Radioactive Dating Methods

Abstract

Radioactive dating methods involve radioactive isotopes of various elements and, of the 1400 to 1500 nuclides known presently, more than four-fifths are radioactive although most of them do not occur naturally because of their very rapid rates of radioactive decay. To obtain the ages of rocks and minerals, naturally occurring radioisotopes are used which continued to exist long after the Big Bang because of their extremely slow decay rates. This is the case with ²³⁸U, ²³⁵U, ²³²Th, ⁸⁷Rb and ⁴⁰K. However, some arise from the decay of long lived, naturally occurring radioactive parents, among them ²³⁴U, ²³⁰Th and ²²⁶Ra. And a few may be created by natural nuclear reactions, for instance ¹⁴C (radiocarbon), ¹⁰Be and ³H (tritium). While today, artificial radioisotopes have been introduced into the environment by thermonuclear testing and the operation of nuclear fission reactors and particle accelerators. Whatever its source, radioactivity is significant with regard to geochronology and radioactive dating researches really began in an attempt to determine the age of the Earth. This was not long after Henri Becquerel’s discovery of it in 1896 and the work of Marie Curie who found that thorium and uranium minerals emit radiation and later identified two new elements, polonium and radium. In fact, it is from the latter that the word “radioactivity” derived. So by 1913, it became possible for serious investigations to be made regarding our planet’s age, a matter referred to by Arthur Holmes as indelicate, adding that they were in progress anyhow because “Science knows no shame”. Subsequently, dramatic developments have taken place and determining the ages of minerals, rocks, archaeological and historical objects and so on is now routine. The major methods for achieving this are discussed in this chapter of which the main aim is to provide a brief perspective of the subject which is actually vast in scope. These are listed alphabetically and include four radiation damàge techniques which are electron spin resonance, fission track dating, pleochroic haloes and thermoluminescence. No effort has been made to describe new approaches still being developed such as the La/Ce isotope scheme which constitutes a potentially powerful adjunct to the Sm/Nd method for petrogenetic studies but which is impeded because of the conflict between the best determination by counting of the β-decay half-life of La (3.02x10¹¹ a⁻¹) and geological half-life assessments based on La/Ce and Sm/Nd mineral isochrons. In addition, it has been necessary to exclude information apropos recent research progress because of space restrictions. Also because readers will have different scientific requirements and most may not be involved in radiometric dating concerned with changes in the radioactivities of samples. Anyone interested in acquiring more detailed knowledge can consult the author’s “Isotopes in the Earth Sciences” (Chapman & Hall, London, 1994) or a second edition of this now prepared and published in 1997 called “Radioactive and Stable Isotope Geology” which has a co-author, H.-G. Attendorn, and published by Chapman & Hall. Nevertheless this chapter offers a useful and compact synopsis of radioactive dating methods for non-specialist professionals and moreover for students of the earth sciences too.