Over time, this signature goes away, eventually relaxing to a condition wherein the disequilibria are no longer detectable.The Houtman Abrolhos Islands, situated at the western passive margin of the Australian continent, consist of a series of shelf-edge coral reefs.
Uranium/Thorium dating of ferricretes from mid- to late Pleistocene glacial sediments, western Tasmania, Australia.
The differing chemistries and half-lives of these nuclides (with timescales ranging from seconds to billions of years) make them exceptionally useful chronometers for variety of natural processes and materials Perhaps the most important and commonly used isotopes are Ra, the first three of which are commonly used to date the formation of carbonate minerals and skeletal materials (e.g., corals and cave deposits) and the full suite of which are used to date volcanic materials, such as lavas and the crystals they contain.
Because these isotopes are related to each other by a radioactive decay chain, a material left chemically untouched for a long period of time (a “closed system”) exhibits a special condition called secular equilibrium, wherein the activity of each isotope in the chain is the same (activity is defined as the numbers of decays per unit time, which is equal to the number of atoms of that element times its decay constant).
Materials that can be dated using U-series techniques are those that either (A) form by a process that causes disequilibrium, which results when the isotopes in the decay chain become separated (“fractionated”) in some way, or that (B) form by a process that records an existing disequilibrium in the material they form from.
For instance, when crystals form in a magma, Th, U and Ra in the magma enter the different materials in different proportions, producing radioactive disequilibrium.