Natural uranium and thorium series disequilibrium: new approaches to geochemical problems

Many geological processes at the earth's surface, such as weathering, erosion, and sedimentation, involve chemical and physical reactions which result in disequilibrium in the natural radioactive decay series, 238U, 235U, and 232Th. If the average lifetime of a given daughter nuclide is of the same order of magnitude as the duration of a geochemical system, then the changes in radioactivity of the disequilibrated nuclide can be used as a chronometer for the system. A daughter nuclide with a greater lifetime cannot be used to date such a system, but its abundance relative to other isotopes may serve as a 'fingerprint' of origin, inasmuch as disequilibration processes can cause very large variations in isotopic ratios. A nuclide with a very short lifetime will not be useful in studying geologic processes; it will be quickly re-equilibrated with a longer lived parent. However, nuclides with lifetimes of a few days, such as radon, can be used to study geologically rapid processes such as soil gas diffusion. (Auth.)