Carbon in the Earth’s core

Yingwei FeiGeophysical LaboratoryCarnegie Institution of Washington

Carbon Budget

Carbon in the solar system Relatively abundant (e.g., 12xSi)

Carbon in the meteorites Iron meteorites (0.01-0.6 wt%) Carbonaceous chondrites (~3.2 wt%)

Carbon in the Earth Range from 0.07 to 1.5(?) wt%

Carbon in the core

• Uncertain

octahedrite

cohenite

Key factors affecting carbon budget in the Earth and

core

Earth formation models

Element volatility trend

Core formation models

Mantle/core carbon partitioning

The relative abundances of elements in the Earth and various carbonaceous chondrites vs. the log of the 50% condensation temperature at 10-4 atm pressure

McDonough [2003] => 0.07 wt% C in the Earth

Other considerations

Pressure effect

Planetary accretion and differentiation

Carbon added during and after accretion

=> Higher C in the Earth (>1.5 wt%)Wood [1993]

Carbon in the core

Carbon in the mantle?

Carbon partitioning between mantle and core?

Carbon partitioning between inner and outer cores?

Magma Ocean

Core

Geophysical constraints 6-10% density deficit

(outer core) ~2% density deficit

(inner core)

FeNi alloy + 8-12 wt% light elements S, C, O, Si, H…

14

13

12

11

10

9

Density, g/cm

3

350300250200150

Pressure, GPa

CMB

ICB

PREM

hcp Fe (300K)

Hugoniot

7000K

Earth core

Li and Fei [2007]

Criteria for light elements

Density consideration - PVT data

Density-velocity relationship - velocity measurements

Inner-outer core density difference - element partitioning btw solid and

liquid

Temperature - melting relations

Birch’s law - velocity vs. density

FeS2

FeSi

FeO

FeS

Pure Fe

PREM

Fiquet et al. [2008]

Melting relations in the Fe-C System at High Pressure

Shterenberg et al. [1975] Tsuzuki et al. [1984] Wood [1993] Fei et al. [2007]

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1 bar

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Weight % Carbon

Melting relations in the Fe-C system at 20 GPa

Fe

Liquid

Fe+Fe3C

Fe3CFe

Fe-C Melt

Fe

Fe+liq

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Weight % Carbon

Melting relations in the Fe-C system at 20 GPa

Fe

Liquid

Fe+Fe3C

Fe3CFe

Fe

Fe3C

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Weight % Carbon

Melting relations in the Fe-C system at 20 GPa

Fe

Liquid

Fe+Fe3C

Fe3CFe

Fe-C Melt

Fe3C

Fe3C+L

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Weight % Carbon

Melting relations in the Fe-C system at 20 GPa

Fe

Liquid

Fe+Fe3C

Fe3CFe

Fe-C Melt

10µm

Fe-C System at High Pressure

Fei et al. [2007]

1 bar

Core temperature

Inner core mineralogy

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Weight % Carbon2200

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Weight % Carbon

Weight% Carbon

5 GPa

10 GPa

Fe Fe7C3Fe3C

Tem

per

atu

re, K

Effect of pressure on eutectic temperature

Fe melting

Fe-C eutectic melting

Fe-S eutectic melting

Challenges

Effect of carbon on liquid and solid iron densities at outer and inner core conditions, respectively.

Melting relations at IOC boundary (329 GPa)

Partitioning of C between silicate and metallic iron up to CMB conditions

Multi-component systems including other light elements such as S, O, and Si

Solutions TEM

NanoSIMS

Laser-heating DAC

5µm

FIB

Synchrotron X-ray

Field emissionmicroprobe

Multi-anvil lab

Melting in the Fe-C-S system

1.0 GPa 3.6 GPa

4.8 GPa 6.2 GPa

25µm

Melting in the Fe-C-S system

C

O

S

Melting in the Fe-C-S system

P = 20 GPa,T = 1375 ˚C

Fe-C-S melt

C-bearing Fe

Core stratification may occur in small planetary bodies.

Implications:

The solid inner core is nearly S-free, but it could contain significant amount of carbon, whereas the liquid outer core would be S-rich and C-poor.

Fe-C-S melt

C-bearing

Fe

Magma Ocean

Core>Melting over a wide pressure range

Differentiation of planetary bodies (large or small) occurs through extensive melting

Melting composition change as a function of pressure

4.8

4.4

4.0

3.6

3.2

2.8

2.4

2.0

1.6

1.2

0.8

0.4

0.0

C content in Fe, wt%

2520151050

Pressure, GPa

Eutectic C

C solubility in metallic Fe

Wood, EPSL, 1993

Conclusions The eutectic temperature of Fe-C system

increases with increasing pressure

Carbon solubility in metallic iron increases with increasing pressure whereas eutectic composition remains constant

If carbon is an important component of the Earth’s core, the inner core would crystallize as C-bearing Fe, rather than iron carbide such as Fe3C

In the Fe-C-S system, we found liquid miscibility gap closure at high pressure. Metallic Fe crystallizes with significant amount of C and negligible S, implying that C is more likely in the solid inner core than S

Solutions Extend pressure range

Use of laser-heating diamond anvil cell

Nano analysis

Multi-Anvil Apparatus Capable of generating pressures up to 27 GPa

and reaching temperatures above 2500 K

TC

Al2O3

Fe

Fe-C Melt