NORTHWEST AFRICA 12241

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 was initially classified as an olivine gabbro with a cumulate texture, specifically an orthocumulate—defined as containing 85–75% 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 rare absence of maskelynite indicate a low shock stage. Notably, the unique depleted, poikilitic shergottite Asuka 12325 also contains abundant crystalline plagioclase (Takenouchi et al., 2019, 2020). However, although the bulk Mg# of 0.42 for Asuka 12325 is consistent with a mafic rock in the classification scheme proposed by Irving et al. (2010), it has an anomalous CaO content of 4.1 wt% (see diagram below).

It was presumed by Udry et al. (2021 #1033) that the shock experienced by NWA 12241 is comparable to that experienced by the plagioclase-bearing augite basalt NWA 8159 estimated to be 15–23 GPa. Takenouchi et al. (2020 #1326) estimated a similar peak shock pressure of ~16–22 GPa (possibly initially ~26–31 GPa) for depeted poikilitic shergottite Asuka 12325. Lapen et al (2019) determined that NWA 12241 belongs to the intermediate subgroup of shergottites (see diagram below).

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). Variable abundances and sizes of pyroxene oikocrysts among the diverse suite of poikilitic shergottites reflects differences in crystallization time at depth prior to magma ascent and near-surface crystallization of the non-poikilitic areas (Yamazaki et al., 2022 #2153).

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 analogous to the emplacement of NWA 12241, except the latter is derived from a geochemically intermediate source instead of an 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). In addition, due to the significantly lower peak shock level (16-22 GPa) experienced by A-12325 compared to other poikilitic shergottites, Takenouchi et al. (2019) inferred that it may have been ejected in a distinct impact event by a slower impactor (6 km/s).

Further investigation by Udry et al (2021 #1033) with respect to Crystal Size Distribution (CSD), bulk mineral and trace element compositions, and oxygen fugacity variability among the dual textures led to a revised classification for NWA 12241 as an intermediate poikilitic shergottite. The CSD plot for NWA 12241 is similar to the bimodal texture of enriched poikilitic shergottite NWA 10169, while the oxygen fugacities of the dual lithologies and the mineral compositions are similar to those of the intermediate poikilitic shergottites LEW 88516 and ALH 77005. However, because of the low degree of shock experienced by the meteorite, as indicated by the presence of crystalline plagioclase without maskelynite, they consider that NWA 12241 may have been launched from Mars in a distinct impact event from the other poikilitic shergottites (see the NWA 1950 page for further information regarding the poikilitic shergottites).

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 >>