Introduction

Abstract

More than 1,000 papers have been written concerning lead isotopes (Doe, 1968 b) since Aston’s first abundance measurements were made by the mass spectrograph (Aston, 1927, 1929). Aston’s first measurements were exciting as they resulted in the discovery of ²⁰⁷Pb by Aston and discovery of ²³⁵U, estimation of its half-life within better than a factor of two, and the estimation of the age of the earth within a factor of two by Rutherford (1929). Precision measurements by mass spectrometric methods (Nier, 1938, 1939; Nier et al, 1941) introduced modern lead isotope studies. The isotopic composition of lead varies because of the radioactive decay of ²³⁸U to ²⁰⁶Pb, ²³⁵U to ²⁰⁷Pb, and ²³²Th to ²⁰⁸Pb (see Appendix A for decay chains). One isotope, ²⁰⁴Pb, has no long lived radioactive parent. Thus the process producing isotopic differences in lead isotopes should not be confused with the physico-chemical fractionation processes that cause isotopic differences in stable isotopes of light elements such as carbon, oxygen and sulfur. The prime factors controlling partitioning of isotopes are the mass separation divided by the atomic weight and changes in the oxidation state. Sulfur, for example, has a complex geochemistry characterized by many oxidation states and a range of ³²S/³⁴S in nature of about 15 percent, whereas silica, with approximately the same percent mass range for ²⁸Si/³⁰Si but with a very simple geochemistry, has less than 1/10 the range found for sulfur isotopes.