--> ABSTRACT: Applicability of Refined K-Ar Age Analysis of Ultrafine Illitic Material from Mudrocks for Constraining Burial History, by S. Kelly Sears, Reinhard Hesse, Hojatollah Vali, W. Crawford Elliott, and James L. Aronson; #91019 (1996)

Datapages, Inc.Print this page

Applicability of Refined K-Ar Age Analysis of Ultrafine Illitic Material from Mudrocks for Constraining Burial History

S. Kelly Sears, Reinhard Hesse, Hojatollah Vali, W. Crawford Elliott, and James L. Aronson

K-Ar apparent ages of untreated <0.05 µm illitic material separated from mudrocks of the Reindeer D-27 well, Beaufort-Mackenzie area, Arctic Canada, increase with increasing depth of burial. XRD patterns and HRTEM images of octadecylammonium ion (nc = 18) exchanged samples have shown that a component of illite, referred as "expandable illite," can be depleted of its interlayer K but still have the high-charge characteristic of illite. K-Ar apparent ages after nc =18 exchange show two distinct age shifts compared to the original untreated samples. Ages of the 0.05-0.1 µm fraction decreased after nc = 18 exchange in two shallow samples from 120 ± 2 to 115 ± 3 Ma (1920.2 - 1935.5 m) and from 95 ± 2 to 55 ± 3 Ma (2026.9 - 2072.6 m), respectively. This is due to depletion of interlayer constituents in "expandable detrital illite," which may be similar to weathered illite found in soils. However, the ages are greater than the depositional age as the non- expandable illitic material is detrital in origin. The age of the deepest sample (3825.2 - 3840.5 m) increased slightly from 100 ± 2 to 106 ± 3 Ma. Although the proportion of authigenic illite has increased in this sample, the expandable illite component is still largely detrital. K-Ar ages of the <0.05 µm subfraction increased from 19 ± 3 to 92 ± 4 Ma at shallow depth and from 65 ± 3 to 97 ± 3 Ma in the deepest sample. This suggests that the exchange of K by nc = 8 affected a young expandable illitic component of authigenic origin. The age change is less in the deeper sample because it is dominated by an authigenic illite component. This study represents an alternative approach to better constrain the timing of post-depositional events in burial history through the distinction between diagenetic and detrital illitic populations.

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