CR6
(Primitive achondrite in MetBull 89)
Purchased June 2003
no coordinates recorded
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 did not fit 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 genetically related to the CR carbonaceous chondrite group. Studies of the REE pattern for NWA 3100 demonstrate a similarity to the CR-related chondrite LEW 88763 (Bunch et al., 2008, and reference therein).
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; LPSC XXXVI [2005], #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 may 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 Tafassasset, both have O-isotopic ratios that plot with the CR chondrite group and may 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 metasomatic processes. The specimen of NWA 3100 shown above is a 3.1 g partial slice.
