NORTHWEST AFRICA 12455


Ténéréite (typical)*
or Tafassite Clan
(CR7 in MetBull 108)
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Purchased Nov 2018
no coordinates recorded

A relatively fresh stone weighing 611 g was found in the desert of Northwest Africa and obtained by meteorite dealer Habib-naji Naji (pers. comm.). A sample of the stone was ultimately sent by Ben Hoefnagels for analyses and classification to the University of Washington in Seattle (A. Irving and S. Kuehner) and Washington University in St. Louis (P. Carpenter), and it was determined that NWA 12455 is a completely recrystallized, texturally evolved CR7 chondrite.

*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.

Northwest Africa is composed of olivine, orthopyroxene, and plagioclase grains forming triple junctions, along with lesser FeNi-metal (kamacite and taenite), FeS (troilite and pentlandite), chromite, and minor terrestrial alteration products. No relict chondrules were observed in the studied sections. The O-isotopic composition for NWA 12455 was investigated at the University of New Mexico (K. Ziegler), and the values for six subsamples clearly plot along the CR trend line. Similarly, the Cr-isotopic composition was investigated at the University of California, Davis (M. Sanborn and Q. Yin), and the 54Cr value was determined to be the same within uncertainty as that for Renazzo.

A further metamorphic category in the textural continuum—type 8—has been proposed by Irving et al. (2019) to distinguish between those highly metamorphosed meteorites in which relict chondrules can still be discerned (e.g., NWA 12272 [LL7]) and those which exhibit a completely recrystallized texture (e.g., NWA 3133 [CV7]). The designation of type 8 was also suggested for other chondrite groups with members having similar completely recrystallized textures, including CR (e.g., NWA 12455), CK (e.g., NWA 8186 [Achon-ung]), and H (e.g., NWA 4226 [H7]), as well as certain meteorites within the acapulcoite and winonaite groups (see Irving et al., 2019 #6399).

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).

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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?'

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Diagram credit: Dr. Carl Agee, IOM Seminar Sept 1, 2020
'Dr. Carl Agee: Some New Meteorites from the Sahara Desert'

Ma et al. (2021, 2022) 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 the NWA 011 and NWA 6704 grouplets of 1.5 and 1.7 m.y. after CAIs, respectively, they argued that these meteorites derive from one or more additional parent bodies associated with a common reservoir (see top diagrams below). 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 metallic core, achondritic mantle, and chondritic crust (see schematic illustration below).

ε54Cr vs. Δ17O for Tafassites and the NWA 011 and NWA 6704 grouplets
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Diagrams credit: Ma et al., Geochemical Perspectives Letters, vol. 23, pp. 33–37, fig. S-13 (2022 open access link)
'Early formation of primitive achondrites in an outer region of the protoplanetary disc'
(https://doi.org/10.7185/geochemlet.2234)

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Schematic illustration credit: Jiang et al., 84th MetSoc, #6062 (2021)

A more comprehensive investigation of the suite of four ungrouped primitive achondrites (NWA 3250, NWA 11112, NWA 12869, and Tafassasset) was undertaken by Jiang et al (2023) with an expanded team having relevant expertise in Cr and O isotope systematics, Mn–Cr chronometry, nucleosynthetic anomalous isotopes, and geothermometry. Employing advanced petrographic and mineralogical techniques, including high resolution X-ray tomographic microscopy, their analyses led to the conclusion that NWA 3250, NWA 11112, and NWA 12869 compose a grouplet of primitive achondrites that derive from a small parent body (tens of km in diameter) which accreted very early (<1 m.y. after CAIs) from a nebular reservoir that would later produce the CR chondrite parent body. Importantly, they determined that Tafassasset should be removed from inclusion in this grouplet due to significant mineralogical differences in comparison with the other three members (see diagrams below). Therefore, a potential 'tafassite clan' comprised of up to 4 parent bodies, each of which formed early in the CR reservoir, may be represented in our collections as (1) Tafassasset grouplet, (2) Jiang et al. grouplet, (3) NWA 011 basalt grouplet, and (4) NWA 6704 orthopyroxene grouplet.

Triple Oxygen Isotopes for CR-like Primitive Achondrites
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ε54Cr vs. Δ17O for CR-like Primitive Achondrites
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Diagrams credit: Jiang et al., GCA, In Press (2023)
'Tracking and dating incipient melting of a new grouplet of primitive achondrites'
(https://doi.org/10.1016/j.gca.2023.01.022)

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.

ε54Cr vs. Δ17O Diagram for AhS 202
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Diagram credit: Miller et al., 52nd LPSC, #2360 (2021)
'Stalking a Large Carbonaceous Chondrite Asteroid Using ε54Cr–Δ17O
Isotope Systematics of the Unique Xenolith Almahata Sitta 202'

The specimen of NWA 12455 shown above is a 0.30 g partial slice. The top photo below shows a slice of this meteorite with a larger surface area, while the bottom images are petrographic thin section micrographs showing the recrystallized structure of NWA 12455.

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Photo courtesy of Ben Hoefnagels

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Photo courtesy of Dr. Anthony Irving—University of Washington in Seattle

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Photo courtesy of Mirko Graul

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Photo courtesy of Peter Marmet