Isotope abundances of rare-earth elements in meteorites: 1. Implications of samarium, europium, and gadolinium to the early history of the solar system

The isotope abundances of samarium, europium, and gadolinium have been measured in two normal chondrites, an achondrite, a carbonaceous chondrite, and a volcanic basalt originating from a depth of 40 km in the Kilauea-Iki vent in Hawaii. The meteoritic isotope abundances are similar to the terrestrial abundances within experimental errors, and ‘general’ or ‘special’ anomalies for these three elements in meteorites do not seem to exist. There is no depletion of the isotopes with large thermal-neutron capture cross section (Sm149, Eu151, and Gd157) relative to the earth. With reference to a recent theory which attributes the terrestrial and meteoritic abundances of D, Li, Be, and B to spallation and thermal-neutron irradiation, our data can be discussed as follows: (1) the neutrons produced in spallation processes were not thermalized, so that the rare-earth isotope abundances were unaffected; (2) if the neutrons were thermalized, the ratio of irradiated material to nonirradiated material is exactly the same for the earth and meteorites; (3) there was no significant thermal-neutron irradiation. These three alternatives are discussed with reference to some recent studies of meteorites.