Obtaining accurate information from these decay systems for the purposes of determining the age of a mineral or rock requires: (1) the decay constant of the parent nuclide is accurately and precisely determined; (2) closed system behavior, which can be simply stated to mean that the Parent/daughter ratio has only changed by radioactive decay; and (3) the initial daughter nuclide, if present, can be precisely and accurately accounted for.
In this section we outline the basic principles of the various radio-isotopic geochronometers, differentiating the U-Pb system applied to U-bearing accessory minerals from the isochron geochronometers (Re-Os, Lu-Hf, Pb-Pb etc.) applied to chemical precipitates and organic residues.
Zircon is ideal for U-Pb dating because U has a similar charge and ionic radius to Zr it substitutes readily into the zircon crystal structure (in modest amounts, typically in the 10’s to hundreds of parts per million (ppm) range) whereas Pb has a different charge and larger ionic radius leading to its effective exclusion from the crystal lattice.
Therefore at the time of crystallization (t0) there is effectively no Pb present in a crystal (although mineral and fluid inclusions may contain Pb) and the present day Pb is the direct product of in-situ U decay since t0 (see section 3.1 for further details).
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.
Zircon (Zr Si O) is a common accessory mineral in silicic volcanic rocks ranging from lavas to air-fall tuffs to volcaniclastic sedimentary rocks and is a nearly ubiquitous component of most clastic sedimentary rocks.