Clay Reaction Kinetics; Bentonites, Shales and K-Ar ages

B. Velde

The conversion of smectite to illite gives rise to a dating tool in sedimentary basin studies. The rate of conversion is dependent upon time, temperature and the chemical composition of the sediments. Bentonites and shales show different rates of smectite conversion. Using a two step kinetic formulation for reaction progress, one can extract kinetic constants for shale and bentonite reactions. Simulations of reaction progress indicate that in young and hot basins there will be a significant difference between shale and bentonite reaction progress whereas the two materials tend to become concordant in old, cooler basins. These observations indicate that shale and bentonite "ages" will not give exactly the same result even when affected by the same burial events.

Use of bentonites for dating should be favoured for several reasons:

1. Bentonites give more homogeneous K-Ar dates as a function of grain size, hence less difference will be observed for different operating procedures in dating.

2. Bentonites respond more rapidly to brief thermal events, allowing the determination of specific episodes in basin history.

3. Bentonites contain few detrital minerals in the clay fraction which eliminates problems of contamination.

It should be remembered that potassic phases (essentially illites) in both; shales and bentonites are formed over significant periods of time and hence the K-Ar "age" determined is necessarily an integration of Argon radiogenically generated over a period of the basin history. Because they are generated at different rates, use of both shale and bentonite dates should give more information on the history of a sedimentary basin.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California