Carbon in the Earth’s core Yingwei Fei Geophysical Laboratory Carnegie Institution of Washington.
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Transcript of Carbon in the Earth’s core Yingwei Fei Geophysical Laboratory Carnegie Institution of Washington.
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]
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
1 bar
2400
2300
2200
2100
2000
1900
1800
1700
16008.07.06.05.04.03.02.01.00.0
Weight % Carbon
Melting relations in the Fe-C system at 20 GPa
Fe
Liquid
Fe+Fe3C
Fe3CFe
Fe-C Melt
Fe
Fe+liq
2400
2300
2200
2100
2000
1900
1800
1700
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Weight % Carbon
Melting relations in the Fe-C system at 20 GPa
Fe
Liquid
Fe+Fe3C
Fe3CFe
Fe
Fe3C
2400
2300
2200
2100
2000
1900
1800
1700
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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
2400
2300
2200
2100
2000
1900
1800
1700
16008.07.06.05.04.03.02.01.00.0
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
2100
2000
1900
1800
1700
1600
1500
1400
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Weight % Carbon2200
2100
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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