(LL7 in MetBull 97)
no coordinates recorded
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) proposed that the acapulcoite/lodranite meteorites should be divided based on metamorphic stage:
primitive acapulcoites: near-chondritic (Se >1213 ppm [degree of sulfide extraction])
typical acapulcoites: FeNiFeS melting and some loss of sulfide (Se ~512 ppm)
transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
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, olivinepyroxene 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.
click on image for a magnified view
Diagram credit: Wittke et al., 74th MetSoc, #5222 (2011)
'Northwest Africa 5131: Another Tafassasset-Like Metachondrite Related To The CR
Chondrite Parent Body'
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).
Diagram credit: Agee et al., 51st LPSC, #2292 (2020)
'Northwest Africa 12869: Primitive Achondrite From the CR2 Parent Body or Member of a New Meteorite Group?'
Diagram credit: Dr. Carl Agee, IOM Seminar Sept 1, 2020
'Dr. Carl Agee: Some New Meteorites from the Sahara Desert'
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 (Hamilton et al., 2020) 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 which was previously unrepresented in our collections.
ε54Cr vs. Δ17O Diagram for AhS 202
click on image for a magnified view
Diagram credit: Miller et al., 52nd LPSC, #2360 (2021)
'Stalking a Large Carbonaceous Chondrite Asteroid Using ε54CrΔ17O Isotope Systematics of the Unique Xenolith Almahata Sitta 202'
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).