Because these elements (and their isotopes) all have different half-lives, elements with longer half-lives will be present in greater quantities than those with shorter half-lives.
Therefore, when equilibrium is disrupted (for example when a mineral breaks down leading to gain or loss of uranium and/or thorium), we can calculate the degree to which equilibrium has been restored between parent and daughter isotopes.
By Aimee Komugabe Uranium series (U-series) dating is based on the uranium and thorium radioactive decay chains.
These decay chains involve a series of different elements and may be as long as 35 steps, before reaching the stable end product – lead.
Thus, the amount of ionium in sediments can be used as a rough measure of the age of sediment.
Accurate dating by measurement of ionium alone requires that the rate of sedimentation of ionium be constant with time, an assumption that does not hold for many sediments; any thorium-232 present in seawater will also precipitate, and the decay of the ratio of ionium to thorium-232 can be used as a measure of time.
This age is in good agreement with the age of the meteorites and the age of the Moon as determined independently.Finally, there must not be a process that causes the thorium to shift its position within the sediment.If these assumptions are correct, this dating technique can produce accurate results.Primordial lead is thought to have been formed by stellar nuclear reactions, released to space by supernovae explosions, and incorporated within the dust cloud that constituted the primordial solar system; the troilite (iron sulfide) phase of iron meteorites contains lead that approximates the primordial composition.The lead incorporated within the of uranium and thorium isotopes.Likewise, both thorium-230 and thorium-232 are assumed to precipitate out in a constant ratio; no chemical process favors one form over the other.