Uranium decay dating
Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density.
Other minerals sometimes used for uranium-lead dating include monazite, titanite and two other zirconium minerals, baddeleyite and zirconolite.
Uranium-Lead dating is a radiometric dating method that uses the decay chain of uranium and lead to find the age of a rock.
As uranium decays radioactively, it becomes different chemical elements until it stops at lead.
With 238U the Pb/U ratio grows much more slowly with age, but the idea is the same.
If you took rocks of all ages and plotted their two Pb/U ratios from their two isotope pairs against each other on a graph, the points would form a beautiful line called a concordia (see the example in the right column). First, its chemical structure likes uranium and hates lead.
Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge.
In these cases, the concordia diagram is a valuable tool.
The straight line takes the zircons off the concordia. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched.The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay).In a 704-million-year-old rock, 235U is at its half-life and there will be an equal number of 235U and 207Pb atoms (the Pb/U ratio is 1).In a rock twice as old there will be one 235U atom left for every three 207Pb atoms (Pb/U = 3), and so forth.