What is uranium series dating

03 Jan

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.

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.

This method is particularly useful for dating submarine lavas, where traditional radiocarbon methods (dating of eruption–related charcoal) is not possible.

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.

Corals forming from seawater record the steady state Th disequilibrium of the water, which is recorded in the coral when it forms.

How long this takes depends on the precision and accuracy of our measurements and the size of the original disequilibria (bigger disequilibria last longer).

In practice, we can usually detect U-series disequilibria for 5 to 7 half-lifes.