Earth and Planetary Sciences ETDs

Publication Date

12-11-1978

Abstract

Chondrules in thin' sections of 18 different H3 to H6 group chondrites were analyzed by 100 µm broad beam electron probe techniques. Chondrule textures, crystal, and chondrule sizes were determined microscopically. Results indicate a decrease in variance or standard deviation from chondrules of petrologic types 3 to 6 of FeO, MgO, TiO2, MnO, K2O, and P205 content. Mean TiO2 and Cr2O3 content decrease from petrologic types 3 to 6 and mean FeO, Al2O3, CaO, and Na2O increase slightly from lower to higher petrologic types. Al2O3 and Na2O are correlated in all petrologic types but more so in types 4, 5, and 6. Al2O3 and K2O show a progressive increase in correlation from types 3 to 6 and AI2O3 and TiO2 show a progressive decrease in correlation from types 3 to 6. Al2O3 and CaO show a slight correlation in type 3 but not in the higher petrologic types. MnO and Al2O3, Na2O, and K2O are inversely correlated in types 4, 5, and 6, but not in type 3. MnO and Cr2O3 show some correlation in petrologic type 3 but not in types 4, 5, and 6. Normative fayalite and plagioclase content of chondrules become increasingly more homogeneous from petrologic types 3 to 6, with chondrules of petrologic type 3 showing a wide variation in range and content. Excess Na and K found to occur frequently in chondrules of type 3 chondrites decreases in amount and frequency from types 3 to 6. Chondrule textures were found to be distinct compositionally. Chondrule diameters increased progressively from petrologic types 3 to 6. Chandrule diameters of porphyritic chondrules correlate with maximum phenocryst size in type 3 chondrites, but not in types 4, 5, and 6; however, for a given chondrule diameter the phenocryst size is larger in the higher petrologic types. The following conclusions can be drawn from the results of this study. The decrease in standard deviation of chondrule composition from petrologic types 3 to 6 results from equilibration between chondrules and matrix with progressively higher degrees of either progressive- or autometamorphism. The progressive depletion or enrichment of an oxide in chondrules from petrologic types 3 to 6 can be explained by the same mechanism of metamorphic redistribution and equilibration between matrix and chondrules. The larger phenocrysts in chondrules of a given diameter in the higher petrologic types can be attributed to grain growth during recrystallization. It is clear from this study that no gross differences exist in chondrules from the different petrologic types of chondrites, and the gentle, continuous trends observed can be attributed to metamorphic processes and need not be the result of primary differences. It is not possible to determine if metamorphism resulted from reheating or if it resulted from different rates of cooling of hot chondritic material. Chondrule textures were determined to be more compositionally unique than previously surmised. It is suggested that composition is an important factor in determining chondrule texture, although events in the thermal history of the chondrule cannot be ruled out.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Klaus Keil

Second Committee Member

Douglas Gridley Brookins

Third Committee Member

Rodney Charles Ewing

Fourth Committee Member

Albert Masakiyo Kudo

Language

English

Document Type

Thesis

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