Magma Oceans, core
formation and the
differentiation of the Earth
B.J. Wood
• How to build core and
mantle-- the experimental
view.
• Silicate earth (primitive
upper mantle) has
approximately CI chondritic
ratios of refractory
lithophile elements.
• It is depleted in Si relative to
CI reference, meaning It has a
high Olivine/pyroxene ratio.
• If the depletion were due to
Si entering the core, then core
would contain ~7% Si (Allègre
et.al. 1995).
We need to estimate how siderophile elements are partitioned between
core and mantle in order to apply experiments to process of
of core formation.
.
CORE-MANTLE PARTITIONING assuming chondritic ratios
of refractory elements in bulk earth.
Di 
Refractory
DFe
13.6
D
Ni
DCo
DV
DCr
DW
DNb
23-27
23-27
1.5-2.2
3-4
15-22
0-0.8
i  core
i  mantle

i  metal
i silicate
Volatile
DMn
DSi
0.2-2.0
0.1-0.35
These should place strong constraints on
the P,T conditions of accretion and core
formation. The P,T conditions also need
to be consistent with ~10% light element
in the core and the current oxidation
state (oxidised Fe content) of the mantle.
•
Metal-silicate partitioning is a redox process, depending on oxygen
fugacity and valence n:
MO n/2  M 
in silicate
•
n
4
O2
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). Coremantle 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).
(Thibault & Walter, 1995
Li & 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.
Core formation end-members
•
•
•
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
Deep magma ocean model of core formation
silicate--liquidus
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 wellmixed 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’.
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 of
core ~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 in
core
Add Crystals to the Silicate
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%
•
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 perovskite
dissolution 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.
Principal
experimental
uncertaintyno data at very high
pressures.
The last 20% of accretion is most important for setting Ni
and Co contents of silicate Earth. But almost all experimental data
refer to pressures < 26 GPa, or < 50% accretion. Less important for
pressure-insensitive elements like V or Cr.
Affect of deep magma ocean with progressive oxidation
on timescales of accretion and core segregation.
182Hf(lithophile)-182W(siderophile)
system (t1/2=9 Myr)
Fraction accreted
~11 Myr for constant
DW (met/sil)~20
e.g Yin et. al. (2002)
Kleine et. al. (2002)
Ft=1-exp(-t/)
With progressive
oxidation and complete
re-equilibration changes
from 11 M.yr to 10.5 M.yr
What about other potential chronometers of core formation?
238,235U- 206,207Pb t
1/2 = 4.5 Gyr; 0.7 Gyr
Estimates of the Pb isotopic composition of the silicate earth
and 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 (as
expected). Core formation with
time constant of 11 M.yr does
not yield observed Pb isotopic
composition of silicate
Earth.
• Extraction of sulphide
50-150 M.yr later shifts the
silicate Earth into the observed
region, but a substantial amount
is required.
Given that core-mantle
partitioning requires
extraction of metal at high
P and T, can we see any
evidence of silicate
fractionation in such a
magma ocean stage?
We all learn that large
bodies of inviscid silicate
melt undergo fractional
crystallisation.
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 is
provided by partitioning
of elements between
the major lower mantle
perovskite phases
magnesium perovskite
(80%) and calcium
perovskite (5%) and
liquid silicate mantle.
Calculated effect on the
upper mantle (PUM) of
fractionally crystallising
Mg-perovskite and Caperovskite
in different ratios.
Maximum amount of
fractionation which would be
invisible is ~8% of a 90:10
mixture.
ie a very small small fraction
of the lower mantle.
Melt fraction as f(depth) Abe (1997)
• 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.
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.
• The age of the Moon, generally considered to
be formed by a giant impact, is >50 Myr after
origin of solar system.
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 chemical imprint of core formation on the silicate earth
By late 1960’s the region of high gradients had been resolved
as 2 pronounced discontinuities at 410 and 660 km depth.
CaSiO3perovskite
CORE-MANTLE PARTITIONING assuming chondritic ratios
of refractory elements in bulk earth
Di 
Refractory
DFe
13.6
D
Ni
DCo
DV
DCr
DW
DNb
23-27
23-27
1.5-2.2
3-4
15-22
0-0.8
i  core
i  mantle

i  metal
i silicate
Volatile
DMn
DSi
DCu
DAg
DPb
DZn
DTl
0.2-2.0
0.1-0.35
~10
~20
~20
~0
~10
Volatile elements estimated by comparing
to lithophile elements of similar volatility
Core formation and terrestrial magma ocean
• Composition of the Upper Mantle. Strong
compositional affinity with chondritic meteorites, both the
CI chondrites and ordinary chondrites.
• No compositional layering of Mantle
Upper mantle = Whole mantle compositionally
Primitive 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.
• 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.
A more sensitive test of fractionation
• 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.
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).