Pb pb dating
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).But now imagine that some geologic event disturbs things to make the lead escape.That would take the zircons on a straight line back to zero on the concordia diagram.If a constant supported (radiocarbon) is a common way to obtain age estimates for organic material, ranging back up to 50,000 years of age.The radiocarbon community has built sophisticated chronology models, which rely on equally sophisticated age-depth functions, with the objective of properly quantifying the uncertainty of the resulting chronology.
The results from these zircons therefore plot along that straight line, establishing what is called a discordia. If a 1500-million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the concordia, always pointing to the age of the disturbance.
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).
Our model can extend beyond the current CRS approach, deal with asymmetric errors and mix is the element’s radioactive decay constant) to infer the ages, it results in a logarithmic approximation to a series of dates.
This logarithmic approximation heavily restricts the age-depth model.
The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years.