The lead incorporated within the of uranium and thorium isotopes.

Thus, the lead isotopic composition of any mineral or rock depends upon its age and the environment from which it was formed; that is, it would depend upon the ratio of uranium plus thorium to lead in the parent material.

Uranium 235 decays to lead 207, and Thorium 232 decays to lead 208.

In addition there is another stable isotope, lead 204, that is entirely primordial and does not form via radioactive decay at all.

, method of establishing the time of origin of a rock by means of the amount of common lead it contains; common lead is any lead from a rock or mineral that contains a large amount of lead and a small amount of the radioactive progenitors of lead— isotope thorium-232.

The important characteristic of common lead is that it contains no significant proportion of radiogenic lead accumulated since the time that the mineral or rock phase was formed.

, method of establishing the time of origin of marine sediments according to the amount of ionium and thorium they contain.

Because uranium compounds are soluble in seawater, while thorium compounds are quite insoluble, the thorium isotopes produced by the decay of uranium in seawater are readily precipitated and incorporated in sediments.

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 basic principle of uranium – thorium series dating in corals is that uranium in oxygenated environments is about ten thousand times more soluble in seawater than thorium is.

As corals grow, they incorporate significant seawater uranium and virtually no thorium into their skeletons.

A closed system is one in which there is no free exchange of elements in a material with the outside environment.

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.