ランタン-セリウム法の地球化学

抄録

Applications of lanthanum-cerium (La-Ce) isotope systematics in the geochemistry are reviewed in this article. Rare earth elements (REE) behave coherently in the geological phenomena and are strongly partitioned into the liquid phase relative to mafic minerals through the magmatic processes. The incompatibility of REE regularly increases from heavy to light REE in the igneous processes. Therefore, REE have been used extensively along with other incompatible elements to understand igneous petrogenesis. Based on the above characteristics of REE, time-integrated REE fractionation in the mantle and crust have been broadly discussed using Sm-Nd isotope systematics. In 1982, Tanaka and Masuda first observed the variation of ¹³⁸Ce/¹⁴²Ce ratio caused by β^--decay of ¹³⁸La in the terrestrial materials and established a La-Ce isotope systematics as a geochemical tracer and a geochronometer. The time-integrated light REE (LREE) fractionation had been interpreted by the extrapolation from the results of Sm-Nd systematics. The more detailed observation of LREE evolution has been enabled by the introduction of La-Ce systematics. However, the data of Ce isotope analyses have been limited because only a few scientists have been involved in this study. Therefore, further Ce isotope analyses of the terrestrial and meteoritic materials with Nd isotope analyses will provide more detailed information on the LREE evolution in the Earth and the solar system.