Solar System Time Scales from Long-Lived Radioisotopes in Meteorites and Planetary Materials

Long-lived radioactive isotopes, defined here as those that have half-lives in excess of a few hundred million years, have been utilized for chronology since the early part of the twentieth century. The past several decades have seen significant advances in the precision and accuracy of chronological information based on the decay of long-lived radioisotopes, particularly in the mass spectrometric techniques for measurement of isotope ratios and in the determination of relevant decay constants. Chronometers based on the decay of radioisotopes essentially date the time of isotopic closure following a chemical event that fractionated the parent element from the daughter element. Assuming that parent/daughter isotope ratios can be determined accurately and precisely and that the decay constant is known, meaningful age information based on such chronometers may only be obtained if (1) there was complete equilibration of the isotopic composition of the daughter element prior to fractionation of the parent element from the daughter element and (2) there has been no disturbance of isotope systematics following the isotopic closure event that is to be dated.