Uranium series dating peerblock not updating
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.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).
When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.Secular equilibrium can only occur in a radioactive decay chain if the half-life of the daughter radioisotope is much shorter than the half-life of the parent radioisotope, as typical of the uranium series decay chains.Uranium series disequilibrium: Unequal radioactivity of the intermediate radioisotopes (e.g., Ra) in the U-series decay chains resulting from changes in the respective elemental ratios (or called elemental fractionation) during a geological or environmental event or process.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.
So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero.
In these cases, the concordia diagram is a valuable tool.