THE AGE OF THE EARTH

Abstract

Forty years ago, Patterson in his pioneering work on lead isotopes defined the age of the Earth and of the meteorites with 4.55 Ga. We reconsider the question of the age of the Earth and its relation to the age of the meteorites, and similar to Patterson's approach, we use lead isotope systematics.U-Pb investigations of three meteoritic objects on which early thermal and/or chemical events generated large U/Pb fractionations are outlined in this article: refractory inclusions of the Allende meteorite, phosphates in ordinary chondrites and basaltic achondrites. All these samples contain lead with highly radiogenic compositions (206Pb204Pb < 150) and therefore their 207Pb-206Pb age is almost independent of the isotopic composition associated with the measured 204Pb to within the precision of a few million years. The Pb-Pb ages of the most radiogenic compositions measured in Allende refractory inclusions range from 4.568 to 4.565 Ga, the Pb-Pb ages of secondary phosphates in equilibrated ordinary chondrites vary from 4.563 to 4.504 Ga, and basaltic achondrites show ages between 4.558 and 4.53 Ga.These age determinations indicate precise time constraints for the classical scenario concerning the formation of the first planetary objects of the solar system. Formation of the Allende refractory inclusions occurs at 4566 +2-1 Ma. Accretion of chondritic bodies occurred at a maximum of 3 Ma later. Eight million years after formation of the Allende inclusions, magmatic activity, including partial melting, magma segregation, and eruption occurs on planetary bodies. During the next 200 Ma thermal processing, shock perturbation, and heating takes place.The meaning of an ''age of the Earth'' is evaluated in relation to the major early processes, end of accretion, core formation, and atmosphere extraction. A similar value for the age of the Earth is found based on lead isotopes and I-Xe systematics; this age is about 0.1 Ga younger than that of primitive meteorites. Pb-Pb and I-Xe terrestrial ages are interpreted as mean ages of core segregation and of atmosphere outgassing, respectively. Within this framework, the ''age of the Earth'' corresponds to the end of its accretion and to its early differentiation.