Uranium series dating of quaternary deposits dating a man going through a bitter divorce
Regardless of how the disequilibrium formed, over time the U-series isotopes will tend to return to a state of secular equilibrium so long as the material remains in a state that does not allow chemical exchanges in or out of it, remaining a “closed system”.
The time frame of this process, which is exploited to determine a U-series disequilibrium date, is governed by the shorter of the two half-lifes in any U-series isotope “parent-daughter” pair.
A modification of the internal isochron approach uses Ra-Th dating of minerals in historical eruptions to deduce the timescale over which the minerals themselves grew (by comparing their ages to the known eruption age).
Another volcanic rock dating method using these isotopes looks at variations in daughter-parent isotope ratios between the whole-rock compositions of volcanic units of different ages at one volcano, and through a series of assumptions deduces the relative time between eruptions.
Ra dating of igneous materials takes several different forms and usually require a chemical normalization using a stable or long-lived isotope of the daughter isotope, much like is in used other radiometric systems like Rb-Sr and U-Pb. This method is often called an “internal isochron” because the age of one rock is determined from the variations between minerals within it.
While such ages are often used to infer when an eruption occurred, the event that is actually being dated is not the eruption per se, but the formation of minerals in a cooling magma, which happened sometime earlier.
The largest radioactive disequilibria are always found in the youngest materials.
Over time, this signature goes away, eventually relaxing to a condition wherein the disequilibria are no longer detectable.
In practice, we can usually detect U-series disequilibria for 5 to 7 half-lifes.
Th dating of biogenic and abiogenic carbonate materials is a widely used application in paleoclimatology and paleooceanography.
For instance, U-series dating of coral skeletons that grew in a specific environment and depth range relative to sea level can be used to reconstruct the history of sea level changes.
The half-lifes of Ra are roughly 250, 75 and 1.6 thousand years, so that these isotopes are useful for looking at events that happened in the past thousand to million years.
This is a very important time period of Earth history (the Pleistocene and Holocene) and a time period that very few other geochronometers can address.