Lead Isotopes and the Age of the Earth

Abstract

RATIOS of the abundances of isotopes of lead from a considerable number of ores have been given by A. O. Nier1 and discussed by A. Holmes². They are expressed as numbers of atoms of Pb²⁰⁶, Pb²⁰⁷, Pb²⁰⁸ per atom of Pb²⁰⁴. The last is not of radioactive origin; the others are presumably partly original and partly derived from the decay of the radioactive elements U²³⁸, U²³⁵ and Th²³². Their abundance would therefore be expected to increase with time, and this is found to be the case; they are more abundant, always relative to Pb²⁰⁴, in ores more recently formed than in old ones. Holmes has used the variation to derive an estimate of the age of the earth, which he gives as about 3,350 million years. This is substantially more than had previously appeared probable, though no evidence definitely contradicts it. On the other hand, several geophysicists have found Holmes‘s argument difficult to follow; the following account is partly based on a suggestion made privately by Bullard.