Carbonaceous chondrites of the Ornans type: A metamorphic sequence

C3(O) chondrites comprise a metamorphic sequence. The following order reflects increasing grade: Kainsaz. Felix. Ornans. Lance. Isna, Warrenton. Assignment of Karoonda to the Ornans subtype is uncertain, but it is certainly of higher petrologic type than C3. Average olivine and pyroxene compositions in the metamorphic sequence change progressively from Fo₁₂. Fs₃ to Fo₃₄, Fs₁₁, respectively, and per cent mean deviation decreases. Kamacite and taenite change composition with increasing grade, reflecting higher equilibration temperatures. Blurring of textural features and Fe/Mg exchange between matrix and inclusions are also evident. As in the ordinary chondrites. contents of rare gases and possibly volatiles correlate with degree of metamorphism, but the effects are small. The meteorite Ornans presents an intriguing paradox. Observed chemical enrichment and depletion patterns reflect a higher metamorphic grade than do petrographic properties. The data suggest that abundance patterns of volatile components were not generated by metamorphism, but may represent primary differences. Strong correlations present in other C3(O) chondrites indicate some genetic link between metamorphism and composition, although the relationship is probably not causal. The autometamorphism model of Larimer and Anders (1967) appears to be the most straightforward explanation, but an observed negative correlation between the amount of matrix and content of volatiles suggests a re-examination of the two-component model. The decoupling mechanism required for Ornans is uncertain.