A single meteorite weighing 136 g was purchased in Rissani, Morocco. The stone was analyzed at Northern Arizona University (T. Bunch and J. Wittke) and was initially thought to represent a very fine-grained, completely recrystallized L7 chondrite (Bunch et al., 2005). Two very small relict chondrules were identified in the thin sections studied. This meteorite has been shocked to stage S1 and terrestrially weathered to grade W2.
Despite its similarities to an L7 chondite, certain elemental ratios such as Fe/Mn and Ca/Na were inconsistent with those known from any ordinary chondrite group, and therefore an O-isotopic analysis was conducted. Based on the values from the completed analysis, conducted at the University of Western Ontario (T. Larson and F. Longstaffe), it was demonstrated that the O-isotope plot of NWA 3100 falls along the trend line of the CR chondrites. This suggests that NWA 3100 is likely genetically related to the CR carbonaceous chondrite group. Studies of the REE pattern for NWA 3100 also demonstrate a similarity to the CR-related, FeO-rich achondrite LEW 88763 (Bunch et al., 2008 and reference therein); however, new analyses of LEW 88763 by Day et al. (2015) led them to propose its reclassification as an anomalous achondrite, possibly related to the ungrouped achondrite NWA 6704 and pairings.
A cooperative study was undertaken of a number of previously ungrouped achondrites, primitive achondrites, and silicated irons which have O-isotopic compositions that plot within the CR field (Northern Arizona University, University of Washington, and University of Western Ontario; 36th LPSC, #2308 ). Of the meteorites that were studied, including NWA 3100, NWA 801, Tafassasset, NWA 011/2400, LEW 88763, Sombrerete, and NWA 468, it was determined that some or all of them might have originated in the core, mantle, crust, and regolith of a large, at least partially differentiated, CR-type parent body, which was subsequently disaggregated.
Northwest Africa 3100 is a recrystallized, texturally evolved chondrite with an elevated Fe/Mn ratio and Ca-rich plagioclase (features possibly reflecting metasomatism), and an O-isotopic composition that plots within the CR chondrite field. These features may be most appropriately associated with the newly proposed group of carbonaceous metachondrites (Irving et al., 2005). The more highly fractionated, CR-related chondrite NWA 2994 and the equilibrated CR-an (or CR7) Tafassasset, both meteorites with O-isotopic ratios that plot with the CR chondrite group, may each represent lithologies on the CR parent body that experienced a higher degree and/or a longer duration of thermal metamorphism (Bunch et al., 2008) as well as metasomatism.
Continued research on this topic has been ongoing (e.g., Bunch et al., 2005, [#2308]; Floss et al., 2005, [MAPS vol. 40, #3]; Irving et al., 2014 [#2465]; Sanborn et al., 2014 [#2032]). As provided in the Sanborn et al. (2014) abstract, a Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships among meteorites (see diagram below). Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. The specimen of NWA 3100 shown above is a 3.1 g partial slice.
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)