Potassium Metasomatism and Diffusion in Cretaceous K-Bentonites From the Disturbed Belt, Northwestern Montana and in the Middle Devonian Tioga K-Bentonite, Eastern u.s.a. (Illite/smectite, Clays, Diagenesis, United States)
Altaner, Stephen Paul
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https://hdl.handle.net/2142/71139
Description
Title
Potassium Metasomatism and Diffusion in Cretaceous K-Bentonites From the Disturbed Belt, Northwestern Montana and in the Middle Devonian Tioga K-Bentonite, Eastern u.s.a. (Illite/smectite, Clays, Diagenesis, United States)
Author(s)
Altaner, Stephen Paul
Issue Date
1985
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
Abstract
This thesis presents X-ray diffraction, elemental, K/Ar, and petrographic data of K-bentonites and enclosing shales from the Marias River Formation (Late Cretaceous) in the disturbed belt of Montana. The objective of this study is to determine the kind and extent of chemical exchange between bentonite beds and shale host rocks during K-bentonite formation.
One 2.5 m thick K-bentonite bed and five thinner K-bentonite beds (1.1 m thick and less) are zoned mineralogically and chemically, with illite- and potassium-rich upper and lower contacts and a smectite-rich potassium-poor middle. In all cases, the formation of smectite-rich clay minerals appears to be due to a deficient supply of potassium and not lower temperatures. For K-bentonites that were originally vitric tuffs, potassium appears to be the only major element that is metasomatically derived, however, the exact source of potassium was not determined. To maintain electroneutrality calcium may have moved from the bentonite into the enclosing shale. Variations in the amount of layer silicates in shale surrounding bentonite beds are interpreted to be due to depositional variations and not due to chemical exchange during illitization.
I/S from the 2.5 m thick K-bentonite bed is zoned with respect to K/Ar age, with the contacts giving ages 3-4 my older than the center. This difference in apparent age is interpreted to indicate that potassium transport in the bentonite was diffusion controlled. Solution of the coupled equations for chemical transport and reaction (illitization) for the 2.5 m thick bentonite yields a diffusion coefficient for potassium transport of about 5-8 x 10 cm/sec. Solutions assuming this value predict I/S zonation observed in thinner bentonite beds.
The Tioga (Middle Devonian) K-bentonite, studied from outcrop localities in New York, Pennsylvania, Virginia, West Virginia, Indiana, and Illinois, shows a broader range in I/S layer composition and stacking order than reported in previous studies of this unit (Weaver, 1956; Droste and Vitaliano, 1973; Johnsson, 1984). The reason for the difference is that this study examined rocks from a larger region and therefore from a wider range of thermal grades than previous studies.
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