Scientists have many ways to discover how old rocks and fossils are.
The ratio of lead versus uranium in the zircon is what is used to determine the age of the rock. As you know, radioisotopes do not decay directly into a stable state; rather they go through stages of radioactive decay until reaching a stable isotope.
Dislocations that move through the lattice accumulate U and other trace elements.
Pipe diffusion along dislocation arrays connected to a chemical or structural sink results in continuous removal of selected elements (for example, Pb), even after deformation has ceased.
and most refined of the radiometric dating schemes.
It can be used to date rocks that formed from about 1 million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range.
The assumption has been that the parent (U) and daughter (lead, Pb) remain locked in the tight crystal lattices of zircon, so that mineralogists can accurately measure ratios of the elements resulting from radioactive decay.
Comparison of the age populations of the lower allochthon with those of the rest of the allochthonous and autochthonous units of NW Iberia suggests that the terranes to the footwall of the Variscan suture should not be considered as exotic elements, but contiguous pieces of the same continental margin transported onto the adjacent Gondwana mainland in Variscan times.First, the impact: Our findings have important implications for the use of zircon as a geochronometer, and highlight the importance of deformation on trace element redistribution in minerals and engineering materials.Now, the reasons for the concern: Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes. This means that any lead found in zircon minerals was made by radioactive decay, after the formation of the mineral. Zircon also concentrates Uranium (You) and (although less so) Thorium (Th) and excludes lead (Pb), which means it has a very high You/Pb ratio.