PETROLOGY OF FEO-POOR, PORPHYRITIC PYROXENE CHONDRULES IN THE SEMARKONA CHONDRITE

The mineralogy and petrology of FeO-poor, porphyritic, pyroxene- and olivine-rich chondrules in the Semarkona (LL3.0) chondrite are described in detail. In an extension of the textural classification scheme, these chondrules are designated types IAB and IB. In type IAB chondrules, the proportion of olivine phenocrysts is between 20-80%, and in type IB chondrules, olivine constitutes <20% of the phenocryst assemblage. All the chondrules studied are FeO-poor and contain olivine and low-Ca pyroxene phenocrysts in varying proportions. Olivine is present both as chadacrysts enclosed in low-Ca pyroxene and as larger phenocrysts. Ca-rich pyroxene occurs commonly as rims on low-Ca pyroxene phenocrysts. Lamellar zoning in low-Ca pyroxene, obseved in backscattered electron images, is interpreted as a primary growth feature. Apparent partition coefficients between phenocrysts and mesostasis for major and minor elements are consistent with crystallization of an essentially molten chondrule at rapid cooling rates (100-1000-degrees-C/h). Within the entire type I series, there are continuous changes in textural and compositional properties that suggest common origins for all chondrules in this series. These properties include proportions of olivine and pyroxene phenocrysts, FeO contents of olivine and pyroxene phenocrysts, and a complementary relationship between the proportions of refractory elements and Si-, Fe-rich precursor components in chondrule bulk compositions. Observations of the behavior of Na suggest that evaporation and recondensation of volatile elements was not an important process during formation of type I chondrules, and that abundances of volatile elements were largely controlled by the abundance of a volatile-rich precursor component.