NORTHWEST AFRICA 1839

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.

Historically, 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 which contains 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 of 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: the term ‘metachondrite’ has been suggested by Irving et al. (2005) 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 several existing chondrite groups (e.g., CV, H, L, and LL).

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 (see oxygen 3-isotope diagram). In addition, the bulk elemental composition shows a definitive match to the CO chondrite Kainsaz (A. Irving, pers. comm). Therefore, NWA 1839 is considered to be the first CO metachondrite.

With a grade of W2, NWA 1839 shows only moderate weathering and is well preserved. It exhibits an equigranular texture with no relict chondrules apparent, and has a shock features corresponding to stage S1, perhaps indicating a long period of annealing following peak shock pressures. 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.