C2-ungrouped (CM-like)
(C3.0-ungrouped in MetBull #99)
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Purchased September 2009

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 anaysis 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 chondrules with larger C aggregates in the matrix and olivine in NWA 5958. They found only a few small CAIs 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 the meteorite by Jacquet et al. (2016). Rare fine-grained, carbonaceous chondrite xenoliths were only observed by Bunch et al. (2011).

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, 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.

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Diagram credit: Bunch et al., 42nd LPSC, #2343 (2011)

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 below. It demonstrates that NWA 5958 plots near some C2-ungrouped meteorites such as Acfer 094 rather than with C3-ungrouped meteorites. Together with other similar meteorites, NWA 5958 could sample a separate CM-like parent body (Jacquet et al. (2017).

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click on photo for a magnified view
Diagram credit: Jacquet et al., MAPS, vol. 51, p. 862 (2016)

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); NWA 5958 plots slightly below the ungrouped achondrite (CV-clan-related) NWA 782217O = ~ –4‰).

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Diagram adapted from Sanborn et al., 46th LPSC, #2259 (2015)

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.

In a study of presolar grains present in 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, more consistent with a type 2. 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 144 mg fragment, while the image below shows an excellent interior close-up, shown courtesy of Greg Hupé.

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click on photo for a magnified view
Photo courtesy of Greg Hupé