Meta-CO or -CV
(L7 in MetBull 89)
no coordinates recorded A 121.8 g meteorite was found in the Sahara Desert and later purchased by American collector N. Oakes in Rissani, Morocco in June 2003. This meteorite was analyzed at Northern Arizona University (T. Bunch and J. Wittke) and was initially determined to be a highly recrystallized L7 chondrite. Notwithstanding this L7 classification, a subsequent oxygen isotope study (D. Rumble III, CIW) showed that NWA 1839 plots on the Carbonaceous Chondrite Anhydrous Mineral (CCAM) line within the field of CO chondrites.
click on image for a magnified view Diagram credit: Irving et al., 79th MetSoc, #6461 (2016) Chromium vs. Oxygen Isotope Plot
click on diagram for a magnified view Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018) Northwest Africa 1839 is well preserved and exhibits only moderate weathering. It has an equigranular texture with no relict chondrules visible. The low-Ca pyroxenes contain an abundance of CaO (Wo2.3), which by definition (see below) surpasses the maximum amount found in type 6 chondrites (ave. 1.3–1.8 mol%). Curiously, the high-Ca pyroxenes in NWA 1839 do not follow the defined CaO abundances of a type 7 chondrite—that is, a lower abundance for a higher petrologic type. Instead, in contrast to type 6 chondrites which have an average value of ~Wo45, NWA 1839 contains clinopyroxene with Wo47 (Mittlefehldt and Lindstrom, 2001). Northwest Africa 1839 exhibits a shock stage of only S1, perhaps indicating a long period of annealing following peak shock effects. Type 7 ordinary chondrites were originally defined by Dodd et al. (1975) according to specific petrographic characteristics. They listed three metamorphic criteria to distinguish between petrographic types 6 and 7:
- the presence of poorly defined chondrules in type 6, but only relict chondrules in type 7
- low-Ca pyroxenes in type 6 contain no more than 1.0 wt% CaO (1.0 wt% = ~1.9 mol% Wo), but more than 1.0 wt% in type 7; conversely, the CaO content of high-Ca pyroxenes decreases from type 6 to type 7
- feldspar grains gradually coarsen to reach a size of at least 0.1 mm in type 7
For those meteorites that experienced metamorphic temperatures high enough for metal–sulfide melting to occur, which most commonly occurs as a result of impact events, an igneous texture would be produced (Mittlefehldt and Lindstrom, 2001). In these cases the use of the Van Schmus–Wood classification scheme is no longer valid, and these meteorites could instead be referred to as primitive achondrites or metachondrites, or they might be impact melts.Two views of a 1.43 g slice of NWA 1839 are shown above. The photo below shows a thin section viewed in partially polarized light in which the equigranular texture and absence of chondrules and metal is evident.
Photo courtesy of T. Bunch—Northern Arizona University