3 - Radioactivity, isotopes and dating

Summary

Preamble

Radioactive decays of certain naturally occurring isotopes are widely used to date terrestrial and meteoritic materials and to trace their evolution. Long before the discovery of radioactivity in 1896, it was understood that geological events occurred in a recognisable sequence, but attempts to fit them to a time scale were very insecure and contentious (see Section 4.2). Sedimentation and the fossil record are still central to geological history but now the fossil-based geological periods are linked to isotopically dated events. The principles of dating by radioactive decay require precise measurement of isotopic abundances. Isotopic methods have become so sensitive that very small variations in isotopic ratios of light elements, arising independently of radioactivity, are also routinely measured (Section 3.9).

We distinguish three categories of radioactive isotope that are of interest (Tables H.1, H.2, H.3 of Appendix H). Table H.1 lists the isotopes that are not produced in the Earth or the atmosphere by any continuing process, and must be accounted for in the inventory of elements in the Earth's original accretion. In only one important case (²³⁵U) is the half-life less than 10⁹ years and then only marginally so (a very rare isotope, ¹⁴⁶Sm, has a half-life of 10⁸ years). Many shorter-lived species would have been produced at the same time but have now disappeared. This is a clue that the last of the nuclear synthetic events that produced the material of the Solar System occurred several billion years ago.