The compositional classification of chondrites: VI. The CR carbonaceous chondrite group

Abstract

New analytical data combined with recent studies by other researchers allow the definition of a Renazzo (CR) group of carbonaceous chondrites. We analyzed nine CR chondrites (Acfer 187, Acfer 209, El Djouf 001, Elephant Moraine 87747, Elephant Moraine 87770, Elephant Moraine 87847, MacAlpine Hills 87320, PCA91082, and Yamato 793495) constituting at least five independent fall events by instrumental neutron activation analysis for twenty-seven elements. Along with previously analyzed Renazzo, six or more closely related fall events are represented. Key CR properties include refractory lithophile abundances ~1.0 × CI levels, Zn/Mn ratios ~0.3 × CI levels, metal contents of 100–160 mg/g, (unusually high for a carbonaceous chondrite group), relatively large chondrules (mean size ~ 700 μm), and the presence of magnetite framboids. Al Rais is a close relative but too different in chemical and isotopic composition to be considered a normal member of the CR group; we suggest that it be treated as an anomalous member (CR-an), but that its properties not be included in CR ranges and means. MAC87320, PCA91082, EET87770, and Acfer 187 were studied petrographically along with Renazzo and Al Rais. Renazzo has a lineation possibly caused by fluid-lubricated, impact-induced shearing. The CR chondrites experienced some reduction during weak thermal metamorphism; the heating must have taken place prior to hydrothermal alteration. We suggest that formation of magnetite (and framboidal magnetite in particular) in CI and CR chondrites is due to hydrothermal alteration of metal-rich (or opaque-rich) precursors, and that the low abundance of magnetite in CM chondrites relative to CI indicates that the CM precursors were metal poor. Some carbonates in CI and CR chondrites may have formed by H 2O reaction with cohenite or poorly crystallized graphite.