Potassium—argon ages of sedimentary and pyroclastic rocks

Abstract

SummaryIt is rarely possible to obtain reliable potassiumargon ages from sedimentary rocks. Glauconites generally give dates that are too low and very scattered, though the largest of a group of such measurements may be useful to define minimum ages of sediments that cannot be dated in any other way. Illite dates are subject to opposing systematic errors: because of this they may coincide roughly with the age of the sediment, but they cannot be regarded as reliable. Sylvines lose argon easily, though they may give good potassium-calcium dates.From bentonites and volcanic ashes biotite, sanidine, and low-potassium feldspars have all been successfully dated. Contamination by old detritus is sometimes a problem, but the dates obtained from different minerals generally agree well, showing that the results are reliable. The wide geographical distribution of bentonites gives them a special value for time-scale work.