A Study of Compositional Zoning in Garnets From the Ducktown Mining District, Tennessee: Implications for Mathematical Modelling of Garnet Growth
Babb, Robert Frederick, II
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https://hdl.handle.net/2142/68065
Description
Title
A Study of Compositional Zoning in Garnets From the Ducktown Mining District, Tennessee: Implications for Mathematical Modelling of Garnet Growth
Author(s)
Babb, Robert Frederick, II
Issue Date
1981
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Language
eng
Abstract
Garnets from the Ducktown mining district were examined for compositional zoning. Unzoned garnets are common in the sphalerite rich areas--the garnets are 50% spessartine--and in the calc-silicate nodules--the garnets show extreme sieve texture. Normal (convex manganese profile) zoning is prevalent among most of the remaining garnets, but clusters of garnets with inverted or reverse zoning have also been found. The most probable origin for these clusters is nucleation on preexisting fragments. The profiles are examined in relation to the surrounding minerals (all unzoned except plagioclase) and in relation to existing models of zoning formation. A new zoning model based on diffusion independence of growth is presented. The effects of slight variations in temperature and in composition are found to be extremely important and must be included. The presence of many variables and no single rate determining step more realistically models natural garnet growth and requires a numerical solution for the model.
From this model and the observations of the natural garnets, it is seen that the differences in the profiles of most of the garnets at constant metamorphic grade appear to be controlled by two major factors. The bulk chemical composition of the local environment is responsible for the absolute magnitude of the manganese in the garnet and may also play a decisive role in the nature of the calcium zoning. The relative rates of diffusion and surface growth are responsible for the shape of the manganese profile. Minor factors which can cause modifications of the profile include the availability of fluid during the retrogessive stage of metamorphism, the initial rates of nucleation and the number of nuclei, the rate at which manganese is supplied to the fluid from the matrix minerals, and the presence of excess (nonremoveable, inclusion-forming) impurities such as quartz and sulphides.
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