Accretion of the anomalous CR2 chondrite Northwest Africa 14674: Implications for the complexities of the CR parent bodies

Abstract

An understanding of the differences between ungrouped, or anomalous, and normal carbonaceous chondrites could provide information on the population of parent bodies required to explain a chondrite group and on first solid accretion and evolution in the outer protoplanetary disk. The CR chondrites are key in this respect, as they display a unique formation history that distinguishes them from other groups. They are known to have formed between 4.1 and 4.6 Myr after CAI, with two generations of chondrules. Northwest Africa (NWA) 14674 is a CR2 anomalous (CR2-an) chondrite with very similar oxygen isotope composition, dark inclusion (DI) content, and serpentine-magnetite matrix to Al Rais (CR2-an). Both are petrologic subtype 2.3 with fresh magnesian olivine, and partly altered ferroan olivine, pyroxene, and metal. Additionally, NWA 14674 contains residual GEMS-like material at the nanoscale within preserved moderately altered areas. DI and matrix in NWA 14674 are mineralogically similar but they have different fabrics, and matrix is more porous than both DI and fine-grained rims (FGR). Matrix has aligned framboidal magnetite aggregates swathing the chondrules, suggesting slight compaction of the chondrite. Some DI have inner chondrule fragments and concentric layers richer and poorer in magnetite, indicating formation as accretionary pellets and lapilli: they are pebbles rather than clasts. The framboidal magnetite abundance is consistent with an alkaline alteration fluid potentially due to NH3 ice mixed with the more common water ice, which implies late distal accretion. Comparison with the CR chondrites Bells (regolith-like) and NWA 801 (with high-pressure clasts) indicates that a complex history involving inward drift, disruption of the grandparent body, and reaccretion of debris along with chondrules, DI pebbles, and dust is required to explain CR chondrite formation. The diverse facies observed in CR chondrites may be explained by the formation of relatively large parent bodies, comprising distinct layers (core to regolith). Some material has been inherited from a chondritic protoplanet that formed during the oligarchic growth phase of planetary formation. Subsequently, this initial body underwent disruption and partial reaccretion into the CR parent body.