Major Causes of Age Distortion in Uranium-Lead Isotopic Radiogeochronology

Abstract

The causes of isotopic age distortion that may occur during the dating of endogenous geological processes (rocks) by the uranium-lead isotopic method are considered. Three groups of reasons are distinguished: mineralogical, geochemical and analytical. The main mineralogical reason for the distortion of the U-Pb isotopic age is the multistage crystallization of geochronometer minerals, which is manifested, for example, in zircon, in the anatomy of their crystals. It was concluded that in order to obtain reliable information about the time course of geological processes for complex crystals (primarily zircon), local uranium-lead isotope dating methods ("SHRIMP", LA-ICP-MS, etc.) should be used. The geochemical reasons include the discrepancy between the isotopic composition of impurity lead and the isotopic composition of corrective lead (abnormal isotopic composition of ordinary lead) and the polystage history of the development of the uranium-lead isotope system. It is noted that the most probable reason for the violation of the uranium-lead isotope system by zircons in the hypergenesis zone is the entrapment of uranium by defects in the crystal structure and cracks, and the predominant loss of uranium in monazite. At the same time, the loss of uranium by monazites depends on the composition of the acids. It is indicated that washing monazites in a weak solution of nitric acid leads to the appearance of a significant reverse discordance, while no loss of lead is observed. The same operation in a weak solution of hydrochloric acid leads to the preferential leaching of ordinary lead. For analytical reasons, the lowest accuracy of determining the prevalence of the 204Pb isotope (204Pb/206Pb ratio) is indicated. The impact of contamination of samples dated (method TIMS) by lead and uranium from reagents is considered. It is clear that the contamination of multi-grain samples (1-2 mlg) of minerals with uranium and lead from reagents with a modern isotopic composition, in a blank test of lead 10–9 g (the ratio of the mass of Pb of the sample to the mass of Pb from the reagents of 40 to 1) is not significant affects dating results (isotopic ratios of 207Pb/206Pb, 207Pb/235U and 206Pb/238U). A blank sample of uranium is usually 2 orders of magnitude smaller (10–11-10–12 g). A strong inverse relationship between the degree (proportion) of radiogenic lead contamination of radiogenic lead aliquots on the isotopic composition of lead and the calculated values of the lead content in the sample was revealed. When an aliquot for determining the content of uranium and lead is contaminated with ordinary lead from the reagents, the smallest distortion of the calculated value of the lead content occurs when the ratio of sample lead to tracer lead is 1:1, while a slightly smaller relative distortion of the lead content is noted with increasing age of the radiogenic lead of the samples.