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EXTRACTION OF DIAGENETIC AND DETRITAL AGES AND OF THE ⁴⁰K_detrital/⁴⁰K_diagenetic RATIO FROM K-Ar DATES OF CLAY FRACTIONS

Marek Szczerba^* and Jan rodo

Institute of Geological Sciences Polskiej Akademii Nauk, Senacka 1, 31-002 Kraków, Poland

^* E-mail address of corresponding author: ndszczer{at}cyf-kr.edu.pl

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 2M₁ is considered detrital. A plot of the function: exp(t)–1 (where t is time and 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_detrital and %K_diagenetic). In order to do that, the detrital mass fraction (wt.%_detrital) should be recalculated into the percentage of detrital K (%I_d(K)):

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 ⁴⁰Ar*/⁴⁰K vs. %I_d(K) for a precisely determined ratio of ⁴⁰K_detrital/⁴⁰K_diagenetic 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 ⁴⁰K_detrital/⁴⁰K_diagenetic ratio using genetic algorithms.

The errors in diagenetic and detrital illite mass-fraction determination mean that the ⁴⁰K_detrital/⁴⁰K_diagenetic 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 ⁴⁰K_detrital/⁴⁰K_diagenetic ratio) improves the results significantly.

Evaluation of data obtained from the literature using the proposed approach leads to the conclusion that the ⁴⁰K_detrital/⁴⁰K_diagenetic 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 ⁴⁰K_detrital/⁴⁰K_diagenetic ratio is different from 1.00.

Key Words: Diagenetic Age • Genetic Algorithms • Illite Age Analysis • K-Ar • Mixed Ages