40Ar/39Ar Dating of Illite-to-Mica Minerals in Diagenetic-to-Hydrothermal Conditions: Potentials and Limits
Horst Zwingmann and Norbert Clauer
Technical and scientific aspects of a 40Ar/39Ar study on authigenic illite of Rotliegendes (Permian) gas-bearing sandstones in Northern Germany are presented. The illite distribution which reduced the reservoir qualities of the rocks, relates to facies and tectonic setting in horst-graben positions. Timing of the illite formation was obtained by K-Ar dating which provided ages between 220 and 160 Ma. The 220-200 Ma ages predominate in the horst areas and the 180-160 Ma ages are common in the graben areas.
To unravel the 60-Ma-long duration of illite authigenesis, the samples were packed in quartz vials under high vacuum conditions to control the 39Ar recoil during irradiation. The 39Ar recoil ranges from about 20% of the total 39Ar of the < 0.2µm fractions to about 10% in the 2-6 µm fractions. Theoretical modelization of the measured 39Ar recoil of different grain sizes suggests that the best suited grain size has to have at least 20 µm to minimize the recoil effect. Decreasing 39Ar recoil relates also to increasing illite crystallinity, which means that size is not the only determining factor for recoil.
The total 40Ar/39Ar gas ages of the illite fractions yield values comparable to the K-Ar ages of the same aliquots (identical within analytical uncertainty), indicating that the technique is well controlled. Beyond the recoil effect, real advantages exist among the 40Ar/39Ar results: plateaus of about 200 and 180 Ma ages were obtained, indicating two distinct illite generations in the clay mineral assemblages during the considered time span. The 39Ar release patterns of coarser size fractions also gave "staircases" suggesting internal reorganization inducing 39Ar recoil and potential occurrence of detritals.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California