Unusual dark clasts in the Vigarano CV3 carbonaceous chondrite: Record of parent body process

Abstract

Abstract— Two unusual dark clasts found in the Vigarano CV3 chondrite were examined using an optical microscope and a scanning electron microscope (SEM). Both clasts lack chondrules, Ca‐Al‐rich inclusions, and coarse‐grained mineral fragments; they, instead, contain abundant inclusions that consist of fine grains (<1 μm) of homogeneous Fe‐rich olivine, thus resembling the fine‐grained variety of dark inclusions in CV3 chondrites. The external shapes of inclusions in the clasts bear a close resemblance to those of chondrules and chondrule fragments; some of the inclusions are surrounded by dark rims similar to chondrule rims. Our SEM observations reveal the following unusual characteristics: 1) the inclusions are not mere random aggregates of olivine grains but have peculiar internal textures, that is, assemblages of round or oval shaped outlines, which are suggestive of pseudomorphs after porphyritic olivine chondrules; 2) one of thick inclusion rims contains a network of vein‐like strings of elongated olivine grains; 3) an Fe‐Ni metal aggregate in one of the clasts has an Fe‐, Ni‐, S‐rich halo suggesting a reaction between its precursor and the surrounding matrix; and 4) olivine in the clasts commonly shows a swirly, fibrous texture similar to that of phyllosilicate. These characteristics suggest that the dark clasts in Vigarano are not primary aggregates of dust in the solar nebula but were affected by aqueous alteration and subsequent dehydration by heating after accretion to the meteorite parent body. The fine olivine grains in these clasts were presumably produced by thermal transformation of phyllosilicate, as is the case with those in the two thermally metamorphosed Antarctic CM chondrites, Belgica‐7904 and Yamato‐86720. From textural and mineralogical similarities, some of the dark inclusions and clasts previously reported from CV3 chondrites and other types of meteorites may have origins common with these clasts in Vigarano.