Radiogenic isotope dating
Although it can accommodate several 100’s of ppm U in the crystal lattice, it also incorporates variable amounts of Pb.Often this results in several analyses of the same unzoned titanite grain that define a discordia (on a Tera-Wasserburg concordia diagram) anchored with a Pb ratio of common-Pb on one end and the closure age.Silicic air-fall tuffs are the most common volcanic rocks in fossil-bearing sequences and are found in layers that range in thickness from a millimeter to many meters and are commonly preserved in marine settings.In most of these rocks the primary volcanic ash has been altered, probably soon after deposition, to clay minerals in a process that does not affect zircon.
The rate at which radioactive elements decay is governed by the exponential decay constant.
For example, common-Pb (or the natural occurrence of non-radiogenic lead) includes radiogenic isotopes (206, 207, 208) as well as non-radiogenic isotopes (204).
If the ratio of these is constant or measurable, then we can separate the radiogenic from the non to extract meaningful age information.
The amount of time required for half of a given quantity of a parent radioactive element to decay into the daughter product is referred to as the half-life.
It is important however, that the half-life of an element is defined in terms of probability and is not the time required for exactly 50% of a given quantity to decay (e.g.
Radioisotope geochronology in its present form is made possible by radioactive decay.