Kamacite and olivine in ordinary chondrites: Intergroup and intragroup relationships

The three principal groups of ordinary chondrites (OCs), H (high total Fe), L (low total Fe), and LL (low total Fe, low metallic Fe), constitute ∼80% of all meteorites observed to fall. In going from H to L to LL chondrites, the abundances of siderophile elements decrease and the degree of oxidation increases. The proportion of oxidized Fe (i.e., FeO in silicates) increases at the expense of metallic Fe. Because Fe is more readily oxidized than Ni or Co, bulk metal becomes increasingly rich in Ni and Co (e.g., Prior, 1916). Equilibrated LL chondrites are thus characterized by high FeO/(FeO + MgO) ratios in olivine and low-Ca pyroxene, high taenite/kamacite ratios, and the occurrence of Co-rich kamacite. The present study reports high-precision electron microprobe analyses of olivine and kamacite in a large suite of OCs. The purpose of the study was sixfold: (1) rigorously define the compositional ranges of these phases for each OC group, (2) identify anomalous OCs whose olivine and/or kamacite compositions lie outside the established ranges, and hence may not belong to the three main OC groups, (3) characterize the phases in the chondritic clasts of the Netschaevo iron meteorite and determine how closely related Netschaevo is to OCs, (4) determine if there are intragroup variations of olivine and kamacite compositions with petrologic type, (5) identify those OCs as fragmental breccias that contain some olivine and/or kamacite grains with aberrant compositions, and (6) search for new metallic Fe-Ni phases with extreme compositions. An expected by-product of this investigation was that a few meteorites that previously had not been well-characterized would be reclassified