Background

The various chemical elements have different properties and can therefore be readily separated from each other by igneous processes. The various isotopes of a given element are not so easily separated. The abundances of the radioactive isotopes in the crust and mantle, and their decay products, are not constant in time. Elemental compositions of magmas and residual mantle are complementary; isotopic compositions are identical, but they diverge with time. Therefore, the information conveyed by the study of isotopes is different in kind than that provided by the elements. Each isotopic system contains unique information, and the radioactive isotopes allow dating of processes in a planet's history. The unstable isotopes most useful in geochemistry have a wide range of decay constants, or half-lives, and can be used to infer processes occurring over the entire age of the Earth (Table 17.1). In addition, isotopes can be used as tracers and in this regard they complement the major- and trace-element chemistry of rocks and magmas. Isotopes in magmas and gases, however, cannot be used to infer the depth or location of the source.

Studies of isotope ratios have played an important role in constraining mantle and crustal evolution, mixing and the long-time isolation of mantle components or reservoirs. Isotope studies derive their power from the existence of suitable pairs of isotopes of a given element, one a ‘primordial’ isotope present in the Earth since its formation, the other a radiogenic daughter isotope produced by radioactive decay at a known rate throughout geological time.