Chondrules in H3 chondrites - Textures, compositions and origins

Compositional and textural variations among chondrules in unequilibrated (type 3) H-group chondrites are examined in order to determine possible relations between chondrule compositions and textures. Bulk compositions of polished thin sections of 90 individual chondrules and 16 compound chondrule sets from the Sharps, Tieschitz and Bremervorde chondrites were measured by broad-beam electron probe analyses; the chondrules were also classified petrographically as barred olivine, porphyritic olivine, porphyritic pyroxene, barred pyroxene, radiating pyroxene or fine-grained. The mean compositions of each type are found to be distinct as verified by discriminant analysis, despite a large scatter, with the olivine-rich chondrules characterized by low SiO2 and high FeO and MgO contents, greater concentrations of TiO2, Al2O3, Na2O and K2O, and lower Cr2O3 and MnO relative to pyroxene-rich chondrules. Data suggest that composition, together with cooling rate, has played a conspicuous role in producing observed chrondrule textures, and are consistent with chondrule formation from mixtures of differing fractions of high-, intermediate- and low-temperature nebular condensates that underwent melting in space.