Numerous friable, fusion-crusted stones having a combined weight of 875 g were purchased from a Moroccan source by several meteorite dealers. Samples were sent for analysis and classification to the University of Washington in Seattle (A. Irving and S. Kuehner) and Arizona State University (L. Garvie), and it was determined that NWA 12416 is an ungrouped carbonaceous chondrite of petrologic type 3.
The meteorite has a relatively high abundance (~50%) of fine-grained matrix containing small chondrules, mineral fragments, and AOAs. In addition, minor kamacite and pentlandite are present, but no CAIs or phyllosilicates were observed. The oxygen isotope composition of NWA 12416 was obtained by laser fluorination at the University of New Mexico (K. Ziegler), and the meteorite has unique oxygen isotope values distinct from those of the CV, CK, and CM groups, but similar to those for a number of previously classified carbonaceous chondrites, including the C-ungrouped NWA 8781, the C3-ungrouped Telakoast 001, and the C2-ungrouped NWA 13167 (see triple oxygen diagrams below).
Diagram from MetBull 108
Diagram from MetBull 107
Diagram from MetBull 110
Diagram from MetBull 109
Diagram credit: Greenwood et al., GCA, vol. 277, p. 381 (2020, open access link)
'Linking asteroids and meteorites to the primordial planetesimal population'
(https://doi.org/10.1016/j.gca.2020.02.004)
Zhu et al. (2022) determined the Δ17O, ε54Cr, and ε48Ca values for NWA 12416 to be –5.93 (±2.66) ‰, 1.4 (±0.05), and 0.28 (±0.29), respectively, which support a unique CC group classification (see diagrams below). The ε48Ca value is the lowest known among CC chondrites, and the Δ17O value is among the lowest known of any meteorite.
Δ17O, ε54Cr, and ε48Ca Relationships for NWA 12416
blue triangles = ungrouped NC chondrites; red triangles = ungrouped CC chondrites
click on image for a magnified view
Diagrams credit: Zhu et al., GCA, vol. 342, pp. 156–168, fig. 2a,c (2023, open access link)
'Chondrite diversity revealed by chromium, calcium and magnesium isotopes'
(https://doi.org/10.1016/j.gca.2022.12.014)
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)
click on image for a magnified view
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
click on photo for a magnified view
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.
Information about other members of the CT group can be found on the following pages: Chwichiy 002 (CT3.00), NWA 8781 (CT3), Qued Mya 002 (CT2[3]), and tentative member NWA 13167 (CT2). The specimen of NWA 12416 shown above and below is a small 0.85 g fragment acquired from Mark Lyon.
