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, CIW) showed that NWA 1839 plots on the Carbonaceous Chondrite Anhydrous Mineral (CCAM) line within the field of CO chondrites.
The similarly classified meteorite NWA 3133/2643 was found to have FeO/MnO ratios in olivine and pyroxene which are higher than those for ordinary chondrites, and was found to contain other elemental ratios most consistent with a CV carbonaceous chondrite composition (Schoenbeck et al., 2006). These factors led to a tentative revised classification of NWA 3133/2643 by NAU scientists as an ungrouped primitive achondrite. Subsequent to this revision, a change in the nomenclature previously used to describe those primitive achondrites which are texturally evolved chondrites was proposed by Irving et al. (2005). The term 'metachondrite' was suggested to describe those chondrule-free stony meteorites which are actually texturally-evolved chondrites with completely recrystallized and highly equilibrated textures resulting from high degrees of metamorphism or partial melting, and which have elemental ratios and O-isotopic compositions demonstrating affinities to existing chondrite groups (e.g., CV, H, L, and LL). The unique meteorite NWA 2788 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 TFL; this meteorite is considered by Bunch et al. (2006) to be a metachondrite associated with an unknown carbonaceous chondrite parent body (see diagram below). If a chondrule-bearing representative of this parent body is found and identified in the future, it was suggested that it be termed a 'CT chondrite'.
While NWA 1839 and NWA 3133/2643 are both metachondrites with carbonaceous chondrite affinities, and both are purported to have originated from the same nomad collector source, initial O-isotopic studies showed that they are not paired. Further O-isotopic analyses conducted at The Open University, UK, plot within the CO group overlapping the highly extended CV field. In addition, the bulk elemental composition (e.g., Mn/Mg and Al/Mg ratios) shows a close similarilty exists with the CO chondrites (Sanborn et al., 2015). In their continued analyses of this meteorite, Sanborn et al. (2015) demonstrated that NWA 1839 plots very near to the CV chondrites on a coupled Δ17O vs. ε54Cr diagram. Therefore, in consideration of all the data established thus far, NWA 1839 is considered to be genetically related to the carbonaceous chondrites, and could possibly represent the first CO metachondrite.
Diagram credit: Irving et al., 79th MetSoc, #6461 (2016)
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. 1.31.8 mol%). Curiously, the high-Ca pyroxenes in NWA 1839 do not follow the defined CaO abundances of a type 7 chondritethat 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 metalsulfide 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 SchmusWood 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. BunchNorthern Arizona University