Introduction

Early in the twentieth century, Rutherford and his colleagues developed the use of the fixed lifetimes of radioactive nuclei as a chronometer to measure ages of terrestrial and meteoritic rocks, and in 1929 Rutherford extended their use to make arguments about the age of the elements since their mean epoch of creation, related to the age of the Galaxy and the Universe. In the 1950s and afterwards, Fowler and Hoyle and others refined these arguments on the basis of improved understanding of nucleosynthesis; complications raised by questions related to Galactic chemical evolution will form a major topic of this chapter. In recent years, the discovery of dead short-lived radioactivities (e.g. from the isotopic anomalies mentioned in Chapter 3) has led to further inferences of timescales related to the formation of the Solar System. Some of the relevant species are listed in Table 10.1.

Age-dating of rocks

The basic idea in radioactive age-dating of rocks (from the Earth, Moon and meteorites) is to find the ratio of daughter to parent in an isolated system. Thus the age inferred is usually the ‘solidification age’ which is the time since the last occasion when chemical fractionation was halted by solidification.