Intercalibration and Comparison in Two Laboratories of Measurements Incident to the Determination of the Geological Ages of Rocks

Abstract

In an effort to extend the scope of geological age measurements based on the accumulation of helium in igneous rocks, researches were initiated which indicated disagreement when compared with previously published results from the same geological horizons. Cooperative investigations, undertaken by the authors, show that the radium determinations used in formulating the previous helium time scale are incorrect by more than a factor of two. The magnitude of the necessary downward revision of ages varies, depending upon the Th/U ratio of the individual rock specimens. Helium age determinations have been made on a number of igneous rocks by two entirely different techniques, the alpha-helium method and the radon-thorium-helium method. The alpha-helium method is independent of radioactive standards, and its results are in agreement with the new measurements by the radonthorium-helium method. These researches also represent the first comparison of helium age measurements on the same specimens by two or more observers. While the helium ages of many individual geological horizons are lowered by these new results, the total span of geological time remains unreduced. Precision radium standards in the region of ${10}^{\ensuremath{-}12}$ gram have been verified.