...species with the same atomic number but different mass numbers), decay with time.These include elements with an atomic number greater than 83—of which the most important are uranium-235, uranium-238, and thorium-232—and a few with a lower atomic number, such as potassium-40.and most refined of the radiometric dating schemes.
Therefore, one can assume that the entire lead content of the zircon is radiogenic, i.e.
it is produced solely by a process of radioactive decay after the formation of the mineral.
To understand this, one needs to know that though uranium-238 ( ..a special type of dating method that makes use of a microscope rather than a mass spectrometer and capitalizes on damaged zones, or tracks, created in crystals during the spontaneous fission of uranium-238.
In this unique type of radioactive decay, the nucleus of a single parent uranium atom splits into two fragments of similar mass with such force that a trail of crystal damage is left in... Petrzhak in uranium-238, is observable in many nuclear species of mass number 230 or more.
method of age determination that depends on the production of helium during the decay of the radioactive isotopes uranium-235, uranium-238, and thorium-232.