NORTHWEST AFRICA 5958

Numerous fragments of a fusion-crusted carbonaceous chondrite having a combined weight of 286 g were found in Algeria and purchased by G. Hupé in Morocco. A sample was submitted to Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington at Seattle (A. Irving and S. Kuehner) for analysis and classification. Northwest Africa 5958 was designated an ungrouped carbonaceous chondrite of petrologic type 3.0, with a low shock stage of S1 consistent with other carbonaceous chondrites.

In their study of magnetic susceptibility, Elmaleh et al. (2012) identified abundant Fe-rich phyllosilicates such as the serpentine cronstedtite, indicative of a low degree of parent body hydrothermal alteration. Moreover, the observation of partially altered chondrules led them to consider revising the petrologic type from 3.0 to 2.9. Although the meteorite experienced low terrestrial weathering (W1), NWA 5958 has experienced significant loss or gain of some elements during its residence in the desert, with more extreme alteration observed toward the surface (Ash et al., 2011). Subsequent analyses of the magnetic properties of NWA 5958 conducted by Jacquet et al. (2016) indicate the presence of significant magnetite (1.2 wt%) compared to kamacite (0.42 wt%), which attests to a degree of parent body aqueous alteration. Utilizing infrared spectroscopy to investigate the hydrous phases in NWA 5958, Jacquet et al. (2016) identified a significant abundance of phyllosilicates indicative of weak to moderate aqueous alteration, presenting a spectral range that matches well that of CM2 LEW 85311.

In the first compositional analysis conducted by Bunch et al. (2011), it was found that NWA 5958 consists of a wide variety of small objects (0.05–2.5 mm) set in a dark, porous, fine-grained matrix. Notably, matrix material constitutes 76 vol% of the meteorite, a higher abundance than in most CM chondrites, and it contains a relatively low abundance of chondrules composing 19 vol% (Jacquet et al., 2016). Silicates, phyllosilicates (e.g., the Fe-serpentine cronstedtite), and partially equilibrated Fe-sulfides were observed in the matrix by Stroud et al. (2014). Similar to CM chondrites, relatively small chondrules of mostly Mg-rich POP, enstatite PP, and forsterite PO types are present and host multiple accretion rims (up to five), with each successive layer composing material of larger grainsizes. In their study, Jacquet et al. (2016) identified accessory metal, sulfides, and poorly characterized phases (PCP, now determined to be tochilinite-cronstedtite intergrowths [TCI]). The TCI composition in NWA 5958 reflects a relatively high mean "FeO"/SiO2 ratio of 5, which is consistent with very minimal aqueous alteration compared to other CM group members (Rubin et al., 2007). Contrariwise, the relatively low mean S/SiO2 ratio (0.05) for NWA 5958 is consistent with a high degree of aqueous alteration. At the same time, the low abundance of FeNi-metal (< ~0.2 vol%) is more consistent with a moderate degree of aqueous alteration comparable to petrologic type 2.2–2.5 in the Rubin et al. (2007) scheme.

Bunch et al. (2011) reported µm-sized hexagonal carbon grains present in some NWA 5958 chondrules with larger C aggregates in the matrix and olivine. Rare fine-grained carbonaceous chondrite xenoliths were also observed. In addition, only a few small CAIs were found in the meteorite (2 vol%), and it was ascertained that these do not contribute significantly to the unique 16O-rich composition of the bulk meteorite (Ash et al., 2011). A larger number of AOAs were observed in NWA 5958 by Jacquet et al. (2016). The AOAs were determined by Marrocchi et al. (2019) to be of the compact type rather than porous, with Mg# of 98.9–99.9. They further assert that the AOAs condensed from the protoplanetary cloud over very short timescales measured in days to weeks, and agglomeration and thermal annealing occurred over a successive period of a year to a decade or more, respectively.

An oxygen 3-isotope diagram based on the initial values determined by Bunch et al. (2011) is shown below. Given these values, NWA 5958 falls along an extension of the carbonaceous chondrite anhydrous mineral (CCAM) line of slope-0.94, but is distinct from other C chondrites in having values even closer to initial solar values exemplified by the slope-1 line (D. Rumble III, CIW; see O-isotope plot). The CCAM line represents a linear array on an oxygen three-isotope diagram that is defined by the ratio plots for a mixture of all minerals that constitute CAIs. It was determined by Young and Russell (1998) that the most primitive Solar System materials defined a linear array with a slope of 1.00 (Y&R line). These primary materials initially had heterogeneous 16O contents, but later mass fractionation and oxygen exchange processes resulted in material with higher 17O and 18O contents, generally evolving towards the CCAM line.

Further analyses of NWA 5958 were conducted by Jacquet et al. (2016), and they obtained O-isotopic values (Δ17O = –4.26‰) different from those determined previously (Δ17O = –7‰). A new oxygen 3-isotope diagram based on these new values is shown in the diagram below. It demonstrates that NWA 5958 plots near some C2-ungrouped meteorites such as Acfer 094 rather than with C3-ungrouped meteorites, although terrestrial alteration in cold and hot deserts is a factor that needs to be considered. Together with other similar meteorites, NWA 5958 could sample a separate CM-like parent body (Jacquet et al., 2017).

On a coupled ε54Cr vs. ε50Ti diagram, Torrano et al. (2021) demonstrated that NWA 5958 plots in a unique location away from all other known carbonaceous chondrite groups. With respect to Δ17O vs. ε54Cr and ε50Ti, they determined that NWA 5958 plots close to the CO chondrite field (see diagrams below). Their data support the classification of this meteorite as C2-ungrouped (CM-like). In addition, the Cr and Ti systematics indicate that the CM and CO chondrites derive from separate parent bodies, while the CM- and CO-like ungrouped chondrites derive from one or more additional parent bodies, all of which accreted within close proximity at a heliocentric distance near ~3.7 AU.

Other CM-like meteorites were identified by Greenwood et al. (2019) that may be related to NWA 5958 and perhaps represent a separate CM-like parent body, including EET 87522, GRO 95566, LEW 85311, MAC 87300, MAC 88107, NWA 7821, NWA 11556, and Y-82054 (see diagram below). In a separate study, Prestgard et al. (2022 #6184) also found that ungrouped MAC 87300 and MAC 88107, together with ungrouped NWA 13689, share similar petrographic characteristics and oxygen isotopic compositions, and they suggest these three may represent a unique grouplet. An in-depth analysis of the CM2.7 LEW 85311 conducted by Lee et al. (2019) led them to conclude that its high abundance of refractory inclusions, and thus its anomalous REE and bulk O-isotopic composition, is more consistent with a separate CM-like parent body potentially including NWA 5958. On the other hand, a single large isotopically-heterogeneous CM parent body could be the source for all of these meteorites, and the variability in aqueous alteration observed among them might be attributed to differences in their water:rock ratio, temperature, and/or other factors.

A new method was employed by Franchi et al. (2019) to obtain more reliable O isotope values. They found that components in both NWA 5859 and LEW 85311 have δ18O values that plot along the entire CO–CM trend line (slope ~0.7) linking the CO and CM groups (see diagram below). These results support the model for a mixture of anhydrous (CO3), intermediate (C2-ung), and aqueously altered material (CM2) which is derived from the core, mantle, and outer zone, respectively, on a common asteroid (Greenwood et al., 2014).

Sanborn et al. (2015) presented a Δ17O vs. ε54Cr coupled diagram in their analyses of NWA 5958. Utilizing the initial Δ17O value determined by Bunch et al. (2011), along with the 54Cr value determined by Göpel et al. (2013) of +0.973 (± 0.153)—this value being close to the mean for all carbonaceous chondrites—they demonstrated that the meteorite plots in a distinct region (bottom orange circle in diagram below). However, given the new O-isotope value determined by Jacquet et al. (2016) of Δ17O = –4.26‰, the meteorite is within the trend of other carbonaceous chondrite groups (top orange circle in diagram below); it plots slightly below the ungrouped achondrite (CV-clan-related) NWA 7822 (Δ17O = ~ –4‰).

The initial bulk chemical and trace element compositions calculated for NWA 5958 were found to resemble that of CI chondrites, including a high volatile content, with the 187Os/188Os ratios having the largest value (the most radiogenic) of any measured carbonaceous chondrite (Ash et al., 2011). Subsequent bulk chemical and trace element compositional analyses were conducted by Jacquet et al. (2014). After accounting for the typical altered signature caused by an extended residence in a desert environment, NWA 5958 shows very close similarities to the CM chondrite Paris. Measurement of the Cr2O3 content in chondrule olivine for NWA 5958 is comparable to that for the CO3.03 ALHA77307 and the CM2.7–2.9 Paris, which is indicative of very limited thermal metamorphism at temperatures <300°C.

Comparative analyses of the known asteroid types and a suite of ungrouped and rare meteorites in multiple forms (bulk, powder, polished section), including the ungrouped carbonaceous chondrite NWA 5958, were conducted by Krämer Ruggiu et al. (2021) utilizing petrographic, spectroscopic, and albedo data. They concluded that this meteorite best matches the Ch-type asteroids (see diagram below).

Using improved analytical techniques, Marrocchi et al. (2018, 2019) identified type I porphyritic chondrules in NWA 5958 (and CV3 Kaba) that contain inherited relict olivine grains showing affinities to primitive AOA-like condensates, and which likewise represent some of the earliest solids to form in the protoplanetary disk. These relict grains, located in the interior of the host chondrules, have cores that are both Ca–Al–Ti-poor and 16O-rich compared to the surrounding host (in situ) olivine crystals. The oxygen isotope values for these relict grains plot along the primary chondrule minerals (PCM) line, thus predating chondrule formation (see diagram below).

In a study of presolar grains in the NWA 5958 matrix conducted by Nittler et al. (2012), a small abundance of 13C-rich grains (45 ppm) and O-anomalous grains (~100 ppm) were identified. The low abundance of O-rich presolar silicates (~50 ppm) observed by Stroud et al. (2014), including an Al,Mg-spinel and an enstatite grain, is thought to be due to loss as a result of parent body hydrothermal alteration. These values suggest a slightly higher degree of hydrothermal metamorphism compared to typical type 3.0 carbonaceous chondrites and is more consistent with a type 2. In their continued investigation of presolar grains in NWA 5958, Nittler et al. (2020) determined that the average matrix presolar SiC abundance for the analyzed samples is 18 (+15/–10) ppm, while the average matrix O-rich presolar grain (silicates and oxides) abundance is 30.9 (+17.8/–13.1) ppm. The isotopic composition of most of the presolar grains indicates a probable derivation from low-mass (1.5–4 M_⊙) AGB stars, with only a small number of the O-rich presolar grains originating in Type II supernovae (Ek et al., 2019). These presolar grain abundances in NWA 5958 are similar to those in CM chondrites. The relatively low abundance of presolar silicate and oxide grains found in NWA 5958 again attests to a significant degree of aqueous alteration.

Northwest Africa 5958 is a unique primitive sample from the early Solar System, having many characteristics intermediate between CM and CO chondrites. The specimen of NWA 5958 shown above is a 1.62 g crusted fragment, while the image below shows an excellent interior close-up, shown courtesy of Greg Hupé.