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Smectite to illite conversion and K-Ar ages

Published online by Cambridge University Press:  09 July 2018

B. Velde  and
C. Renac
B. Velde
Affiliation:
Laboratoire de Géologie, UR 1316 CNRS, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris, France
C. Renac
Affiliation:
CRSCM CNRS, la rue de la Ferollerie, 45071 Orléans Cedex 2, France
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Abstract

Interpretation of K-Ar determinations of the apparent age of crystallization of diagenetic potassic layer silicates should take into account the current ideas concerning the smectite to illite conversion of clay minerals. These minerals are the major K-bearing neogenic phases in sedimentary rocks and, therefore, it is important to keep in mind their origin and the pathway of their evolution. Another important contributor to K and Ar content are detrital minerals which are not considered in detail in this paper. The radiogenic Ar retained by diagenetic illitic clay minerals (illite and illite-smectite interstratified minerals with a low smectite content) will be a function of: (1) the amount Ar lost through the destruction of mixed-layer and small grained illitic material dissolved to produce new, larger, illite-rich crystals; and (2) the amount of Ar produced and retained in the growing, stable illite mineral grains. Hence the K/Ar ratios in diagenetic illites depend on several variables. In samples where the detrital contribution is small, when the illite growth reaction is rapid, i. e. occurring over a short period of time, the accumulated radiogenic Ar can be used to date the geologic event which caused this crystal growth. However, if the process occurs over a significant period of time, as is usually the case in shales, the age deduced by radiogenic Ar content will indicate the changes in mineralogy and grain-sizes of the different participating phases. These changes will be the result of time and temperature variables of the sedimentation and burial of the samples studied. The radiogenic Ar accumulated will reflect a sequence of mineral changes over a period of time.

Type
Research Article
Information
Clay Minerals , Volume 31 , Issue 1 , March 1996 , pp. 25 - 32
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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