Earth and Planetary Sciences ETDs

Publication Date

12-6-1974

Abstract

A series of experiments, involving molten magnesium silicate spherules that were subjected to high initial cooling rates and allowed to nucleate at high temperatures (985-1350°C), were performed so as to compare their thermal histories to the resulting internal features. Such comparisons could be used to decipher the thermal history of chondrules and the conditions surrounding their formation based on their mineralogy, crystal morphology, and overall texture. Various features of the spherules were correlated with the initial high cooling rate process, nucleation temperature, and the maximum temperature attained during the recalescance to demonstrate the restrictions resulting from or their sensitivities to specific thermal events. To extend parts of this comparison to spherules nucleating at temperatures lower than those of the high cooling rate experiments (i.e. 800-950°C), several devitrified magnesium silicate spherules were also included in the study. The only crystalline phase nucleated in these experiments is forsterite-protoenstatite field. The residual melt is quenched to a glass. Forsterite crystal morphology changes from a bar-like structure to dendrites, fibers, and submicroscopic crystals, respectively, with greater undercooling for a given bulk composition while the crystal width generally decreases under the same conditions. The textures of these spherules are two main classes, spherulitic and those derived from excentroradially arranged, wedge-shaped grains, and can be differentiated on the basis of nucleation temperature and bulk composition. Those spherules consisting of excentroradiating grains appear to be nucleating from the coolest portion of the melt, i.e. near the surface. Diagrams in which crystal morphology, crystal size, and texture are superimposed on nucleation temperature, with respect of bulk composition, are potential references for determining choldrule nucleation temperatures. There is a temperature realm in which nucleation of the metastable melt is most likely to occur. Once the nucleation temperature of a chondrule is known, specific parameters can be sought that would restrain nucleation until such undercoolings have been reached, thereby providing a means to test various models that attempt to explain the formation of chondrules.

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

Albert Masakiyo Kudo

Project Sponsors

NASA grants NGL 32-004-063 and 32-004-064

Language

English

Document Type

Thesis

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