A fine-grained dark inclusion in the Vigarano CV3 chondrite: Record of accumulation processes on the meteorite parent body

Mineralogy and petrography of an unusual dark inclusion (DI) from the Vigarano CV3 chondrite (AMNH 2226-7) has been studied in detail. Formerly, this DI was briefly described by JOHNSON et al. (Geochim. Cosmochim. Acta, 54,819,1990). Unlike most common types of DIs, AMNH 2226-7 consists almost exclusively of fine grains (< μm in diameter) of mainly Fe-rich olivine and is devoid of chondrules, large mineral fragments and Fe-rich olivine aggregates (chondrule pseudomorphs). The Fe-rich olivine grains are mostly granular and equidimensional in morphology, and acicular to fibrous olivine grains, common in other types of DIs, are absent. Lesser amounts of relatively coarse grains (10-60μm) of Mg-rich olivine, Fe-rich olivine, enstatite, spinel, melilite, and perovskite are dispersed throughout the DI. A most distinguished feature of this DI is a network of arcuate bands (each 10-500μm wide and 250μm-3.5mm long) which exhibit peculiar textures. The bands consist mainly of olivine grains which are slightly finergrained and more Fe-rich than those in other areas. Several bands commonly occur roughly parallel to each other with equal spacing, forming a set of consecutive parallel bands. Different sets of parallel bands crosscut one another, some almost orthogonally. These mineralogical and textural features differ from the major group of DIs which contain chondrules and/or chondrule pseudomorphs, and indicate that the formation of AMNH 2226-7 cannot be explained by the direct replacement of a precursor chondrite. Probably, the origin of this DI is not directly related to that of the major types of DIs. We support the interpretation of JOHNSON et al. (1990) that the arcuate bands resemble fluvial sedimentary deposits and may reflect particle settling processes on the meteorite parent body. We suggest AMNH 2226-7 to be the product of reaccumulation of a fragmented and comminuted C3 chondrite. The predominant occurrence of fine-grained components in this DI may have resulted from either a grain size sorting process during reaccumulation or the disaggregation of already fine-grained precursor material such as type B DIs