Age Discordantity in Uranium-Lead Radiogeohronology

In uranium-lead radiogeochronology, discordance is a difference in the numerical values of the age calculated by the isotopic ratios: 206Pb/238U, 207Pb/235U and 207Pb/206Pb. Discordance is calculated by the formula: D (%) = 100 * [t (207/206) – t (206/238)] / t (207/206). Discordance is usually associated with a violation of the closure of the isotopic system. There are two types of discordance. Direct discordance (D > 0 %) is due to losses of lead or capture of uranium. There may be one more case of "apparent" direct discordance, when two (or more) uranium-lead isotopic systems are mixed, even when they are not disturbed. The reverse discordance (D < 0 %) is due to the loss of uranium or the capture of radiogenic lead. In this communication, we consider the possibility of the appearance of an "apparent" not real reverse discordance due to anomalous (depleted isotope 235U) isotopic composition of uranium, provided that the isotopic system was not disturbed. The assumption of the possibility of entering the isotopic system of uranium with anomalous isotopic composition is based on the phenomenon of a natural nuclear reactor of the Oklo type. The analysis of entry into the isotopic system of uranium with anomalous isotopic composition shows that the age values calculated from the ratios of 206Pb/238U, 207Pb/235U do not depend on the isotopic composition of uranium, while the isotopic ratio of 207Pb/206Pb and the age calculated from it have inverse dependence on the 238U/235U ratio. In practice, lead-uranium isotopic ratios are calculated based on the content of total lead, uranium and the isotopic composition of lead. At the same time, it is assumed in the calculations that the ratio 238U/235U = 137.88. The latter leads to underestimation of the 238U isotope content and overestimation of 235U, in the TIMS method. Something else is observed during SIMS dating. In the case when the content of uranium is determined by the intensity of the isotope 238U, the ratio 206Pb/238U will reflect the real age, and the ratio 207Pb/235U will be rejuvenated. If the uranium content is determined by the intensity of the 235U isotope, the 207Pb/235U ratio will reflect the real age, and the 206Pb/238U ratio will be overestimated. To overcome this invalid "visible" discordance, and at the same time to detect manifestations of natural nuclear reactors in the early history of the Earth, it is possible with one more additional measurement in the course of uranium-lead analytical studies, by simultaneously determining the isotopic composition of uranium (in the aliquot in which the isotopic composition of lead is determined) in using the TIMS method and measuring the content of both uranium isotopes (238U and 235U) in the methods SIMS.