PRINCIPAL COMPONENT ANALYSIS OF THE BULK CHEMICAL COMPOSITION OF ORDINARY CHONDRITES

Ordinary (O) chondrites are the most abundant class of meteorites and are composed of varying proportions of chondrules, refractory inclusions, metal/sulfide grains and matrix. They are divided into 3 groups (H, L, LL) by bulk composition (e.g., total Fe, total metallic Fe, total oxidized Fe), with each group assumed to be from a single/similar parent body. Meteorites from all 3 groups are thought to have formed by accumulation of materials present in the primitive solar nebula at c. 4.56 Ga and are in effect sedimentary rocks [1]. In this study we used Principal Component Analysis (PCA) and a large bulk compositional data set [2] to test the idea that O chondrites formed by accumulation of distinct components in the solar nebula. The analysis returned results generally consistent with a “sedimentary” origin. The first 2 principal components (PC1, PC2) explain over half of the compositional variation in O chondrites. PC1, explaining 38.0% of variation, shows the importance of Fe-Ni metal, and oxidized (i.e., silicate) components in determining O chondrite composition. PC1 successfully breaks out the H, L, and LL groups although there is a minor overlap between H and L, and L and LL groups. PC1 also shows that sulfide grains accumulated independently of metal grains, perhaps related to their lower condensation temperatures. PC2, explaining 12.6% of variation, shows the importance of chemically bound water, which is linked to C and inversely correlated with FeS – we attribute this observation as confirmation of the importance of the matrix component dominated by low temperature condensates. Our PCA generally does not break out petrologic types (although the importance of dehydration is evident in the L class) nor refractory inclusions. The former result suggests that metamorphism of parent bodies representative of each class was approximately a closed system excluding H₂O. Ref:[1] Scott, E.R.D. (2007) Ann Rev Earth Planet Sci; [2] Jarosewich, E. (1990) Meteor. 25: 323