2006 Wood et al., Accretion of the Earth and segregation of its core (1119308)
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Vol 441|15 June 2006|doi:10.1038/nature04763REVIEWSAccretion of the Earth and segregation ofits coreBernard J. Wood1, Michael J. Walter2 & Jonathan Wade2The Earth took 30–40 million years to accrete from smaller ‘planetesimals’. Many of these planetesimals had metalliciron cores and during growth of the Earth this metal re-equilibrated with the Earth’s silicate mantle, extractingsiderophile (‘iron-loving’) elements into the Earth’s iron-rich core. The current composition of the mantle indicates thatmuch of the re-equilibration took place in a deep (.400 km) molten silicate layer, or ‘magma ocean’, and that conditionsbecame more oxidizing with time as the Earth grew. The high-pressure nature of the core-forming process led to theEarth’s core being richer in low-atomic-number elements, notably silicon and possibly oxygen, than the cores of thesmaller planetesimal building blocks.rom observations of newly formed stars it appears that theSun’s planetary system formed from a flattened disk of dust andgas which accreted rapidly (,104 yr) into ‘planetesimals’,10 km in diameter1.
Gravitational interactions and collisionsbetween these bodies generated Moon-to-Mars-sized planetaryembryos in 105–106 yr and planetary bodies on a timescale of10–100 million years (Myr)1 . Tungsten isotope anomalies inmeteorites2 demonstrate that their asteroidal parents segregatedcores within a few million years of the origin of the Solar System.Applied to the Earth, the same techniques point to protracted(30–40 Myr) metal re-equilibration during planetary growth. Comparison of the composition of the Earth’s mantle with that ofpresumed protoplanetary material3 indicates high-pressure, hightemperature conditions of core segregation4–6, consistent with theexistence of a deep silicate ‘magma ocean’ during much of the Earth’sgrowth.
Once the Earth had attained the size of Mars, however,crystallization of silicate perovskite in the lower mantle led toincreasingly oxidized conditions6,7, which ultimately halted thesegregation of metal and may have led to a latest phase of sulphideaddition to the core8,9. This drawn-out process generated a corecontaining, in addition to the major components Fe (,85%) and Ni(5%), 1.9% S, 4–5% Si and possibly .1% O.FAccretion and core formation in planetary objectsThe planets of the Solar System originated as dust and gas in theyoung Sun’s protoplanetary disk.
The mechanisms of initial growthtowards large bodies are poorly understood, but whether by gravitational instability or simple ‘sticking together’ of aggregates, theprocess must have formed a large number of 10-km-sized objectsrapidly if the condensed materials were to avoid being dragged intothe Sun10–12. Once bodies reached this critical size, gravitationalperturbation became the dominant mechanism for further accretionthrough collision.
Although some planetesimals were destroyed incollisions, others would have continued to accrete, provided impactvelocities were low enough, or impact angles shallow enough, forgravity to recombine impact fragments and ejecta1,13–15. Models showthat once a population of larger planetary embryos emerged, theyaccreted rapidly at the expense of smaller objects in a period of‘runaway growth’ driven by dynamical friction and gravitationalfocusing1.
Accretion eventually became self-regulated by gravitationalinteractions among planetary embryos and surviving planetesimalsin a period of oligarchic growth16. The few tens of planetary embryosthat survived this accretionary phase of about one million years wereMoon-to-Mars-sized objects, with masses of the order of 0.01–0.1times the mass of the Earth17.The evidence for early core formation in planetary embryos andplanetesimals (radius .
30 km) is substantial. Many meteorites aresamples from the asteroid belt between Mars and Jupiter, and areprobably a remnant of the early days of accretion. The occurrence ofdistinct populations of iron and achondritic silicate meteoritesattests to early differentiation in planetesimals that were subsequently destroyed during collisions18. Tungsten (W) isotopesconfirm early core formation and place strict time limits on thetiming of segregation of metal from silicate19.
182Hf decays to 182Wwith a half-life of about nine million years. Hafnium (Hf) islithophile, having a pronounced preference for silicate phases overmetal, whereas W is siderophile, preferring the metal phase. So whena metal core segregates from silicate, the Hf/W ratio becomes elevatedin the silicate but is near zero in the metal. If metal segregationoccurred while 182Hf was ‘alive’ (,45 Myr after the origin of theSolar System), then silicate reservoirs exhibit positive 182Wanomaliesand metal reservoirs exhibit negative anomalies relative to undifferentiated proto-nebular material such as the chondritic meteorites.The ‘depleted’ 182W isotopic composition of iron meteorites relativeto the chondrite standard constrains metal segregation in precursorobjects to within a few million years (,5 Myr) of the origin of theSolar System2,19.
This time frame is consistent with statistical modelsin terms of how long it takes to grow objects large enough (.30 km)to retain their heat15. Heat retention is important because metal doesnot segregate from silicate unless it is molten and efficient segregationalso requires melting of the silicate20,21.Several heat sources contributed to melting of protoplanets in theyoung Solar System.
Decay of the short-lived radionuclides 26Al(half-life t 1/2 < 7 £ 105 yr) and 60Fe (t 1/2 < 3 £ 105 yr) could havesupplied sufficient heat to completely melt an early formed planetesimal21–24. Even in the absence of radioactive decay, the kineticenergy supplied by impact among small, porous planetesimals, andespecially from collisions involving larger objects, would have1Macquarie University, Department of Earth and Planetary Sciences, North Ryde, New South Wales 2109, Australia.
2University of Bristol, Department of Earth Sciences, Queen’sRoad, Bristol BS8 1RJ, UK.© 2006 Nature Publishing Group825REVIEWSNATURE|Vol 441|15 June 2006caused substantial melting. Surviving undifferentiated bodies must,therefore, have grown relatively slowly, enabling heat to be radiatedaway and after substantial decay of the most important radionuclides.
In small, differentiated bodies, the pressure at whichmetal and silicate material equilibrated during core segregationwould generally have been less than 2 GPa. Equilibration temperatureis more difficult to define because young objects can reach very hightemperatures from the heating processes discussed above and frompotential energy released during settling of metal21. Metal–silicateequilibration in early-formed objects may therefore have occurred ata wide range of high temperatures (for example, 1,500 to .3,000 K).By the end of the oligarchic growth stage the tens of survivingplanetary embryos were no longer constrained in well-regulatedorbits and began to interact gravitationally, setting up a final,cataclysmic stage of accretion by collision which lasted,107 years1,25. Many of these objects had proto-cores of iron alloy.A large terrestrial planet like the Earth probably sustained a numberof big collisions during accretion, and a late-stage giant impactbetween a Mars-sized object (,1026 g), sometimes referred to as‘Theia’, and the proto-Earth (,1027 g) is the prevailing theory for theformation of the Moon.
This would have provided sufficient energyto melt the proto-Earth completely.In summary, the Earth accreted over a period of at least 107 yearsfrom smaller bodies, most of which had already-segregated metalliccores. The energies of impact and metal separation providedsufficient heat to induce substantial melting episodically throughoutthe accretion and core-formation process. It is in the context ofaccretion, melting and metal segregation that the process of coreformation needs to be viewed.Chemical signature of core formationGrowth of the Earth from planetary embryos and planetesimalsresulted in the substantial partitioning of siderophile elements intothe metallic core, leaving lithophile elements behind in the silicatemantle.
Observed metal–silicate partitioning behaviour can, inprinciple, be used to understand the core separation process provided chemical compositions of the bulk silicate Earth (otherwiseknown as the primitive mantle), the core and the bulk Earth areknown. Primitive mantle is estimated from analyses of mantleperidotites3,26, bulk Earth from the compositions of undifferentiatedprotoplanetary material, represented by the CI carbonaceouschondrite meteorites and the core is obtained by calculating thedifference.Figure 1 shows a plot of the ratios of elemental concentrations inthe bulk silicate Earth divided by those in CI chondrites, normalizedEarthto ½Mg½MgCI ¼ 1:0 to correct for the high volatile contents of themeteorites. The elemental ratios are plotted as a function of thetemperature by which 50% of the element of interest would havecondensed during cooling of a gas of solar composition27.
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