Meteoritic ¹³⁸Ce/¹⁴²Ce ratio and its evolution

Abstract

The nuclide ¹³⁸La decays to ¹³⁸Ce and ¹³⁸Ba by β⁻ and electron capture, respectively, with a total half life of about 1×10⁵ Myr. Rankama¹ suggested the possibility of age determination using this radioactive decay system. Recently, Tanaka and Masuda succeeded in age determination by using the ¹³⁸La–¹³⁸Ce geochronometer for a gabbro from the Bushveld complex, South Africa. This La–Ce isotopic system, coupled with the Sm–Nd system, provides a new isotope tracer^2,3 for the geo- and cosmochemical samples, especially for samples having a Ce anomaly in the rare-earth element patterns. Meteoritic primordial composition of cerium isotopes can serve as a basis for La–Ce systematics. Furthermore, Ce isotopic compositions for meteorites will provide information concerning half-life data of the ¹³⁸La decay system, which remains uncertain. Here, cerium isotope ratios were determined for Juvinas, Pasamonte and ALH-78132 eucrites and the Jilin (H4) chondrite. Neodymium isotope ratios and abundances of lanthanum, cerium, neodymium and samarium were also determined for these meteorites.