, and Ti values among the CR2 carbonaceous chondrites and these ungrouped equilibrated meteorites is coincidental, and that significant geochemical differences (e.g., olivine Fa content and Fe/Mn) and other discrepancies (e.g., petrologic type discontinuity) exist that make a common parent body untenable. They contend that the thermally metamorphosed CC meteorites represent a unique group for which they propose the name 'ténéréites' (see list and diagrams below).

Ma et al. (2021) and Neumann et al. (2021) investigated the suite of ténéréites, for which they proposed the name 'tafassites'. They employed numerical modeling to constrain the formation and thermal history of the parent body, which they found was most consistent with an accretion age of 0.9 (±0.1) m.y. after CAIs—significantly earlier than that of the CR chondrite parent body at 3–4 m.y. after CAIs. In addition, they determined the diameter of the tafassite parent body to be 200–400 km. Moreover, based on stable isotope systematics and the distinct accretion ages obtained for NWA 011 and NWA 6704 of 1.5 and 1.7 m.y. after CAIs, respectively, they argued that these meteorites derive from one or more additional parent bodies. At the other end of the lumping–splitting spectrum, Jiang et al. (2021) contend that the CR parent body once comprised all of the meteorites that are isotopically and geochemically similar, composing a now disaggregated, at least partially differentiated body with a core, achondritic mantle, and chondritic crust (see schematic illustration below).

Miller et al. (2021) utilized a coupled ε54Cr vs. Δ17O diagram (see diagram below) to determine the genetic provenance of the ungrouped carbonaceous chondrite AhS 202, which was found as a xenolithic clast in the Almahata Sitta polymict ureilite. Based on its plot, AhS 202 could represent the unmelted chondritic lid surrounding a Ceres-sized (~640–1,800 km-diameter as indicated by evident prograde metamorphism involving the amphibole tremolite [Hamilton et al., 2020; Hamilton et al., 2021]; Dodds et al., 2022 [#2158]) differentiated asteroid, possibly associated with the proposed ténéréite group (Agee et al., 2020). Alternatively, AhS 202 may derive from an asteroid that formed in the CR reservoir and was previously unrepresented in our collections.

Northwest Africa 5131 is a recrystallized meteorite that is petrographically consistent with a low-degree partial melt of CC precursor material which has retained its metal component. The rock subsequently experienced equilibration processes through an extended period of thermal metamorphism. The specimen of NWA 5131 shown above is a 2.73 g partial slice. The photo below shows a large cut face on the main mass exhibiting a high metal content (see the poster by Wittke et al., 2011).