NORTHWEST AFRICA 2646

A single stone reported to weigh 30.67 g was found in Algeria or Morocco and acquired by Moroccan dealer A. Habibi. A 9.3 g portion of the small meteorite was subsequently purchased by N. Oakes. A sample of the stone was submitted for analysis and classification (T. Bunch and J. Wittke, NAU; A. Irving and S. Kuehner, UWS), and it was determined to be a unique type of rock from Mars—a plagioclase–olivine-clinopyroxenite (also olivine-gabbro or "lherzolitic" shergottite).

This meteorite is related to the small martian "lherzolitic" shergottite group, the members of which are generally considered to be plutonic, ultramafic, olivine clinopyroxenites according to IUGS nomenclature. As is the case for the martian meteorites RBT 04262/261 and NWA 4468, NWA 2646 is a basaltic rock containing less olivine and more plagioclase (in the form of maskelynite) than those amounts required to conform to the IUGS definition for a terrestrial lherzolite. These martian meteorites share some similar mineralogical characteristics to the "lherzolitic" shergottite group (Mikouchi, 2009), and they may have a genetic relationship. Northwest Africa 2646 in particular is related to the martian "lherzolitic" shergottite group through geochemical properties such as trace element abundances and isotopic compositions, and its Rb–Sr age of 167 (±6) m.y. is within the range of ages of the "lherzolitic" shergottite group as well as a number of olivine-free enriched basaltic shergottites and at least one olivine-phyric shergottite (NWA 1068) (Shih et al., 2009).

Interestingly, the unique characteristics observed in NWA 2646, which are inconsistent with those of other established martian meteorite groupings, has led the editor of the Meteoritical Bulletin Database to recommend the use of the less-than-descriptive classification of ‘Martian’. It has been revealed that NWA 2646 bears some close similarities to the Antarctic martian meteorites RBT 04262 and NWA 4468 (see formation information on the NWA 1950 page).

In an effort to resolve the discrepencies that exist between the official IUGS definition of lherzolites and the application of that term to the varied group of "lherzolitic" shergottites, Mikouchi (2009) addressed the need for changing the name of the "lherzolitic" shergottites to one that is more consistent and more broadly applicable. Since a texturally-based nomenclature is already employed for some shergottite subgroups, e.g., olivine-phyric, it was suggested that the term "pyroxene-oikocrystic" shergottites would be an appropriate name with which to encompass all of the various martian "lherzolitic" shergottites that exist in the worldwide collections. This would include both intermediate and enriched "lherzolitic" shergottites as reflected by a geochemical classification scheme, as well as any depleted members that may be recovered in the future.

A modal analysis of NWA 2646 revealed that it has a heterogeneous composition, consisting of the pyroxenes pigeonite (40.7 vol%) and augite (24.3 vol%), together with olivine (21.6 vol%) and maskelynite (11.4 vol%). Minor phases include chromite, ilmenite, merillite, and pyrrhotite. It contains a higher abundance of plagioclase in the form of maskelynite than other members of the martian "lherzolitic" shergottite group, with the exception of RBT 04262 and NWA 4468. Possible pre-terrestrial secondary hydrous alteration products are present, including calcite and chlorides.

The CRE age for NWA 2646 based on 10Be, an isotope which is produced by local irradiation of the proto-Sun, has been calculated to be 2.5–3.1 m.y. This is similar to several other martian meteorites, including NWA 1068 and NWA 1460 (Nishiizumi and Caffee, 2006), and also similar to the "lherzolitic" shergottites (Walton and Herd, 2007). The specimen of NWA 2646 shown above is a 0.42 g fragment with a portion of outer surface. The photos below show two views of the main mass, along with a thin section in cross-polarized light.