Radiometric dating wikipedia the encyclopedia updating ms access from a form
it is produced solely by a process of radioactive decay after the formation of the mineral.
During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay.
This damage is most concentrated around the parent isotope (U and Th), expelling the daughter isotope (Pb) from its original position in the zircon lattice.
The mineral incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead.
Therefore, one can assume that the entire lead content of the zircon is radiogenic, i.e.
Zircon is very chemically inert and resistant to mechanical weathering—a mixed blessing for geochronologists, as zones or even whole crystals can survive melting of their parent rock with their original uranium-lead age intact.