Broad bounds on Earth’s accretion and core formation constrained by geochemical models

Rudge, J. F. and Kleine, T. and Bourdon, B. (2010) Broad bounds on Earth’s accretion and core formation constrained by geochemical models. Nature Geoscience, 3 (6). pp. 439-443. ISSN 1752-0894 EISSN:1752-0908 DOI 10.1038/NGEO872

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Abstract

The Earth formed through the accretion of numerous planetary embryos that were already differentiated into a metallic core and silicate mantle. Prevailing models of Earth’s formation, constrained by the observed abundances of metal-loving siderophile elements in Earth’s mantle, assume full metal– silicate equilibrium, whereby all memory of the planetary embryos’ earlier differentiation is lost1,2. Using the hafnium– tungsten (Hf–W) and uranium–lead (U–Pb) isotopic dating systems, these models suggest rapid accretion of Earth’s main mass within about 10 million years3–6 (Myr) of the formation of the Solar System. Accretion terminated about 303,7 or 1004,5 Myr after formation of the Solar System, owing to a giant impact that formed the Moon. Here we present geochemical models of Earth’s accretion that preserve some memory of the embryos’ original differentiation. These disequilibrium models allow some fraction of the embryos’ metallic cores to directly enter the Earth’s core, without equilibrating with Earth’s mantle.We show that disequilibrium models are as compatible with the geochemical observations as equilibrium models, yet still provide bounds on Earth’s accretion and core formation. We find that the Hf–W data mainly constrain the degree of equilibration rather than the timing, whereas the U–Pb data confirm that the end of accretion is consistent with recent estimates of the age of the Moon8,9. Our results indicate that only 36% of the Earth’s core must have formed in equilibrium with Earth’s mantle. This lowdegree of equilibration is consistent with the siderophile element abundances in Earth’s mantle.

Item Type: Article
Uncontrolled Keywords: 2010AREP; IA60
Subjects: 02 - Geodynamics, Geophysics and Tectonics
Divisions: 02 - Geodynamics, Geophysics and Tectonics
Journal or Publication Title: Nature Geoscience
Volume: 3
Page Range: pp. 439-443
Identification Number: 10.1038/NGEO872
Depositing User: Sarah Humbert
Date Deposited: 21 May 2010 15:49
Last Modified: 23 Jul 2013 09:57
URI: http://eprints.esc.cam.ac.uk/id/eprint/1393

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