Nepheline and sodalite in chondrules of the Ningqiang carbonaceous chondrite: Implications for a genetic relationship with those in the matrix

Ningqiang is an ungrouped carbonaceous chondrite that has a chemical and mineralogical affinity to CV3 chondrites. The Ningqiang matrix has distinctly higher abundances of Na, K, and Al than CV3 matrices. A recent study by Matsumoto et al. (2014) revealed that the major proportions of these elements can be attributed to the presence of nepheline and sodalite. Scanning electron microscopy revealed that all of the Ningqiang chondrules studied show abundant evidence of extensive Na–Fe metasomatism. Only a small proportion of the chondrules contain primary mesostases in their cores, but the mesostases in their mantles were replaced by fine grains of nepheline, sodalite, Fe-rich olivine, and hedenbergite. The mesostases in the majority of the chondrules were completely replaced by fine grains of the same secondary minerals. Most opaque nodules were also largely replaced by various fine-grained secondary minerals. Nepheline/sodalite form veins penetrating the primary mesostases, providing evidence that aqueous fluids were involved in the alteration reactions. The nepheline/sodalite in the mesostases contain various amounts of inclusions of Fe-rich olivine, diopside, hedenbergite, Fe sulfides, and magnetite. The mineralogical features of the nepheline/sodalite in the mesostases are almost identical to those in the meteorite matrix. These results suggest that a significant fraction of the nepheline/sodalite grains in the Ningqiang matrix originated from the nepheline/sodalite produced in chondrules and refractory inclusions and that they were disaggregated and mixed into the matrix. These processes can be explained consistently by the model of the dynamic formation of chondrite lithology in a parent body proposed by Tomeoka and Ohnishi (2015). We suggest that after a Ningqiang precursor with a CV3-like lithology was metasomatized, it was fragmented, causing the disaggregation of the fine-grained host matrix and the fine-grained altered mesostases, including nepheline/sodalite, and opaque nodules in the chondrules. The chondrules were thereby separated into multiple fragments. Subsequently, during transportation in a fluidized state, all these materials were homogenously mixed together and later underwent accumulation and lithification.