Nepheline and sodalite in the matrix of the Ningqiang carbonaceous chondrite: Implications for formation through parent-body processes

摘要

Ningqiang is an ungrouped carbonaceous chondrite that chemically and petrologically resembles CV3 chondrites. The matrix of Ningqiang shows much higher abundances of Na, K, and Al by factors of 4.4, 2.7, and 1.6, respectively, than in CV3 chondrites. Our scanning and transmission electron microscope observations and synchrotron radiation X-ray diffraction measurements reveal that the major proportions of these elements can be attributed to the presence of nepheline and sodalite. Rietveld refinement of X-ray diffraction data shows that the feldspathoids constitute 7.7 vol.% of all crystalline phases in the matrix. Nepheline and sodalite occur mostly as discrete, equidimensional grains 2–5 μm in diameter that are dispersed homogeneously in the matrix. Most of the grains contain inclusions of Fe-rich olivine and minor Ca pyroxene, magnetite, troilite, and pentlandite. Despite the high abundances of Na, K, and Al in the matrix of Ningqiang, the bulk meteorite abundances of these elements are comparable to those of the CV group (e.g., Rubin et al., 1988). This means that the chondrules, which constitute a major proportion of the volume other than the matrix in Ningqiang, are depleted in Na, K, and Al. In fact, our analyses and observations show that the chondrules in Ningqiang overall contain very small amounts of these elements. Our interpretation of these findings suggests that nepheline and sodalite in the Ningqiang matrix were originally formed by Na-metasomatism of the chondrules and Ca–Al-rich inclusions in the meteorite parent body. Afterward, they were likely disaggregated and scattered into the matrix. However, it is difficult to envisage that the disaggregation and scattering occurred in situ in the present setting of the meteorite. Hence, we suggest that the Ningqiang meteorite underwent these processes before final lithification.