U and Th are found on the extremely heavy end of the Periodic Table of Elements. Furthermore, the half life of the parent isotope is much longer than any of the intermediary daughter isotopes, thus fulfilling the requirements for secular equilibrium Section 2. We can therefore assume that the Pb is directly formed by the U, the Pb from the U and the Pb from the Th. The U-Th-Pb method Section 5. The ingrowth equations for the three radiogenic Pb isotopes are given by:
5 The U-Pb system
However, it is important to remember that all radiometric dating methods are based on three main assumptions: The highly speculative nature of all radiometric dating methods becomes apparent when one realizes that none of the above assumptions is either valid or provable. Put simply, none of these assumptions can have been observed to have always been true throughout the supposed millions of years the radioactive elements have presumed to have been decaying. Of the various radiometric methods, uranium-thorium- lead U-Th-Pb was the first used and it is still widely employed today, particularly when zircons are present in the rocks to be dated.
Mark Harrison, Elizabeth J. Reviews in Mineralogy and Geochemistry ; 48 1: The dominant occurrence of phosphate minerals in crystalline rocks is as accessory phases, most notably apatite, monazite, and xenotime.
Uranium—lead dating , abbreviated U—Pb dating , is one of the oldest  and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4. The method is usually applied to zircon. This mineral incorporates uranium and thorium atoms into its crystal structure , but strongly rejects lead when forming.