Isotopic composition of elements in extra-terrestrial materials 1

The present review focuses on the isotropic composition in extra-terrestrial materials. There are many different factors in the variation in isotopic abundance between terrestrial and extra-terrestrial materials. Major factors in unusual isotopic composition are roughly grouped into three categories: factors associated with fractionation of isotopes, nuclear reaction or radioactive disintegration. Fractionation takes place due to differences in mass among various isotopes. There are physical and chemical factors. Physical ones include vaporization and condensation which meteorites may experience during their formation while the chemical ones include chemical reactions and chemical equilibration. There phenomena are mentioned as factors in the variation in isotopic ratio which is associated with nuclear reactions. An important nuclear reaction is the formation of elements that has been occurring continuously in stars since the big bang. The other two phenomena are nuclear spallation caused by high-energy cosmic rays, and capture of low-energy neutrons. The isotopic ratio may be different between terrestrial and extra-terrestrial materials as a result of various nuclides having been taken into meteorites or the earth in different ways or at different times during their formation. (Nogami, K.)