Use of K-Ar Dating and Oxygen Isotopes for Determining the Thermal History of Shales

摘要

A review of the literature suggests that the isotopes of potassium and oxygen can be used to determine the paleotemperatures of shales and other rocks containing clay minerals. During diagenesis K-bearing smectites and illites lose Ar and gain K over a temperature range of 40 exp 0 to 400 exp 0 C. This causes a decrease in the apparent K-Ar age. The release of exp 40 Ar/ exp 39 Ar by stepwise heating affords a series of apparent K-Ar ages which may include both the detrital age and the ages produced by diagenetic changes. The difference between exp 18 O/ exp 16 O ratios of quartz and illite that have reached isotopic equilibrium can be used to calculate maximum burial temperature. The K-Ar apparent age of size fractions of a diagenetic series of illitic shales varies as a function of temperature. It is possible to use exp 18 O/ exp 16 O temperature values to calibrate a K-Ar geothermometer. A K-Ar geothermometer is designed because the measurements are simpler to make and more reliable than exp 18 O/ exp 16 O measurements. (ERA citation 06:014902)