Carbon-rich aggregates in type 3 ordinary chondrites: Characterization, origins, and thermal history

Abstract

Carbon-rich aggregates from 3 type 3.4–3.6 ordinary chondrites and 2 chondritic clasts have been characterized in detail using transmission electron microscope techniques. The aggregates in all the meteorites studied range in size from 5–1000μm and consist of a fine scale intergrowth of poorly graphitized carbon, amorphous carbon, Fe,Ni metal, and minor chromite. Contrary to previous reports, well-crystallized graphite and magnetite are absent. The association of Fe,Ni metal and carbonaceous material suggests that the original carbonaceous material may have formed by Fischer-Tropsch (FT) type reactions (catalytic hydrogenation of CO gas) at low temperatures (< 400 K), possibly in the solar nebula. This carbonaceous material probably consisted of a complex mixture of hydrocarbons, kerogen-like material, and other complex organic molecules. The aggregates were subsequently accreted onto the ordinary chondrite parent bodies and underwent planetary thermal processing which resulted in the catalytic graphitization of hydrocarbons, in the presence of Fe,Ni metal, to produce poorly graphitized carbon. None of the meteorites studied experienced temperatures sufficiently high to produce crystalline, ordered graphite. Using the empirical geothermometer of Rietmeijer and MacKinnon (1985), the measured d 002 spacings of poorly graphitized carbon show that graphitization occurred at temperatures between 300 and 450°C. This range of temperatures is significantly lower than the generally quoted metamorphic temperatures for type 3.4–3.6 ordinary chondrites (∼450–500°C; Dodd, 1981). The presence of poorly graphitized carbon in type 3 ordinary chondrites is also clear evidence that these chondrites must have accreted cold: no carbonaceous material would have been present in these meteorites if they had accreted at high temperature (> 1000 K). Although it is possible that some of the carbon in the type 3 ordinary chondrites could have been produced by recondensation of CO produced by outgassing from a planetesimal interior, evidence for such an origin for the C-rich aggregates is not compelling.