K-Ar ages of 2 : 1 clay minerals, Mackenzie Delta Beaufort Sea region, Arctic Canada: Significance of n-alkylammonium exchange

K-Ar ages of untreated 2:1 clay minerals of the <0.05 mu m size-fraction se parated from argillaceous rocks of the Reindeer D-27 well, Mackenzie Delta - Beaufort Sea region, Arctic Canada, generally increase with increasing de pth of burial, from 19 +/- 3 at ca. 1935 m to 65 +/- 1 Ma at ca. 3832 m. Al though the ages are lower than the stratigraphic ages, XRD patterns and TEM images of the same material suggest that the K-Ar ages are mixed ages. The se result from a mixture of detrital and diagenetic illite. The K-Ar ages o f the 0:05-0.1 mu m fraction generally decrease with increasing burial, but are older than the stratigraphic ages. K-Ar ages of the <0.05 and 0.05-0.1 mu m size fractions after intercalating octadecylammonium cations (n(C) = 18) also vary with burial depth. The ages of the 0.05-0.1 mu m fraction dec reased sightly from 120 +/- 2 to 115 +/- 3 Ma at shallow depth (ca. 1935 m) , and decreased from 95 +/- 2 to 55 +/- 3 Ma at slightly greater depth (ca. 2065 m). A decrease in age may be due to the exchange of K and Ar by the i ntercalation of ne = 18 cations into the interlayers of an illitic phase of detrital origin. At maximum depth (ca.3832 m), the age of the 0.05-0.1 mu m fraction increased slightly from 100 +/- 2 to 106 +/- 3 Ma after treatmen t with ne = 18. The increase in K-Ar age with treatment is more evident in the <0.05 mu m fraction, which increased from 19 +/- 3 to 92 +/- 4 Ma at ca . 1935 m and from 65 +/- 3 to 97 +/- 3 Ma at ca. 3832 m. This suggests that exchange of interlayer K and radiogenic Ar by n(C) = 18 has occurred in a "young" illitic phase of diagenetic origin. The change in age is less signi ficant in the deeper sample because it is dominated by an authigenic illiti c phase, and the removal of K and Ar by n(C) = 18 is incomplete. The K-Ar d ating of IS and illite from argillaceous rocks of a burial-diagenetic seque nce after treatment with long-chain alkylammonium cations may help to bette r constrain the timing of post-depositional events in the diagenetic histor y. It is possible to distinguish between diagenetic and detrital illitic su bpopulations in shales with small amounts of detrital illite, as well as co nstraining the age of the detrital illitic component.