Introduction

The isotopic abundances of cosmic materials may change for a number of reasons. If a substance contains radioactive nuclei, there will be a continual decrease in the parent and a buildup of the daughter isotopes. Bombardment of materials by cosmic rays or other high-energy particles can also alter the isotopic complement of a sample. During radioactive decays or nuclear fission, particles are emitted which can affect the surrounding nuclei. Fission fragments remain in the neighborhood of the parent nuclei. A third possibility is fractionation, by either diffusion or small mass-dependent effects in chemical reactions. All three of these contingencies have been mentioned or intimated previously. We shall now take up certain aspects of these processes in detail.

It will not be possible for us to discuss most of the dating techniques. The interested reader may consult the textbooks of Faure (1986) or Durrance (1986). Richardson and McSween (1989) have an excellent chapter on radioactive dating.

Rubidium–Strontium Dating; Sample and Model Ages

One of the most straightforward methods of age determination makes use of the decay of 87Rb to 87Sr. We shall discuss this particular method here in detail, because of its pedagogical advantages. We shall have time to mention only briefly other methods, some of which are now more actively pursued than rubidium–strontium.

Both rubidium and strontium are geochemically dispersed, that is they occur primarily as impurities in major minerals.