These short-lived radioactive nuclides are intermediate decay products along a radioactive decay chain.
- both parent and daughter isotopes undergo radioactive decay
- secular equilibrium (parent generation balances daughter decay) exists if the parent/daughter ratio remains stable
- abundances of intermediate daughters is proportional to their half-lives
Secular disequilibrium arises if either the parent or daughter isotope is isolated and is necessary for these systems to be used for dating. Provided there are processes that cause fractionation between parent and daughter, these radioactive pairs can be useful for dating processes occurring on time scales from 1 Ma to days.
a. Daughter-deficiency methods– excess of parent (relative to daughter) isotope
The daughter-deficiency method can be used to date carbonates such as corals (for dating Pleistocene sea-level fluctuation studies) and speleotherms (stalagtites, stalagmites) in caves (climate change studies).
b. Daughter-excess methods– excess of daughter (unsupported relative to parent) isotope
The daughter-excess method uses the same system, U234-Th230. The concentration of 230Th in a sediment with depth (concentration profile) may be used to obtain an estimate of deposition rate.