Ténéréite (evolved)* or CR partial melt cumulate
(Achondrite, ungrouped in MetBull 101)
no coordinates recorded
A single brown-colored meteorite weighing 59.8 g was found in Northwest Africa and subsequently purchased by M. Farmer from a Moroccan dealer at the 2011 Denver Mineral Show. A sample was analyzed at the University of Washington in Seattle (A. Irving and S. Kuehner) and NWA 6962 was classified as an ungrouped achondrite having some geochemical affinities to the brachinite group, although olivine in this meteorite is significantly more ferroan.
*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 interior appearance of NWA 6962 has been described as an aggregate of yellowish-brown olivine crystals containing melt inclusions composed of sodic plagioclase + chromite + Ni-free metal, along with an interstitial Na-rich silicate determined by Raman spectra to be nepheline or tridymite. In addition, scattered grains of Ti-poor Al-bearing chromite are present along with minor clinopyroxene (augite), merrillite, and fluorapatite, as well as sparse abundances of FeNi-metal and FeS grains.
Although NWA 6962 shows some mineralogical similarities to brachinites, its geochemical characteristics are inconsistent with a genetic relationship; e.g., Fe-enriched silicates, Fe/Mn ratios, and O-isotopic values (Dunlap et al., 2015). Oxygen-isotope values for NWA 6962 were ascertained (R. Tanaka, Okayama University, Japan), and the plot is near the CCAM line near the resolved fields for ureilites and the acapulcoitelodranite clan (see the oxygen three-isotope plot). Notably, the O-isotopic values for NWA 6962 are very similar to those determined by N. Banerjee et al. (Western University, Canada) for the 124 g NWA 7680 (see the oxygen three-isotope plot). Since these two meteorites were both purchased in 2011 and have similar O-isotopic values, bulk compositions, and geochemical constitutions, it is considered that they are genetically related (Hyde et al., 2013, 2017).
Additional studies of NWA 6962/7680 by Hyde et al. (2017), including major, minor, and trace elemental compositions, led to the conclusion that there is no obvious relationship to either the ureilites or the acapulcoitelodranite clan. Since NWA 6962/7680 plot near the CCAM line and show similarities to some CM and CV clasts thought to originate from the interior of these asteroids, it is considered that NWA 6962/7680 could also derive from the deep interior of a primitive carbonaceous chondrite parent body.
Further studies of samples from both NWA 6962 and 7680 were conducted by Sanborn et al. (2018). Utilizing a coupled Δ17O vs. ε54Cr diagram, they demonstrated that NWA 6962/7680 plots within the CR/CH carbonaceous chondrite field represented by CR2 Renazzo and CH3 NWA 2210 (see diagram below). This plot is also proximate to that of the ungrouped carbonaceous achondrites NWA 6704 and pairings, NWA 011 and pairings, and Tafassasset/NWA 3100, which suggests that a genetic relationship exists among them.
Chromium vs. Oxygen Isotope Plot
click on image for a magnified view
Diagram credit: Sanborn et al., 49th LPSC, #2296 (2018)
Huyskens et al. (2019) derived and compiled chronological data from multiple dating systems for four different achondrite parent bodies that accreted in the CR reservoir, comprising the pairing groups of NWA 011/2976/4587, NWA 6704/6693/10132, Tafassasset/NWA 3100, and NWA 6962/7680. They determined that each of these parent bodies accreted and differentiated early in Solar System history and over a relatively short timespan ~4.5637 to 4.5624 b.y. ago. Each of these CR-like objects have Cr- and Ti-isotopic compositions, that when coupled to the O-isotopic compositions, plot in distinct locations (see diagrams below). Notably, the CR2 chondrite Renazzo plots nearest to NWA 6962/7680 in OCr space, but no comparable Ti isotope data is yet available.
17O vs. ε54Cr and ε50Ti for CR-like Achondrites
click on photo for a magnified view
Diagrams credit: Huyskens et al., 50th LPSC, #2736 (2019)
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'
While this new ténéréite group may be plausible, two of the proposed "evolved" members, NWA 6962 and NWA 7680, have higher olivine core Fa values (44.645.2) compared to the other suggested members. Abe et al. (2021, #1813 proposed an alternative origin for the NWA 6962/7680 achondrites which was validated through partial melting experiments utilizing the CR2 NWA 7184 and the MELTS software. They demonstrated that NWA 6962/7680 have olivine Fa contents as well as olivine MnO and CaO contents that are consistent with formation as a cumulate which crystallized from a CR chondrite partial melt as it cooled from 1200°C to ~11401145°C under oxidizing conditions of ~IW+2.
Northwest Africa 6962 shows evidence for low shock and a low degree of terrestrial weathering. The specimen of NWA 6962 shown above is a 0.547 g slice.