K-Ar dating of illite fundamental particles separated from illite-smectite

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Authors:Clauer, N.; Srodon, J.; Francu, J.; Sucha, V.
Author Affiliations:Primary:
CNRS, Centre de Géochimie de la Surface, Strasbourg, France
Polish Academy of Sciences, Poland
Czech Geological Survey, Czech Republic
Comenius University, Slovakia
Volume Title:Clay Minerals
Source:Clay Minerals, 32(2), p.181-196. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0009-8558
Publication Date:1997
Note:In English. 44 refs.; illus., incl. 1 table, geol. sketch map
Summary:Fundamental particles of illite-smectite from bentonites were separated into classes by high-speed centrifugation after infinite osmotic swelling of mixed-layer crystals, achieved by Na-exchange and dispersion in distilled water. In samples free of detrital contamination, the thinnest fundamental particles yield older K-Ar ages than the thicker fundamental particles. This implies that they do not preferentially lose radiogenic 40Ar due to size, and that the illitization process is a crystal growth mechanism (not nucleation plus growth). As a result, any K-Ar age of fundamental illite particles from bentonites is an integral over longer or shorter periods of time, depending on the thermal history of the rocks. In thick bentonite beds, the measured age difference between the beginning of the illitization process at the contact with the host rocks and the end in the centre of the bed records extremely slow K diffusion in these well compacted rocks. These data explain why measured K-Ar ages of illite-smectite from bentonites are younger than the corresponding age of shale illitization, inferred from the burial history of the basin. The finest technically separable size-fractions of associated shales (<0.02 µm) yield K-Ar dates greater than the stratigraphic age. This observation points to incomplete recrystallization of detrital illite during burial diagenesis.
Subjects:Absolute age; Bentonite; Burial diagenesis; Clastic rocks; Clay mineralogy; Clay minerals; Diagenesis; Geochronology; Illite; K/Ar; Methods; Mixed-layer minerals; Particles; Sedimentary rocks; Sheet silicates; Silicates; Smectite
Abstract Numbers:98M/1109
Record ID:1997064819
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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