The distribution of carbon in C1 to C6 carbonaceous chondrites

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The carbon content and δ^<13>C of carbon in ten carbonaceous chondrites, spanning petrologic grades 1 to 6,have been determined. There is a gradual change in the nature of the major carbonaceous components from C1 to C6 chondrites : C1 and C2 samples contain carbon as organics, whereas in higher petrologic types, carbon is predominantly amorphous or graphitic. This transition is consistent with carbon in C3 and C4 samples being formed either by dehydrogenation of organic materials during metamorphism on the parent body, or nebular heating followed by accretion at higher temperatures than prevailed during formation of C1-2 meteorites. In addition to a major carbonaceous component, ^<13>C-rich interstellar grains are found in C1 and C2 samples and, to a much lesser extent, CV3 meteorites. CO3 and C4-6 meteorites do not appear to contain ^<13>C-rich materials, a distribution controlled by primary accretion processes and not a result of secondary effects on parent-bodies. However, among the C1 to C3 meteorites aqueous activity might have acted to re-distribute ^<13>C-rich grains by either concentrating them into C1 meteorites, or alternatively by transporting them into the source region of CV3 samples.
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