ウラン系列年代測定法  [in Japanese] Uranium-series method of dating  [in Japanese]

The radioactive decay series arising from ^<238>U, namely uranium (or 4n+2) series, contain seventeen intermediate radioactive daughters. Such daughters, isotopes of elements whose atomic numbers range from 81(TI) to 92(U), are isolated from their parent and from each other in the course of various geologic processes. That is, such geologic processes cut the decay chains because of differences in their chemical properties. As a result, some types of radioactived is equilibrium state between uranium isotopes and their radioactive daughters are observed in naturally occurring materials like sea-bottom sediments, manganese nodules, biogenic carbonate minerals, phosphate rocks, silicate minerals in igneous rocks, and so on. The "uranium-series method" is a general term for all methods of dating by utilizing such disequilibrium systems. In this paper, the dating methods, based on the disequilibrium systems between uranium isotopes (^<238>U of ^<234>U) and ionium (^<230>Th) are fully explained concerning their principles, analytical procedures for uranium and thorium isotopes, and the way in which evaluates their reliability. Ionium is a radioactive isotope of thorium and is produced by decay of the immediate parent ^<234>U. It decays to ^<226>Ra by emission of alpha particles with a half-life of 75,200 years. The ionium dating method, using decay of growth of ionium as a geochronometer, is fundamentally divided into three independent method as follows: 1. The excess ^<230>Th (or ^<230>Th/^<232>Th) method, capable of estimating the accumulation rate of marine sediments and/of the accretion rate of manganese nodules. 2. The ^<230>Th-^<238>U isochron method, for dating igneous rocks. 3. The ^<230>Th/^<234>U (or ^<230>Th/^<238>U) method, appropriate to determine the time of death of carbonate-secreting organisms. These methods are valid for determining the age of late Pleistocene sample in the range from approximately 10,000 years B.P. to 300,000 years B.P., filling the gap between the ^<14>C and the U-Pb, K-Ar, and Rb-Sr methods of dating.