The radioactivity, accessory minerals, and possibilities for absolute dating of bentonites
Hamill, Gilmore Semmes
Adams, John A. S.
Master of Arts
Sixty-three samples of bentonite from the Ordovician and 3 from the Devonian of the eastern United States were collected. Two samples from the Ordovician of Norway, and 4 from the Cretaceous of Wyoming were secured from other individuals. The thorium, uranium, and potassium metal concentrations of these samples were determined by gamma-ray spectral and alpha count-fluorometric uranium (only used on 3 Wyoming samples) techniques. The averages and ranges of these values are [see table] This data is interpreted in terms of the manner of formation and occurrence of the bentonites. It is concluded that the redox potential of the environment and the mineralogical composition of the bentonites are the dominant factors in controlling the concentrations and their ranges. A significant portion of the thorium and uranium is tied up in resistate minerals.. The possibility of using the accessory zircons in bentonites. for absolute age determinations was also explored. Criteria were set up to test the contemporaneous pyrogenic origin of all, or most, of the zircons in the sample. The main criteria were a simple restricted mineral suite consisting solely of pyrogenic minerals, and the character and habit of these minerals (they must show no effects of normal subaqueous transport). Several samples were tested, and one was found to satisfy all of the criteria. From the quantitative separation of a pure zircon concentrate from this latter sample an estimate was made of the amount of bulk bentonite necessary for various types of age determinations. Four to 5 kilograms of bentonite should yield approximately 300 milligrams of pure zircon, which is an adequate amount for radiation damage, and total lead-alpha count methods. About 20 kilograms is optimum for lead-206/lead-207 age determinations. A hydrocyclonic mineral separator is described which can readily handle samples of this size. Aside from the zircon a potassium-argon age may be obtained on the biotite.