Magma Oceans, core formation and the differentiation of the Earth B.J. Wood

Magma Oceans, core formation and the

differentiation of the Earth

B.J. Wood

• How to build core and mantle-- the experimental view.

• Silicate earth (primitiveupper mantle) has approximately CI chondritic ratios of refractorylithophile elements.

• It is depleted in Si relative toCI reference, meaning It has a high Olivine/pyroxene ratio.

• If the depletion were due toSi entering the core, then corewould contain ~7% Si (Allègre et.al. 1995).

We need to estimate how siderophile elements are partitioned betweencore and mantle in order to apply experiments to process ofof core formation..

DFe 13.6 DMn 0.2-2.0

DNi 23-27 DSi 0.1-0.35

DCo 23-27

DV 1.5-2.2

DCr 3-4

DW 15-22

DNb 0-0.8

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Di =i[ ]corei[ ]mantle

=i[ ]metali[ ]silicate

CORE-MANTLE PARTITIONING assuming chondritic ratios of refractory elements in bulk earth.

Refractory Volatile

These should place strong constraints on the P,T conditions of accretion and coreformation. The P,T conditions also needto be consistent with ~10% light elementin the core and the current oxidationstate (oxidised Fe content) of the mantle.

• Metal-silicate partitioning is a redox process, depending on oxygen fugacity and valence n:

in silicate metal

• At the end of accretion the Fe content of the core (85%) and the present FeO content of the mantle (8%) gives an oxygen fugacity ~2 log units below Fe-FeO(IW) equilibrium.

• Ni and Co partition much too strongly into the core at low pressures and appropriate oxygen fugacity (DNi~500, DCo~100, both need D~25) to explain their observed mantle abundances (Ringwood, 1966). Core-mantle partitioning on Earth was not inherited from smaller bodies.

• The large light element (Si,O,S) content of the core (~10%) compared to iron meteorites indicates high pressure is important.

• Hf-W chronometry shows that Earth segregated its core over a much longer time period (~30 M.yr) than asteroids (1-3 M.yr) also requiring re-equilibration of metal and silicate in the growing planet (Kleine et.al. 2002; Yin et. al. 2002).

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MOn/2 = M+n4O2

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(Thibault & Walter, 1995Li & Agee, 1996)

Deep magma ocean model of core formation

•Accreting planetesimals break-up.•Droplets of liquid Fe falling through liquid silicate should stabilise with diameters of about 1 cm and should fall at 0.5 cm/s (Rubie et. al. 2003).They will continuously re-equilibrate with the silicate until they reach a depth at which they can form a thick layer.

• Physically unlikely.

Partitioning requires very high pressures and temperatures ~ 40 Gpa/4300K. Most of Earth’s Nb in core.

• Re-equilibration end-member

• Ni and Co too concentrated in core. V, Cr not siderophile enough

Core formation end-members

Deep magma ocean model of core formation

I used experimental data for V, Ni, Co, Cr, Nb, Mn,Si and W with temperature on silicate liquidus and assumed continuous extraction of metal from well-mixed magma ocean. Assumed magma ocean depth is a fixed fraction of depth to CMB and that droplets of Fe in magma (Rubie et.al 2003) continuously equilibrate until isolated in the metal ‘pond’.

silicate---liquidus

Partitioning during continuous extraction of metal from base of homogeneous magma ocean about 30% of depth to CMB. Fixed oxidation state of the mantle.

Si content ofcore ~0.1%

• Ni and Co partitioning depend on pressure but are relatively insensitive to temperature.

• Weak siderophiles such as V and Cr are sensitive to temperature and require the temperature to be increased substantially (e.g Li and Agee, 1996,2001; Chabot and Agee 2003; Righter et.al. 1997; Gessmann and Rubie 2000) to~40 GPa and ~4300K.

• The temperature is >1000K above the silicate liquidus and implies core extraction at the base of a completely molten mantle in a planet only 20% of size of the Earth.

• Data on other elements would not be consistent with such high temperatures e.g. The core would contain around 15% Si and 60% of Earth’s Nb.

Continuous extraction of metal in accreting Earth from base of homogeneous magma ocean. Observed core-mantle partitioning requires increase of oxygen fugacity (oxidised Fe) during accretion.

reduced oxidised

~6% Si incore

Mush of 90% crystals 10% liquid at the base of the magma ocean? W and Nb become more (too?) siderophile. Si not siderophile.

Si in core~0.1%

Add Crystals to the Silicate

• In order to match the siderophile element contents of the mantle particularly V,Cr the earth must have become more oxidised during accretion.

• A similar result is obtained if the magma ocean contains crystals or if it is not well-mixed.• Plausible causes of oxidation are:(a) Addition of more oxidised bodies later in accretion (Wänke, O’Neill).

(b) Si (from SiO2) dissolution in the core (5-7%) would provide more than enough oxygen. The ‘Smoking Gun’ for Si dissolution in core was claimed by Georg et. al. (2007) who found Si isotopic differences between silicate Earth and chondrites. This is now disputed.In latest stages of accretion

(c) Crystallisation of (Mg,Fe)SiO3 perovskite at the base of the magma ocean when the pressure in the earth became >25GPa. Perovskite has such a strong affinity for Fe3+ that it forces disproportionation of Fe2+ (Frost et.al 2004):

3Fe2+=2Fe3++ Fe0

melt pv(mantle) metal(to core)

• With metal being segregated to the core, the mantle ‘self-oxidises’. This is a process which can only happen on planets larger than Mars.

Lower mantle ‘oxygen pump’

The oxygen pump injects Fe3+

into the upper mantle by perovskitedissolution and recrystallisation. This raises oxygen fugacity during core segregation.

Fe3+ generation by perovskite crystallisation

• Explains why silicate Earth, despite having lower FeO/Fe than Moon and Mars has a higher Fe3+/Fe2+.

• Explains why silicate Earth shows no secular change in oxygen fugacity throughout geologic history-- Fe3+ content of the mantle was established towards the end of accretion.

The last 20% of accretion is most important for setting Niand Co contents of silicate Earth. But almost all experimental data refer to pressures < 26 GPa, or < 50% accretion. Less important forpressure-insensitive elements like V or Cr.

Principalexperimental uncertainty-no data at very highpressures.

~11 Myr for constant DW (met/sil)~20e.g Yin et. al. (2002)Kleine et. al. (2002)

Ft=1-exp(-t/)

Fraction accreted

Affect of deep magma ocean with progressive oxidation on timescales of accretion and core segregation.

182Hf(lithophile)-182W(siderophile) system (t1/2=9 Myr)

With progressiveoxidation and completere-equilibration changesfrom 11 M.yr to 10.5 M.yr

What about other potential chronometers of core formation? 238,235U- 206,207Pb t1/2 = 4.5 Gyr; 0.7 Gyr

Estimates of the Pb isotopic composition of the silicate earthand time of U-Pb fractionation (44-143 Myr after solar system

origin).Pb is generally regarded as having entered the core at this time.

• The difference between~ 30 Myr (W) and 44-143 Myr (Pb) may be due to early entry of W into the core and late entry of Pb. But why would that happen?

• W is siderophile, but Pb prefers to enter sulphides. It does not enter metals easily. So the difference could be explained if the last bit of core was a sulphide.

• Progressive self-oxidation of the mantle through perovskite crystallisation should lead eventually to destabilisation of metal and sulphide crystallisation (Wood & Halliday, 2005).

At the latest stages of accretion metal segregation would be suppressed and sulphide precipitated. This could have a dramatic influence on chalcophile elements (e.g Pb) and Pb isotopic composition of silicate Earth (Wood and Halliday, 2005).

• Recent experiments show Pb is not very siderophile (asexpected). Core formation with time constant of 11 M.yr doesnot yield observed Pb isotopiccomposition of silicateEarth.

• Extraction of sulphide 50-150 M.yr later shifts the silicate Earth into the observedregion, but a substantial amountis required.

Given that core-mantlepartitioning requires extraction of metal at high P and T, can we see any evidence of silicatefractionation in such a magma ocean stage?

We all learn that largebodies of inviscid silicatemelt undergo fractionalcrystallisation.

Fractional crystallisation on the Moon

Fractionation in terrestrial magma ocean

• The idea (e.g.Agee and Walker, 1988) comes from the observation that the Earth’s upper mantle is compositionally like CI chondritic meteorites except it is low in Si/Mg meaning it has a higher ratio of olivine to enstatite (or perovskite).

Earth’s Mantle

Unfortunately the properties of the mantle are consistent with it being compositionally the same as upper mantle peridotite until close to the core-mantle boundary. For example, the 410 and 660 km seismic discontinuities behave as isochemical phase transformations ie no strong compositional layering.

A more sensitive test isprovided by partitioningof elements betweenthe major lower mantleperovskite phasesmagnesium perovskite(80%) and calciumperovskite (5%) andliquid silicate mantle.

Calculated effect on the upper mantle (PUM) of fractionally crystallisingMg-perovskite and Ca-perovskite in different ratios.

Maximum amount of fractionation which would be invisible is ~8% of a 90:10 mixture.ie a very small small fractionof the lower mantle.

• Numerical models of crystallisation of a magma ocean (Solomatov and Stevenson 1993, Abe 1997) indicate that the lower mantle would crystallise very rapidly, with little fractionation.

• Recently discovered slightly non-chondritic Sm/Nd ratio (inherited during accretion- Boyet and Carlson, 2005) of upper mantle may be only detectable effect of silicate fractionation.

Melt fraction as f(depth) Abe (1997)

Conclusions• Mantle contents of siderophile elements are

consistent with complete re-equilibration of metal and silicate in accreting Earth.

• Earth became oxidised during accretion, plausibly setting its current oxidation state at the end of core formation.

• Crystal-liquid fractionation within the silicate Earth has had little impact on upper mantle composition except for elevated Sm/Nd.

Progressive oxidation model-- start reduced so that Si,V, Ni, Co etc enter core, then oxidise so that these elements are added only to the mantle.

• The age of the Moon, generally considered to be formed by a giant impact, is >50 Myr after origin of solar system.

The chemical imprint of core formation on the silicate earth

By late 1960’s the region of high gradients had been resolvedas 2 pronounced discontinuities at 410 and 660 km depth.

CaSiO3-perovskite

DFe 13.6 DMn 0.2-2.0

DNi 23-27 DSi 0.1-0.35

DCo 23-27 DCu ~10

DV 1.5-2.2

DCr 3-4

DW 15-22

DNb 0-0.8

DAg

DPb

DZn

DTl

~20

~20

~0

~10

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Di =i[ ]corei[ ]mantle

=i[ ]metali[ ]silicate

CORE-MANTLE PARTITIONING assuming chondritic ratios of refractory elements in bulk earth

Refractory Volatile

Volatile elements estimated by comparing to lithophile elements of similar volatility

• Composition of the Upper Mantle. Strong compositional affinity with chondritic meteorites, both the CI chondrites and ordinary chondrites.

• No compositional layering of MantleUpper mantle = Whole mantle compositionallyPrimitive Upper Mantle=Bulk Silicate Earth

• Mantle Si deficiency explained (using CI chondrite model) by dissolution of ~7% Si in the core (Allègre et. al. 1995) or volatile loss during accretion.

Core formation and terrestrial magma ocean

• Magma Ocean- A layer 100’s of km thick covering Earth’s surface episodically during accretion.

• This idea goes back at least to Safronov(1978) and Kaula (1979) who showed that impact energies would be sufficient to cause substantial melting in later stages of accretion--and now we have the moon-forming giant impact….

• By that time it had been established that the moon had gone through such a stage because it exhibits classic signs of fractional crystallisation.

• Start with the observation that the upper mantle has CI chondrite ratios of refractory lithophile elements.

• Consider how fractional crystallisation of the 2 most important lower mantle perovskite phases CaSiO3 (5%) and MgSiO3 (80%) would affect upper mantle composition if the upper mantle were a product of fractionation.

A more sensitive test of fractionation

Age of the Earth and core• Lord Kelvin calculated an age of 24 Myr based on time

to cool from a molten sphere to the current geotherm.• Recent measurements place the oldest meteorites at

4.567 Gyr based on U-Pb.• The U-Pb age of the Earth is ~80 Myr younger at 4.48

Gyr.• In contrast, the Hf-W system indicates a far more rapid

rate of Earth accretion (about 30 Myr after origin of solar system).