Accretion of the Earth and segregation of its core

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doi: 10.1038/nature04763
Authors:Wood, Bernard J.; Walter, Michael J.; Wade, Jonathan
Author Affiliations:Primary:
Macquarie University, Department of Earth and Planetary Sciences, North Ryde, N.S.W., Australia
Other:
University of Bristol, United Kingdom
Volume Title:Nature (London)
Source:Nature (London), 441(7095), p.825-833. Publisher: Macmillan Journals, London, United Kingdom. ISSN: 0028-0836
Publication Date:2006
Note:In English. 78 refs.; illus., incl. 1 table
Summary:In this review, chemical and isotopic data on the Earth's silicate mantle are collated and combined with astronomical observations of young stars and physical models of planetary development to present a coherent view of the Earth's early history. Substantial evidence for core formation in planetary embryos and planetismals (> 30 km across) include meteorites and samples from the asteroid belt between Mars and Jupiter, and represent a remnant of early accretion. Geochemical evidence supports the accretion of the Earth over > 107 yr from planetismals most of which had segregated metallic cores, and the process of core formation is discussed in terms of accretion, melting and metal segregation with details of core-mantle partitioning defined on refractory elements that are weakly to moderately siderophile and, like Ni and Co, are compatible in solid mantle silicates (core-mantle partition coefficients presented). Tungsten isotope anomalies of iron and silicate meteorites indicate that asteroidal bodies segregated metallic cores within ∼ 5 Ma after the beginning of the Solar System, while the Earth's core formation was more protracted with re-equilibration of asteroidal cores with the silicate 'mantle' and excess siderophiles. With growth through heterogeneous accretion the gravitational energy increased and at ∼ 45% of its current size, significant melting occurred with a periodic > 400 km thick molten outer layer (a magma ocean). Simulations indicate that this extended to the core-mantle boundary and would be relatively short-lived with crystallization of the lower mantle in a few thousand years. Core segregation at the base of the deepening magma ocean occurred under progressively more (self-) oxidizing conditions, supported by evidence that lower-mantle perovskite forces disproportionation of Fe2+ to Fe3+ plus Fe. A chondritic model combined with data for carbonaceous chondrites and the primitive mantle, indicates a core composition of ∼ 85 wt% Fe, 5 wt% Ni, 4-5 wt% Si and ∼ 8 wt% of non-refractory elements; the density of the liquid outer core is estimated as ∼ 8% lower than pure Fe alloy indicating the presence of low-atomic number alloying elements including Si, S, H and C. [R.K.H.]
Sections:Lunar and planetary studies; Petrology
Subsections:General
Subjects:Accretion; Core; Earth; Genesis; Geochemistry; Hafnium; Hf-182; High pressure; Isotopes; Mantle; Metals; Models; Planetesimals; Pressure; Segregation; Silicates; Stable isotopes; Sulfides; Terrestrial comparison; Tungsten; W-182
Abstract Numbers:06M/2945
Record ID:2010020447
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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