Carbon in glass inclusions of Allende, Vigarano, Bali, and Kaba (CV3) olivines

Abstract

Carbon contents and distribution have been measured in glass inclusions in olivines of CV3 carbonaceous chondrites by using the 12C(d,p) 13C nuclear reaction. All olivines from the four studied meteorites had low carbon contents (<70 ppm). Conversely, glasses of glass inclusions in the same olivines had highly variable carbon contents, all above 100 ppm. Glass inclusions in olivines from meteorites of the oxidized group (Allende, Kaba, and Bali) had carbon contents that varied from 100 to about 2000 ppm, whereas those from the reduced group member Vigarano were surprisingly poor in C (averaging 300 ppm). These relative abundances of carbon in these glasses of reduced and oxidized CV3 meteorites are in contrast to the abundances of interstellar SiC in these meteorites. This indicates that glass inclusions in olivines could have behaved, with respect to carbon, as closed systems that have escaped elemental exchange processes. The carbon content of the glasses is, therefore, likely to be primary and reflect the physico– chemical conditions during the formation of the host olivine and glass inclusions. The redox conditions prevailing during secondary processing of the olivines (e.g., metasomatic Fe–Mg exchange, Ca–Na exchange) appear not to have influenced the carbon distribution. Carbon could have been trapped initially as a refractory carbon species (e.g., carbide) by clear glass inclusions. Despite the fact that the nature of neither the primary nor that of the secondary C species was established, the commonly heterogeneous distribution of C in glass inclusions in olivines suggests entrapment of a solid C-bearing precursor.