Significance of the unequilibrated ordinary chondrites

Abstract

Olivine compositions of thirteen olivine-hypersthene and olivine-bronzite chondrites, chosen for their apparent freedom from recrystallization, were determined with an electron microprobe. A parallel petrographic study of these samples, and five others in which Keil and Fredriksson found variable olivine compositions, was used to relate the extent of variation in each sample to textural characteristics. Large variations in olivine composition occur in all of the chondrites examined in which (1) intergrowth between chondrules and matrix is absent or very slight and (2) clear, brown mafic glass occurs in the matrices of barred and porphyritic chondrules. Smaller variations occur in samples which show slightly greater intergrowth of the chondrules and matrix and in which microcrystalline material occurs in place of glass. Dispersion of olivine compositions is absent from samples which contain petrographic evidence of substantial recrystallization. Variations in olivine composition and the presence of mafic glass appear to be characteristic of the least-altered chondrites and are probably characteristic of the material from which the chondrites formed. These properties disappear early during metamorphism, before recrystallization becomes petrographically obvious. Thus Bjurbole, which appears only slightly recrystallized on petrographic grounds, has recrystallized sufficiently to homogenize its olivine. These conclusions conflict with the present models of Wood and of Fredriksson, and they impose a new restriction on modifications of these models as well as on the formulation of other models.