NORTHWEST AFRICA 5717

A single fresh, highly unequilibrated meteorite weighing 7.310 kg was found near Mahbas, Western Sahara. The stone was acquired in 2008 by a meteorite dealer in Erfoud, Morocco, who then sold it to meteorite dealer D. Pitt. A sample was sent for analysis and classification to Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington in Seattle (A. Irving), and NWA 5717 was classified as an ungrouped chondrite of petrologic subtype 3.05.

The meteorite is composed of two different lithologies, usually with diffuse boundaries. As described in (Bunch et al., 2010 #1280), the darker lithology A, considered to be the host rock, is composed of relatively ferroan olivine chondrules (Fa_9.7–20.8 cores) and a fine-grained cataclastic matrix. Most chondrules in lithology A are surrounded by a fine-grained rim consisting of micron-scale mineral fragments, lithic clasts, sulfide nodules, microchondrules, and FeNi-metal grains in a submicron-scale groundmass, indicative of an accretionary origin in a dusty nebula region (Bigolski et al., 2012 #2426, 2013 #5227, #2239; Cuzzi et al., 2017 #2364). The metal textures observed by Weisberg and Ebel (2010 #5402) reflect slightly higher temperatures compared to Semarkona (3.01).

The oxygen isotope composition (Δ¹⁷O = 0.539, 0.554 ‰) is similar to that of H chondrites. In contrast, the lighter lithology B is composed of the more magnesian chondrules (Fa_0.03–3.4 cores) in a similar size range but including ~5 vol% micro-chondrules, and has an oxygen isotope composition (Δ¹⁷O = 0.061, 0.080 ‰) that plots on the TFL similar to E chondrites. In a multi-isotope study, Zhu et al. (2023) found that the Δ17O, ε50Ti, and ε48Ca values for NWA 5717 do not overlap with those of OCs, although its ε54Cr value does; therefore they infer that this meteorite derives from a distinct parent body.

Northwest Africa 5717 plots within a field occupied by a small number of anomalous, ungrouped, non-carbonaceous chondrites, including HaH 180, JaH 846, NWA 960, NWA 2335, and NWA 2336. Together with the NWA-series meteorites 2040 [LL], 2041 [L], 3114 [L], 3127 [LL], 3157 [L], 4294 [LL], 4298 [LL], 4486 [L], 4531 [LL], NWA 7835 [acho-ung; Irving et al., 2014, #5332; photo courtesy of Stefan Ralew], and NWA 10769 [acho-ung; Moggi Cecchi et al., 2017, #2696], their O-isotopic values infer a possible "supra-TFL" genetic grouping which defines a slope distinct from the ordinary chondrites. As demonstrated in the diagrams below, these meteorites plot far away from the trend lines for the H, L, and LL ordinary chondrite groups and probably represent several previously unrecognized parent asteroids (Irving et al., 2014, #5332).

In a particle size distribution analysis of NWA 5717, Simon et al. (2018) concluded that each of the two lithologies represents chondrule aggregation in a separate nebula region prior to their convergence and assembly into a planetesimal, wherein only the dark lithology experienced prior alteration involving accreted metal and ices. This scenario is also consistent with the isotopically heavy nature of the dark lithology and its lack of fresh metal. The Al–Mg age of NWA 5717 was determined for each lithology by Tang et al. (2015 #2245) using two select Al-rich chondrules. They found the formation age to be 1.94 (±0.62) m.y. and 1.56 (±0.49) m.y. after CAIs for lithology A and B, respectively. The meteorite is very weakly shocked to stage S2 and has experienced minor terrestrial weathering to grade W1/2.

Northwest Africa 5717 has been instrumental in resolving the mechanism of aggregation (sticking) of small particles leading to the formation of planetesimals (see Cuzzi et al., 2017 and references therein for further details). The 4,013.4 g NWA 7402 (L3.1-anom, MetBull 104) that was purchased in 2010 is possibly paired with NWA 5717. The photo of NWA 5717 shown above, courtesy of Mendy Ouzillou, is a 10.0 g partial slice exhibiting both lithologies A and B. Also visible are a few larger light-colored objects composed predominantly of enstatite.