The compositional classification of chondrites—I. The carbonaceous chondrite groups

Twenty carbonaceous chondrites were analyzed by instrumental and radiochemical neutron activation analysis for Na, Mg, Al, K, Ca, Sc, V. Cr, Mn. Fe, Co, Ni, Zn, Ga, Ge, As, Se. Br. Ru, Cd, In, Sb, La, Sm, Eu, Yb, Lu, Os, Ir, and Au. Analysis of 2 or more samples of all but 2 chondrites has helped yield a high precision that allowed the resolution of numerous previously unrecognized trends. Refractory lithophile abundances decrease through the sequence CV (1.33 × CI), CM- CO (1.11 × CI) and CI. The abundances of the common siderophiles Fe, Ni and Co follow the order CI > CM > CO > CV, with CV chondrites depleted about 15% relative to CI. Volatile lithophile (Mn to K) and volatile siderophile (As to Ge) abundances decrease in the order CI > CM > CO > CV. The volatile trends in CO and CV chondrites reverse for the more volatile elements (Br to Cd) producing the sequence CI > CM > CV > CO. These three different sequences in the ordering of group elemental abundances can be used to resolve compositionally the four carbonaceous chondrite groups. We define clans to consist of one or more groups formed at a narrow range of heliocentric distances. Quantization of refractory lithophile abundances indicates the existence of three carbonaceous chondrite clans: CI, CM-CO, and CV. Despite similarities in parameters such as volatile abundances and O-isotope compositions differences in chondrule size and refractory abundances suggest that CO and CV chondrites are indeed best placed in separate clans. The relative heliocentric distance at which CI chondrites formed cannot be inferred, thus it seems safer to assign them to a separate clan.