Robert N. Clayton
The University of Chicago
Election Year: 1996
Primary Section: 16, Geophysics
Secondary Section: 14, Chemistry
Membership Type: Member
Many chemical elements show variations in the abundances of their stable isotopes in extraterrestrial materials (e.g., meteorites and lunar samples), which are much greater than variations in terrestrial materials. In some instances, these variations are due to nucleosynthesis, both in stars and in interstellar space. In other cases, by processes of evaporation and condensation in the solar nebula from which the sun and planets formed. For other elements, isotopic variability results from interactions with energetic particles from the sun (solar flares and solar wind) or from the galactic cosmic rays. We use a variety of mass spectrometric techniques to measure isotopic abundances of many elements: H, C, N, O, Mg, Si, K, Ca, Ti, Cr, Fe, Ni, Sr, Zr, Mo, and others. A method currently under development in collaboration with scientists at the Argonne National Laboratory is Resonance Ionization Mass Spectrometry. This ultra-sensitive technique is being used to measure nucleosynthetic effects in minor elements present in individual interstellar grains (stardust) preserved in primitive meteorites. Isotope abundance variations are also powerful tools for investigation of terrestrial natural processes. For example, the geochemical cycling of carbon and nitrogen through the Earth's crust and mantle can be traced by means of their isotopic compositions. Similarly, the movements of aqueous fluids in the Earth's interior are studied through the isotopic ratios of hydrogen and oxygen.