Numerous partially fusion-crusted fragments weighing together 1,150 g were found in Mali, probably near the border of Algeria, and subsequently purchased by M. B. Sueilem from an Algerian dealer. Analysis and classification was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner) and the University of New Mexico (oxygen isotopes, K. Ziegler). The lot was designated NWA 12241 and determined to be a fresh martian olivine gabbro. The following year three additional fragments having a combined weight of 4,147 g were recovered and sold to D. Pitt. This lot was studied at both the University of Washington in Seattle (A. Irving) and Washington University in St. Louis (P. Carpenter), and NWA 12837 was determined to be paired with NWA 12241.
Northwest Africa 12241 is an olivine gabbro with a cumulate texture, specifically an orthocumulate which is defined as containing 8575% accumulated minerals in the groundmass. The meteorite is composed primarily of coarse-grained olivine and clinopyroxene along with ~15 vol% intercumulus plagioclase. Accessory phases include both Ti-rich and Cr-rich chromite, chlorapatite, merrillite, ilmenite, and pyrrhotite. The presence of microcracks in the silicates and the absence of maskelynite indicates a low shock stage.
Rahib et al. (2019) conducted an in-depth study of a comprehensive sampling of shergottites representing different subgroups: gabbroic, poikilitic, basaltic, and olivine-phyric. Based on expansive information including mineralogical, isotopic, and elemental data, crystallization and CRE ages, redox and temperature conditions, and crystal size distribution, spatial distribution pattern, and phosphorus zonation pattern analyses, a model for the formation and emplacement of shergottites was proposed. First, minerals with a poikilitic texture (pyroxene oikocrysts enclosing olivine and chromite chadacrysts) accumulated in ponded magma near the base of the crust. This magmatism involved multiple volcanic systems on Mars, including both geochemically enriched and intermediate source regions. Thereafter, magma ascended towards the surface while crystallization of sequentially more evolved interstitial melts proceeded; textures of these late-stage phases are manifest as non-poikilitic. This stage occurred under more oxidizing conditions, likely due to auto-oxidation, degassing, and/or assimilation of oxidized crust (Combs et al., 2019). Potassium-rich metasomatic fluids were incorporated during magma ascent as identified in NWA 7397, NWA 10169, and LAR 06319. The crystallized rock was ultimately emplaced as both shallow intrusive sills (gabbroic and poikilitic shergottites) and extrusive lavas (basaltic and olivine-phyric shergottites). The emplacement of gabbroic shergottite NWA 7320 shown in green in the diagram below may be analagous to the emplacement of NWA 12241, except the latter is derived from a geochemically intermediate rather than enriched source. Based on this study, Rahib et al. (2019) inferred that basaltic, olivine-phyric, and poikilitic shergottites are petrogenetically linked to a few common source regions on Mars, and that samples from each of these volcanic complexes were ejected in at least two distinct impact events as demonstrated by CRE age data (see schematic illustration below).
Schematic of the Emplacement of Enriched and Intermediate Shergottites
click on image for a magnified view
Diagram credit: Rahib et al., GCA, vol. 266, p. 489 (2019, open access link)
'Mantle source to near-surface emplacement of enriched and intermediate poikilitic shergottites in Mars'
The specimen shown in the photos above is a 0.77 g partially crusted fragment of NWA 12241 showing pale green olivine grains interspersed with white plagioclase and dotted by black chromite grains, overall exhibiting a coarse gabbroic texture due to relatively slow cooling at depth. Some very fine shock veins are also evident. The photo below shows the remnant black fusion crust along one edge of the specimen with some adhering desert soil. These excellent photos are shown courtesy of Azelmat Nor Eddine.
∗ Recent geochemical research on the martian shergottites has led to new petrogenetic models and classification schemes. read more >>
Photo courtesy of Azelmat Nor EddineSaharock Meteorites