In Situ-Produced Cosmogenic Isotopes in Terrestrial Rocks

Abstract

In the late forties, Libby and his collaborators detected the naturally occurring radioactive isotope 14C, produced by cosmic radiation in the Earth's atmosphere (Libby 1946, Anderson et al 1 947). This marked the onset of the search for cosmic-ray-produced (cosmogenic) isotopic changes in terrestrial and extraterrestrial samples and in the cosmic rays themselves. During the four decades since Libby's discovery, some four dozen cosmo­ genic stable and radioactive isotopes in extraterrestrial samples (Reedy et al 1983) and two dozen in terrestrial samples (Lal & Peters 1 967) have been discovered. These isotopes have found applications in cosmic-ray physics (in the study of acceleration and propagation of galactic and solar cosmic rays), in solar system astrophysics (in determining the evolutionary history of meteorites, the lunar surface, and interplanetary dust particles), and in the Earth sciences (in archaeology, meteorology, glaciology and oceanography). For reviews on these topics, reference is made to Simpson (1983); Reedy et al (1983); and Lal & Peters (1967), Oeschger et al (1970), and Faure ( 1986), respectively. The process of discovery continues, pro­ pelled by advances in the techniques used to measure these isotopes. Even now, examples of new isotopic effects are being discovered, and new applications are being made of the various isotopic effects. When one looks at the history of the development of the terrestrial cosmogenic field, one observes that it grew in waves: After an idea is implemented, the field