The prospect of high-precision Pb isotopic dating of meteorites

Abstract

The radiogenic 207Pb/206Pb ratio is the only extant nuclide chronometer with sufficient time resolution for studies of the solar nebula accretion and early asteroidal differentiation and metamorphism. Pb isotopic dates can be used to link the dates obtained from extinct nuclide chronometers to the absolute time scale. The factors that control precision and accuracy of Pb isotopic dates of meteorites: instrumental mass fractionation in isotopic analysis, mass spectrometer sensitivity, removal of common Pb, multi-stage evolution of U-Pb systems, disturbances caused by diffusion, alteration, and shock metamorphism, and uncertainties in decay constants and the natural ratio of the U isotopes are reviewed. The precision of Pb isotopic dates of meteorites attained with currently available techniques and methodology is +/- 0.5-1.0 Myr in favorable cases. The accuracy of time interval measurements is approximately the same. The most serious limitation on precision and accuracy of Pb isotopic dates is placed by the presence of common Pb of uncertain and/or variable isotopic composition. Improvement in precision and accuracy of Pb isotopic dates would be possible through combined advancement of techniques of isotopic analysis (most importantly, better control over instrumental mass fractionation) and more effective techniques for the removal of common Pb, together with a better understanding of the effects of thermal metamorphism, shock metamorphism, and aqueous alteration on the U-Pb system in meteorites.