Carbon in the Earth’s core Yingwei Fei Geophysical Laboratory Carnegie Institution of Washington

Carbon Budget  Carbon in the solar system  Relatively abundant (e.g., 12xSi)  Carbon in the meteorites  Iron meteorites ( 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 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?

Geophysical constraints  6-10% density deficit (outer core)  ~2% density deficit (inner core)  FeNi alloy wt% light elements  S, C, O, Si, H… 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 FeS 2 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] 1 bar

Melting relations in the Fe-C system at 20 GPa Fe Liquid Fe+Fe 3 C Fe 3 CFe Fe-C Melt Fe Fe+liq

Melting relations in the Fe-C system at 20 GPa Fe Liquid Fe+Fe 3 C Fe 3 CFe Fe 3 C

Melting relations in the Fe-C system at 20 GPa Fe Liquid Fe+Fe 3 C Fe 3 CFe Fe-C Melt Fe 3 C Fe 3 C+L

Melting relations in the Fe-C system at 20 GPa Fe Liquid Fe+Fe 3 C Fe 3 CFe Fe-C Melt 10µm

Fe-C System at High Pressure Fei et al. [2007] 1 bar  Core temperature  Inner core mineralogy Weight% Carbon 5 GPa 10 GPa FeFe 7 C 3 Fe 3 C Temperature, 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 emission microprobe

Multi-anvil lab

Melting in the Fe-C-S system 1.0 GPa 3.6 GPa 4.8 GPa6.2 GPa 25µm

Melting in the Fe-C-S system C O S

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

>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 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 Fe 3 C  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 Fe Fe-C Melt