RB-SR SYSTEMATICS IN ACID-LEACHED CLAY-MINERALS

Hydrochloric-acid leaching of four illites, two montmorillonites and one each of mixed-layer illite-montmorillonite and vermiculite increases both the Rb Sr and 87Sr 86Sr ratios. Calculated RbSr dates are higher for the acid-treated samples than those for the corresponding untreated samples. The RbSr dates for three illites are 10-15% higher than and, for one illite, nearly in agreement with their commonly accepted stratigraphic ages. Leachate 87Sr 86Sr ratios for the illites range from 0.7124 to 0.7277, the lowest value being found for the illite with the highest percentage of expandable layers. Leaching of Sr from the clay minerals may produce only a small change in 87Sr 86Sr ratio of the carbonate phase during dissolution of limestones containing less than 20% clay minerals. © 1979.