An advanced stepwise leaching technique for derivation of initial lead isotope ratios in ancient mafic rocks: A case study of Mesoproterozoic intrusions from the Udzha paleo-rift, Siberian Craton

Abstract

Lead isotope systematics is a powerful tool for the interpretation of magma sources. However, a significant problem in the study of Pb and U is their high mobility during weathering and secondary alteration of igneous rocks. Therefore, recalculations of the time-integrated lead isotopic ratios back to their initial values in ancient rocks should be approached with caution. A standard procedure for deriving initial lead isotopic ratios includes the analyses of minerals which have low U/Pb elemental ratios (e.g., potassium feldspar and/or sulfides). However, these minerals may not always be present in the sample of interest, too small and/or intergrown with other minerals, and thereby mechanically inseparable and/or altered. To circumvent the effects of secondary alteration, a conventional approach is the preliminary treatment of minerals by strong acids (e.g., concentrated HNO3 or Aqua Regia) as well as whole-rock samples in order to remove altered components. The residual phases are then analyzed for initial lead isotopic ratios. However, this procedure does not always yield satisfactory results.In this study, we developed an advanced stepwise leaching technique in which the rock powder was first treated by 2.2 N HCl, then by more concentrated HCl, and finally by concentrated HNO3. This procedure allows for the chemical separation of sulfides, which are dissolved by concentrated HNO3, but unaffected by HCl. At least three leaching steps and two isotopic analyses per sample should be performed. We applied this stepwise leaching procedure to mafic intrusions of a known age (1386 ± 30 Ma) from the Mesoproterozoic Udzha paleo-rift in the northern part of the Siberian Craton. The Pb isotope results for the whole-rock and final leachate fractions are presented for three samples as well as their U, Th and Pb whole-rock data, which revealed completely closed U-Th-Pb isotopic behavior despite moderate alteration of the samples, thereby supporting the 1.38 Ga age of the magmatic event. The obtained initial Pb isotope composition of the mafic intrusion clearly indicates an origin from a long-lived enriched source.