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Carbon in the Earth’s core

Carbon in the Earth’s core. Yingwei Fei Geophysical Laboratory Carnegie Institution of Washington. octahedrite. 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%)

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Carbon in the Earth’s core

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  1. Carbon in the Earth’s core Yingwei FeiGeophysical Laboratory Carnegie Institution of Washington

  2. octahedrite 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 cohenite

  3. Key factors affecting carbon budget in the Earth and core • Earth formation models • Element volatility trend • Core formation models • Mantle/core carbon partitioning

  4. 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

  5. Other considerations • Pressure effect • Planetary accretion and differentiation • Carbon added during and after accretion => Higher C in the Earth (>1.5 wt%) Wood [1993]

  6. Carbon in the core • Carbon in the mantle? • Carbon partitioning between mantle and core? • Carbon partitioning between inner and outer cores?

  7. 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… Earth core Li and Fei [2007]

  8. 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

  9. Birch’s law - velocity vs. density PREM FeS2 FeO Pure Fe FeSi FeS Fiquet et al. [2008]

  10. 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

  11. Melting relations in the Fe-C system at 20 GPa Fe-C Melt Fe Liquid Fe+liq Fe Fe+Fe3C Fe Fe3C

  12. Melting relations in the Fe-C system at 20 GPa Fe Fe3C Liquid Fe Fe+Fe3C Fe Fe3C

  13. Melting relations in the Fe-C system at 20 GPa Fe-C Melt Fe3C Liquid Fe3C+L Fe Fe+Fe3C Fe Fe3C

  14. Melting relations in the Fe-C system at 20 GPa Fe-C Melt Liquid 10µm Fe Fe+Fe3C Fe Fe3C

  15. 1 bar Fe-C System at High Pressure • Core temperature • Inner core mineralogy 5 GPa Temperature, K 10 GPa Fei et al. [2007] Fe Fe3C Fe7C3 Weight% Carbon

  16. Effect of pressure on eutectic temperature Fe melting Fe-C eutectic melting Fe-S eutectic melting

  17. 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

  18. FIB Solutions Field emission microprobe TEM 5µm Laser-heating DAC NanoSIMS Synchrotron X-ray

  19. Multi-anvil lab

  20. Melting in the Fe-C-S system 1.0 GPa 3.6 GPa 25µm 4.8 GPa 6.2 GPa

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

  22. Melting in the Fe-C-S system P = 20 GPa,T = 1375 ˚C Fe-C-S melt C-bearing Fe

  23. 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 Implications: • Core stratification may occur in small planetary bodies.

  24. Differentiation of planetary bodies (large or small) occurs through extensive melting >Melting over a wide pressure range

  25. Eutectic C C solubility in metallic Fe Wood, EPSL, 1993 Melting composition change as a function of pressure

  26. 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

  27. Solutions • Extend pressure range • Use of laser-heating diamond anvil cell • Nano analysis

  28. Multi-Anvil Apparatus • Capable of generating pressures up to 27 GPa and reaching temperatures above 2500 K Fe-C Melt Fe

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