A single, fusion-crusted stone weighing 237 g was purchased in Zagora, Morocco, in May of 2001. It was analyzed at several institutions in France, including Université d'Angers, IfremerCentre de Brest, Muséum National d'Histoire Naturelle, and Ecole Normale Supérieure de Lyon. Northwest Africa 1239 was classified as a rare polymict diogenite, a group that comprises only a small number of identified samples, representing 8 out of 182 diogenites studied by Warren et al. (2009).
Polymict diogenites have been defined as orthopyroxene-rich breccias containing small amounts of mafic lithic clasts or other mineral fragments with wide ranges in texture and composition. The diogenite Aioun el Atrouss is polymict, containing large eucritic clasts, while the polymict Garland contains orthopyroxenite from two distinct compositional groupsan FeO-rich, CaO-poor, low-Al and low-Cr group (I), and a low-FeO, CaO-rich, high-Al group (II). Clasts of magnesian norite and basalt have also been reported in Garland, as well as in the Peckelsheim diogenite. Another diogenite, Y-791073, has been tentatively described as polymict. Northwest Africa 1239 is a brecciated rock consisting of more than 90 vol% orthopyroxenite fragments, but contains a multitude of clasts and grains including eucritic pyroxene, calcic plagioclase (ranging to anorthite), and olivine, along with FeNi-metal, and troilite. It may be inferred that the source rock for this diogenite was situated in a regolith setting.
The compositional variation present in NWA 1239 supports the theory that diogenites are orthopyroxene cumulates resulting from fractional crystallization in a number of compositionally distinct basaltic magma sources. These diverse magmas, of which three orthopyroxenite classes have been identifiedI, II, and III (Hewins, 1980)were initially produced through moderate degrees of fractional melting or low degrees of partial melting (Shearer et al., 1997). In contrast, only two distinct classes, corresponding to the two Garland lithologies, were identified through cluster analysis based on orthopyroxene composition data (Harriott and Hewins, 1984).
From these distinct lithologies, two separate CRE age trends have been defined, representing impacts at 14 m.y. and 24 m.y. ago, consistent with a minimal number of mixing events for diogenites. The 14 m.y. trend is consistent with ejection of all three lithologies plus the polymict members, while the 24 m.y. trend appears to have ejected the class III lithology (containing lower Cr compared to class II; e.g., Roda, ALHA77256, and Ibbenbühren) and polymict members.
Further details on the petrogenesis of diogenites can be found on the Johnstown page. An alternative classification system for the diogenites and dunites based on mineralogical and petrographical features has been proposed by Beck and McSween (2010), and modified by Wittke et al. (2011). The photo above shows a 2.54 g partial slice of the polymict diogenite NWA 1239, while that below shows the main mass.