Lead Isotopes and the Age of the Earth

Abstract

Calculations based on comparison of the isotopic composition of lead from iron meteorites with that of various modern terrestrial leads have placed the age of the earth at around 4550 million years. However, recent data from young volcanic rocks reveal that modern terrestrial lead can have a wide range in isotopic composition. The variations in its coinposition mean that one or more of the assumptions used in the age calculation have been violated. We modified the usual approach by comparing meteorite lead with lead from rocks 2700 million years old, from the Canadian Shield. Using this method and the same constants and assumptions utilized in the earlier calculations we calculated an age of 4750 ± 50 million years for the earth. The earth may be approximately 200 million years older than previously thought; alternatively, primordial terrestrial lead may not have had the same isotopic composition as lead in iron meteorites does.