Several fusion-crusted stones having a combined weight of 380 g were found near Foum Zguid, Morocco and purchased by R. Chaoui. These were subsequently acquired by a number of meteorite dealers, including material purchased by G. Fujihara from R. Chaoui's brother. Sample analyses were conducted at the University of Washington in Seattle (A. Irving and S. Kuehner), Arizona State University (L. Garvie), and the University of New Mexico (oxygen isotope analysis; K. Ziegler). The meteorite was classified as an ungrouped carbonaceous chondrite.
The silicates identified in NWA 8781 include olivine, orthopyroxene, and diopside, along with the opaque minerals kamacite and troilite. The geochemistry writeup in MetBull 107 states that the ferroan olivine component contains 0.20.4 wt% Cr2O3, which is consistent with a subtype 3.0 (Ruggiu et al., 2021 and references therein). The fine-grained matrix material constitutes ~50 vol% and no phyllosilicates were observed. Chondrules are very small (210 [±60] µm) compared to those in CV and CK chondrites. The apparent absence of CAIs is also inconsistent with CV as well as CO chondrites, and the lack of magnetite is inconsistent with CK chondrites. In addition, the O-isotope composition of NWA 8781 is more 16O-rich than that of CM chondrites and plots in a space distinct from other known carbonaceous chondrite groups, but is similar to the C3-ungrouped Telakoast 001, NWA 12416, and NWA 14200, and the C2-ungrouped NWA 13167 and NWA 13479 (see diagrams below).
Diagram from MetBull 107
Diagram from MetBull 110
Diagram from MetBull 108
Diagram from MetBull 110
Diagram from MetBull 109
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'
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 typesMilton (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.
OCr 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.
Comparative analyses of the known asteroid types and a suite of ungrouped and rare meteorites in multiple forms (bulk, powder, polished section), including the ungrouped carbonaceous chondrite NWA 8781, were conducted by Krämer Ruggiu et al. (2021) utilizing petrographic, spectroscopic, and albedo data. They concluded that the best matches to this meteorite are the Xe- and Xk-type asteroids (see diagram below).
Comparison of Meteorite and Asteroid Spectra
Diagram credit: Krämer Ruggiu et al.Icarus, vol. 362, art. 114393 (2021, open accesslink)
'Visible-infrared spectroscopy of ungrouped and rare meteorites brings further constraints on meteorite-asteroid connections'
Northwest Africa 8781 has experienced a low degree of terrestrial weathering, and it has features indicative of very weak shock (S2). The specimen of NWA 8781 shown above is a 0.77 partial slice photographed with a cm-scale cube, courtesy of Gary Fujihara.
Read the Micro Visions article for September 2020 about NWA 8781 in Meteorite Times Magazine, written by John Kashuba and including his always excellent thin section photomicrographs.