Loss of daughter lead nuclides during the radioactive decay of uranium to lead, in which 238U decays to 206Pb, and 235U decays to 207Pb (see decay series). These lead isotopes, together with that produced by the 232Th to 208Pb series, are moderated by the presence of non-radiogenic 204Pb in any calculations of age. After correcting for the original lead in any analysis, if the mineral being investigated has remained a closed system, the 235U:207Pb and the 238U:206Pb ages should be concordant. On a graph of 235U:207Pb against 238U:206Pb the loci of all concordant ages define a curve called concordia (see concordia diagram). If they do not agree, they are discordant and the ratios do not fall on the concordia curve. Because daughter atoms tend to escape from the system, especially when heated or otherwise disturbed, discordant ages are generally on the young side. Because 207Pb and 206Pb are chemically identical, they are not fractionated by natural processes. As a result of this any lead loss from a mineral is in the same isotopic proportion as that in which it occurs in the mineral. The lead loss would be the same for all parts of the rock body, and the plots of 238U:206Pb against 235U:207Pb fall on a straight line below the concordia curve. The two points where this straight line intersects the concordia curve give the age of the rock (higher value) and the time of the lead loss (lower value). In some systems there is not a single time of lead loss but a more continuous process of diffusion. The lower of the two intersects described above may then be further depressed by this continuous diffusion and the otherwise straight-line relationship may be lost especially at the lower value end.
Subjects: Earth Sciences and Geography.