Anomalous Galenas and the Continuous Diffusion of Lead

Abstract

IN the investigation of the isotopes of ‘common lead’ it was found¹ that anomalous galenas from a given area produce lead isotope ratios which, when represented on a ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb plot, delineate a straight line. Russell and Farquhar suggested that this result would be produced if common lead with normal or non-anomalous isotopic composition were mixed with radiogenic lead. Thus the position of a point along the line would be determined by the ratio of common lead to radiogenic lead. Assuming this process to have occurred, then Russell and Farquhar gave arguments to set limits bounding the time interval within which uranium could have generated radiogenic lead of the requisite kind. They further showed that if one could say that the uranium started producing new radiogenic lead at a time t₁, then at a definite subsequent time t₂, determined by the slope of the line in the ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb plot, this radiogenic lead must be garnered from its parent uranium and at some date mixed with the normal lead. It was assumed that the mixing was essentially contemporaneous with the separation of radiogenic lead at t₂.