NORTHWEST AFRICA 13167


CT2
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Purchased 2019
no coordinates recorded

Five partially fusion-crusted stones having a combined weight of 1,027 g were purchased from a Moroccan source. Analyses and classification was conducted at the Natural History Museum in Berlin, Germany (A. Greshake) and the University of New Mexico (oxygen isotope analysis; K. Ziegler), and it was determined that NWA 13167 is an ungrouped carbonaceous chondrite of petrologic type 2.

NWA 13167 has a black, highly porous matrix composed of phyllosilicates, carbonates, Fe-sulfides, and forsteritic olivine, the latter containing FeNi-metal inclusions. Chondrules are small (~300 µm) compared to those in CV and CK chondrites, and CAIs are rare. An oxygen isotope analysis was conducted at the University of New Mexico (K. Ziegler) employing laser fluorination. Northwest Africa 13167 has an O-isotopic composition that is more 16O-rich than that of CM chondrites and is distinct from both CV and CK chondrites (see diagram below).

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Diagram from MetBull 109

Multiple analytical techniques were applied by Irving et al. (2022 #2046) and Garvie and Irving (2022 #2217; XRD data) to a number of previously classified carbonaceous chondrites. High-precision oxygen isotope data obtained by Irving et al. (2022) for a broad sampling of carbonaceous chondrites enabled them to define two new CC region trend lines that are nearly aligned with those previously known (see top diagram below). The data points delineate a new carbonaceous group of at least 16 meteorites that was termed 'CT chondrites', a toponym for the type 3 group member Telakoast 001. This group currently comprises petrologic type 2 and type 3 members. With further analyses, including nucleosynthetic anomalous 54Cr values, it may be determined that other variously classified stones (e.g., NWA 13167, NWA 13249 [C2-ung], NWA 13456 [C2], NWA 13455 [CO3-an], and NWA 13479 [C2-ung]) with petrographic, chemical, mineralogical, magnetic susceptibility, and isotopic similarities to those in the initial study actually belong in the CT group.

Oxygen Isotope Plot for Carbonaceous Chondrite Groups
(note: Qued Mya 002 has a nearly identical plot as NWA 11699)
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Diagram credit: Irving et al., 53rd LPSC, #2046 (2022)
'CT Chondrites: A Newly Recognized Carbonaceous Chondrite Group With
Multiple Members, Including Telakoast 001, Chwichiya 002 And Cimarron'

The other new trend line demonstrated by Irving et al. (2022) delineates a potential carbonaceous grouplet of 4 that was termed 'CZ chondrites' (see diagram above). In addition, a trend line previously proposed by Irving et al. (2019 #2542) that they termed 'CX' initially comprised four meteorites from the CC region representing different textural types—Milton (pallasite), NWA 10503 (metachondrite), NWA 12264 (dunitic breccia), and NWA 11961 (unequilibrated chondrite). However, 54Cr isotope data obtained for all of these meteorites have resolved both NWA 11961 and NWA 12264 as potential new carbonaceous parent bodies distinct from that of NWA 10503 and Milton, the latter two previously considered possible members of the CV-clan (see diagram below). To date, no other meteorites have been demonstrated to have a genetic link to either NWA 11961 or NWA 12264.

O–Cr Diagram to Distinguish 'CX' Trend Meteorites
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Diagram credit: Irving et al., 50th LPSC, #2542 (2019)
'Evidence For A Unique Carbonaceous Chondrite Parent Body ('CX') And Another One With A Dunitic Mantle'

It is noteworthy that this new CT group designation has somewhat of a history in and of itself, which is related to the unique meteorite NWA 2788. This meteorite has a metamorphic texture exhibiting ~120° triple junctions, elevated Fe/Mn and Ca/Na ratios, and an O-isotopic composition that plots very near to the Terrestrial Fractionation Line (TFL). NWA 2788 was considered by Bunch et al. (2006) to be a metachondrite associated with an unknown carbonaceous chondrite parent body. They conjectured that if a chondrule-bearing representative of this parent body is found and identified in the future, it should be termed a 'CT chondrite' (see NWA 2788 photos, abstract, and isotopic plots 1 [ref], 2 [ref]). Now in 2022, one of the co-authors of the 2006 abstract, A. J. Irving, has proposed that this new carbonaceous chondrite group be named 'CT', a toponym for the carbonaceous chondrite Telakoast 001. It is fitting that one of the members of the new CT group, Cimarron, was classified by T. Bunch.

Northwest Africa 13167 has experienced a low degree of terrestrial weathering and has features indicative of very weak shock (S2). The specimen of NWA 13167 shown above and below is a 10.93 g partially crusted cut stone acquired from Philippe Schmitt-Kopplin. The bottom photo is a scan of NWA 13167 that has been color and contrast adjusted to enhance the interior detail.

NWA 13167 10.93 g Section
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Photos courtesy of Philippe Schmitt-Kopplin

NWA 13167 10.93 g Section, Detail Enhanced
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