Pb leakage is the most likely cause of discordant dates, since Pb will be occupying a site in the crystal that has suffered radiation damage as a result of U decay.
A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods.U leakage would cause discordant points to plot above the cocordia. Lunar rocks also lie on the Geochron, at least suggesting that the moon formed at the same time as meteorites. Pb separated from continents and thus from average crust also plots on the Geochron, and thus suggests that the Earth formed at the same time as the meteorites and moon.But, again, exptrapolation of the discordia back to the two points where it intersects the Concordia, would give two ages - t* representing the possible metamorphic event and t and solve for t . This argument tells when the elements were formed that make up the Earth, but does not really give us the age of the Earth. Thus, our best estimate of the age of the Earth is 4.55 billion years.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: ).