New look at the lead isotope growth curve

Abstract

THE isotopic composition of lead changes with time due to radiogenic production from isotopes of uranium and thorium. Two lead isotopes are produced from uranium, ²⁰⁶Pb from ²³⁸U, and ²⁰⁷Pb from ²³⁵U. The paired U-Pb decay schemes are particularly useful since geochronological information can be derived even when there has been a chemical fractionation of U from Pb at some time during the history of the sample. For example, the ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios can be measured in a galena (PbS) sample; by assuming that the parent material for the galena had suffered no chemical fractionation of U from Pb from the time of formation of the Earth until production of the galena, one can calculate a ‘model’ age of formation for the galena. This model is referred to as single stage growth of lead and can be characterised by a μ value [(²³⁸U/²⁰⁴Pb), as measured today] for the parent U-Pb system.