THE AGE OF THE EARTH

Forty years ago, Patterson in his pioneering work on lead isotopes def ined the age of the Earth and of the meteorites with 4.55 Ga. We recon sider 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 i sotope systematics. U-Pb investigations of three meteoritic objects on which early thermal and/or chemical events generated large U/Pb fract ionations are outlined in this article: refractory inclusions of the A llende meteorite, phosphates in ordinary chondrites and basaltic achon drites. All these samples contain lead with highly radiogenic-composit ions (Pb-206/Pb-204 > 150) and therefore their Pb-207-Pb-206 age is al most independent of the isotopic composition associated with the measu red Pb-204 to within the precision of a few million years. The Pb-Pb a ges 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 4.50 4 Ga, and basaltic achondrites show ages between 4.558 and 4.53 Ga. Th ese age determinations indicate precise time constraints for the class ical scenario concerning the formation of the first planetary objects of the solar system. Formation of the Allende refractory inclusions oc curs at 4566 (+2)/(-1) Ma. Accretion of chondritic bodies occurred at a maximum of 3 Ma later. Eight million years after formation of the Al lende inclusions, magmatic activity, including partial melting, magma seg regation, and eruption occurs on planetary bodies. During the next 200 Ma thermal processing, shock perturbation, and heating takes plac e. The meaning of an ''age of the Earth'' is evaluated in relation to the major early processes, end of accretion, core formation, and atmos phere extraction. A similar value for the age of the Earth is found ba sed on lead isotopes and I-Xe systematics; this age is about 0.1 Ga yo unger than that of primitive meteorites. Pb-Pb and I-Xe terrestrial ag es are interpreted as mean ages of core segregation and of atmosphere outgassing, respectively. Within this framework, the ''age of the Eart h'' corresponds to the end of its accretion and to its early different iation.