How Old Is It? How Do We Know? A Review of Dating Methods— Part Two: Radiometric Dating: Mineral, Isochron and Concordia Methods

W ithin a decade of the discovery of radioactivity in 1896, the chemical elements uranium, radium, polonium, and thorium were recognized as radioactive; the basic theory, mechanisms, and mathematics of radioactive decay were established; decay rates of some radioactive chemical elements were determined; and the ages of two mineral specimens were calculated for the first time from their uranium and helium contents.1 Soon thereafter, isotopes were discovered, the decay constants of a few specific isotopes were determined, and mass spectroscopy was developed.2 Ever since these early heady days in the study of radioactivity, numerous radiometric dating methods have been proposed. Some methods were eventually discarded,3 but many others have not only withstood critical scrutiny but have been refined and improved.4 Radiometric Dating To understand how radiometric dating works, consider a crystal of pure rubidium chloride (RbCl). Rubidium (Rb) consists of 72.2% Rb, an isotope that is not radioactive, and 27.8% Rb, an isotope that is radioactive. A cubic crystal of RbCl with a volume of 1 cm contains approximately 10 atoms of Rb. Although we cannot predict exactly when any specific atom of Rb will undergo spontaneous radioactive decay, we know that extremely large quantities of any radioactive isotope will disintegrate in accord with a radioactive decay law that is expressed by the equations: