Radioactive decay and radiometric dating
Since the exact rate at which uranium decays into lead is known, the current ratio of lead to uranium in a sample of the mineral can be used to reliably determine its age.
The method relies on two separate decay chains, the uranium series from Pb) leads to multiple dating techniques within the overall U–Pb system.
It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range. This mineral incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead when forming.
As a result, newly-formed zircon deposits will contain no lead, meaning that any lead found in the mineral is radiogenic.
Zircon has a high hardness (7.5) which makes it resistant to mechanical weathering, and it is also very resistant to chemical weathering. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced.
Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: Principles of Radiometric Dating Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential (Energy) barrier which bonds them to the nucleus.
We can also construct a Concordia diagram, which shows the values of Pb isotopes that would give concordant dates.
The Concordia curve can be calculated by defining the following: ).
For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar.
Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years.