Extraction of diagenetic and detrital ages and of the 40Kdetrital/40Kdiagenetic ratio from K-Ar dates of clay fractions
Szczebra, Marek (Polskiej Akademii Nauk. Institute of Geological Sciences)
Środoń, Jan (Polskiej Akademii Nauk. Institute of Geological Sciences)

Data:	2009
Resum:	Illite age analysis (IAA) is a classical method for extracting diagenetic and detrital ages from mixed ages measured by K-Ar. This approach is based on measuring the masses of diagenetic and detrital illitic components in a few different grain-size fractions of one rock sample and measuring the mixed ages of these fractions. The 1Md illitic polytype is usually considered to be diagenetic, while 2M is considered detrital. A plot of the function: exp(λt)-1 (where t is time and λ is the decay constant) vs. weight percent of the detrital fraction is constructed. On the basis of linear extrapolation to end-member fractions, the diagenetic and the detrital age is obtained. This approach does not take into account various K contents in different polytypes (%K and %K). In order to do that, the detrital mass fraction (wt. %) should be recalculated into the percentage of detrital K (%I): %I = wt. % × %K/wt. % × %K + wt. % × %K × 100 Analytical constraint of the K content of different polytypes is very difficult, so a new approach to this problem has been developed. In the present study, the plot of Ar*/K vs. %I for a precisely determined ratio of K/ K was observed to be linear. On the basis of this observation, a computer program, MODELAGE, was written in the Java programming language using as input a few measured detrital illite mass fractions along with the mixed K-Ar ages of the relevant grain fractions. It then calculates the end-member ages and the K/K ratio using genetic algorithms. The errors in diagenetic and detrital illite mass-fraction determination mean that the K/K ratio and the end-member ages can be evaluated only with some uncertainty. The best results are obtained if the measured mass fractions represent a relatively broad range. Constraining one of the unknowns (particularly the K/ K ratio) improves the results significantly. Evaluation of data obtained from the literature using the proposed approach leads to the conclusion that the K/K ratio is often >1. 00, and some of 1Md illite polytype materials may be of detrital origin. If this is not the case, if a broad range of mass fractions is covered, and if the differences between end-member ages are relatively small, IAA analysis still gives appropriate results, even if the true K/ K ratio is different from 1. 00.
Drets:	Tots els drets reservats.
Llengua:	Anglès
Document:	Article ; recerca ; Versió publicada
Matèria:	Diagenetic Age ; Genetic Algorithms ; Illite Age Analysis ; K-Ar ; Mixed Ages
Publicat a:	Clays and Clay Minerals, Vol. 57, Núm. 1 (2009) , p. 93-103, ISSN 1552-8367

DOI: 10.1346/CCMN.2009.0570109

11 p, 451.8 KB  Accés restringit a la UAB

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