セリウム同位体比測定と年代学への応用における諸問題

抄録

Tanaka and Masuda¹⁾ first applied lanthanum-cerium isotope systematics to geochronology in 1982, using β-decay of ¹³⁸La. In this systematics, it is necessary to measure cerium isotope ratio (ex. ¹³⁸Ce/¹⁴²Ce) with precision better than 0.01% because of long half life and low abundance of ¹³⁸La. Many studies have been done to improve the precision and accuracy of the Ce isotope analysis. But there are some problems in the analysis, such as long data acquisition time and requirement of large amount of cerium. When the La-Ce systematics is used as a geochemical tracer or in geochronology, β-decay constant of ¹³⁸La is a very important factor. However, the decay constant is still in controversy so that age dating results using the La-Ce isotope systematics are less convinced compared with those of Sm-Nd and Rb-Sr systematics.
In this paper, precise measurements of cerium isotope ratios and geochronological applications are reviewed and their problems are pointed out.