NORTHWEST AFRICA 12455


Ténéréite (typical)*
(CR7 in MetBull 108)
(carbonaceous chondrite-related)

standby for nwa 12455 photo
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) 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 the many ungrouped equilibrated meteorites like NWA 12455 is coincidental, and that significant geochemical and other differences exist which 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 oxygen isotope diagram 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 Memeber of a New Meteorite Group?'

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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click on image for a magnified view
Photo courtesy of Ben Hoefnagels

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

standby for nwa 12455 ts photo
click on image for a magnified view
Photo courtesy of Mirko Graul

standby for nwa 12455 ts photo
click on image for a magnified view
Photo courtesy of Peter Marmet