The major differentiation of the Earth at similar to 4.45 Ga | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  The major differentiation of the Earth at similar to 4.45 Ga

Type de publication:

Journal Article

Source:

Earth and Planetary Science Letters, Volume 267, Ticket 1-2, p.386-398 (2008)

ISBN:

0012-821X

Numéro d'accès:

WOS:000254684000034

Mots-clés:

Géochimie et cosmochimie ; N° Contribution : 2304, UMR 7154

Résumé:

The revelation of a small W-182 terrestrial excess relatively to the Hf-182 - W-182 systematics in meteorites has led some authors to claim that the Earth experienced a rapid accretion and an early differentiation, 30 - 40 Ma after the birth of the solar system at 4.567 Ga. This interpretation has since been moderated, but the idea of an early segregation of the core is still widely advocated. We challenge this interpretation with quantitative arguments that concern Hf - W, U - Pb and I - Xe systematics on the Earth. The W isotopic composition of the bulk silicate Earth can be explained by an incomplete isotopic re-equilibration between primitive metal and silicate components during the segregation of the Earth's core. We consider that the primitive metal/silicate differentiation in planetesimals and the segregation of cores of planetary bodies occurred during the first million years and that the segregation of the major part of the Earth's core occurred late in respect to the Hf-182 decay. Consequently, the non-equilibrated fraction of primitive silicate material is estimated to be small, between 6 and 14%, enough however to << open >> the Hf-182 - W-182 chronometer as is presently observed. This significant, but incomplete, metal/silicate re-equilibration only slightly affects the U - Pb chronometer. A reappraisal of the Pb isotope composition of the bulk silicate Earth allows us to define the mean age of the Earth's core's segregation, between 4.46 Ga and 4.38 Ga. This evaluation overlaps the time of outgassing of the atmosphere based on the I-129 - Xe-129 systematics, 4.46 - 4.43 Ga. We consider that the period around 4.45 Ga relates to the major primitive differentiation of the Earth. This scenario coherently and quantitatively explains the Hf-182 - W-182, U-235,U- 238 - Pb-207,Pb- 206, I-129 - Xe-129 and Sm-146 - Nd-142 terrestrial records and it is compatible with the radiometric constraints for the formation of the Moon and coherent with the similar to 10(2) Ma time scale for the accretion of the Earth, as evaluated by current numerical simulations for terrestrial planet formation. (C) 2007 Elsevier B.V. All rights reserved.