GRAIN SIZE AND PETROGRAPHY OF C2 AND C3 CARBONACEOUS CHONDRITES

Abstract

Grain size frequency distributions of the ≥ 0.1 mm size fractions have been measured for 19 C2 and C3 carbonaceous chondrites (7 CM2, 5 CO3, 6 CV3 and 1 CV2). The grain size frequency distribution curves and summary statistics are remarkably similar for the CM2 and CO3 meteorites, with ranges of median grain size from 2.36φ to 2.67φ and graphic mean grain size from 2.20φ to 2.59φ. The CV3 specimens are distinctly coarser than the CM2 and CO3 meteorites and have a wider range of grain size summary statistics. The grain size frequency data for fluid drop chondrules and for lithic chondrules show approximately the same size frequency characteristics as the more abundant particles in each of the petrologic subgroups. If the chondrules and other particles were produced by different mechanisms or in different environments, an extremely effective size sorting process is required to produce the observed grain size frequency distributions. Alternatively, most of the particles and the observed grain size frequency distributions might have been produced by small impacts into an immature, accretionary regolith. The strong similarity between the grain sizes of the CM2 and CO3 meteorites, together with previously recognized chemical similarities, suggest a genetic relationship between the two petrologic subgroups. The grain size frequency distributions of chondrules and other particles in CM2 and CO3 meteorites are different from those found previously in ordinary chondrites