Download Differentiation of the Earth, Core formation

11/2/16
Differentiation 1: core formation
OUTLINE
Reading this week:
White Ch 12
Today
1.  Finish some slides
2.  Layers
3.  Core formation
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Element Relationships: Earth and C1 Chondrites
Most important siderophile and lithophile elements: BULK Earth
has higher Fe/Si and Mg/Si than the chondrites (Sun)
=> if bulk earth ≈ CI chondrite: lower mantle / core must host Si,
or we got < chondrite
Volatiles
Earth is variably depleted in volatile elements (e.g., K, Rb, Cs,
etc.) relative to chondrites
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Crust
Depth (km)
Upper Mantle
Transition Zone
•  Earth is radially zoned, with layers of
increasing density toward the center.
•  Crust and mantle ~1/2 radius
•  Outer and inner core other ~1/2
60
220
410
660
Lower
Mantle
⇒ How do we know?
2898
Outer
Core
(liquid)
5145
Inner
Core
(solid)
6370
Density inferred from the seismological data
Crust
Velocity (km/sec)
0
5
10
Lithosphere
Asthenosphere
Think: higher density =
faster
1000
Mantle
S waves
Mesosphere
2000
P waves
3000
Depth (km)
Outer
Core
4000
Liquid
5000
Inner
Core
S waves
Solid
6000
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Matching seismic velocities to high P-T experiments, starting
with peridotite for mantle, Fe-alloy for core we get x-section
How do we know where the phase changes happen?
Major seismic
discontinuities
Note:
„spinel“ and
„perovskite“ are high
P versions of olivine
Seismic data suggests Fe alloy
like Fe meteorites at high P:
We think ≈85% Fe + ≈5% Ni
Problem: density too high, need
10% „light element“
Primary contenders:
O, S, Si, C, P, Mg and H
No direct evidence, so this is
modeling, experiments,
meteorite analogs
⇒ Still hotly debated
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Core formation in light of accretion
3 scenarios for accretion:
!  homogeneous Earth accretes from materials of the same
composition AFTER condensation, followed by differentiation
!  heterogeneous Earth accretes DURING condensation,
forming a differentiated planet as it grows
!  intermediate between these two end-members
Basic concept of core formation
Both homogeneous and heterogeneous accretion models: core
segregates by:
1) melting of accreted Fe.
2) molten Fe sinks as droplets to the Earth’s center (density)
Details differ between models
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Heterogeneous accretion
Accretion during condensation requires very rapid build up of
earth, ~10Ka from initiation of condensation
•  Early core =
refractory early
condensates (Ca, Al)
time
•  Silicate and metal
phases condense
next, with heating
molten Fe replaces
refractory core
Density
control
Temperature
control
GG325 L32, F2013
Heterogeneous accretion
Pros:
1. Explains relative proportions of refractory, later elements of
inner planets with decreasing nebular temperature outward
2. Some siderophile elements condense later and never
equilibrate with molten iron, remain in the silicate mantle.
Major problem: ultramafic phases have Fe, but Fe should have
segregated to core while still hot: no Fe expected in lower
mantle, but seismology: Fe/(Mg+Fe) ~0.1
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Homogeneous accretion
Condensation first, then Earth builds from cool materials and
becomes hotter Aka: Earth accreted mostly homogeneously after
condensation was complete
Important aspects:
a. Heat builds up as the planet accretes.
b. Sometime afterwards, the core formed by Fe melting,
accompanied by other chemical transformations (see next slide)
entirely
Temperature
controlled
As things heat
up, you lose
volatiles,
moderately
volatiles, etc
GG325 L32, F2013
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Homogeneous accretion
1) Accretion starts with oxidized materials (some volatiles
around, like C1-C3 chondrites)
2) Presence of H2 reduces Fe to metal
3) Later, Earth heats up:
4) Then some Fe2+ incorporated in silicates (olivines, pyroxenes)
Homogeneous accretion
Heavy Fe sinks, light rock “floats”, but volatile loss not 100%
~10% of silicate earth retained volatiles during accretion, other
90% was degassed = primitive mantle
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Homogeneous accretion
Pros:
1. Allows volatiles in the core
2. Explains Fe in core + silicates
3. Provides heat source for early mantle melting/ formation of proto
continents
Cons:
1. Degassing of all but 10% of the volatile elements doesn’t work for
all elements
2. Not all siderophile elements agree with this core formation model
3. Heat for melting Fe comes later: Is there enough heat to melt Fe?
Core Formation
Core formation = closely linked to
accretion and requires:
• Immiscible components (iron
metal and silicate).
• Macro-segregation of
components: at least one of which
was molten or mostly molten.
•  Substantial difference in density
of components.
•  Gravitational settling
Fe melt layer collects, sinks as diapir
(or crack), provides additional
gravitational heat for melting
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Core Formation
Recent model: early
collisional heating => deep
magma ocean (persisted?)
Lower mantle not necessarily
heated: can retain many
siderophile elements as
molten Fe sinks through from
above
Wood et al. (2006)
When? Core Formation Timing
182Hf
to 182W
(T1/2 ~9 Ma = max ~50 Ma extinct)
•  Hf is one of the most refractory
elements => Earth should be
~chondritic
•  Core formation: W into core, Hf
into mantle/crust
•  If core formation happened
AFTER 182Hf-182W was extinct,
Earth should have chondritic W
isotopes
IT DOESN’T => Models suggest
core within 10-30 Ma
Yin et al., 2002
Chondrite
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Compositional caveat
Arguments like W=core, Hf=silicates requires knowledge of
core composition – so what’s in the core?
•  Iron meteorites = 5-10% Ni.
Great: Chondrite – 6% Ni (to core) ≈ primitive mantle
• Density arguments (seismology) require 10% of some light
element(s)
⇒ What light elements are in the core?
Light elements in the core
Contenders: O, S, Si, C, P, Mg and H.
Hotly debated, but many people like S, O:
FeS is miscible with Fe liquid at low and high temperatures
• S more depleted in silicate Earth than similar volatility elements
• Iron meteorites contain FeS (troilite)
FeO miscibility requires high pressures and temperatures
Together with S affects how much sidero/chalcophile elements
enter core: needed to explain mantle
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What the chalcophile elements say
Chalcophiles are depleted in the silicate Earth relative to
chondrites, but not as depleted as many of the siderophiles
are.
⇒ could argue against much S in the core (if there’s more S
loving elements in the mantle than expected, S probably same)
– ongoing problem
What the siderophile elements say
Siderophiles not as low in the mantle as expected from pure
metal-silicate equilibration.
•  5-350 times more enriched than expected
for complete silicate-Fe equilibrium
•  Volatile siderophiles even more enriched
than non-volatile ones.
⇒ 3 possible causes:
1)  incomplete equilibration
2)  an impure Fe phase
3)  addition of a volatile rich component after
core formation, aka “late veneer”
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