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Graphite floatation on a magma ocean and the fate of carbon during core formation

H. Keppler1,

1Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany

G. Golabek1

1Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Keppler, H., Golabek, G. (2019) Graphite floatation on a magma ocean and the fate of carbon during core formation. Geochem. Persp. Let. 11, 12–17.

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Geochemical Perspectives Letters v11  |  doi: 10.7185/geochemlet.1918
Received 21 March 2019  |  Accepted 7 June 2019  |  Published 9 July 2019
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0




Figure 1 Consequences of thermodynamic equilibrium between a magma ocean, graphite, and a primordial atmosphere. Shown are the curves for the oxygen fugacities of the iron-wustite (Fe-FeO) buffer (IW) and for the graphite–CO–CO2 buffer (CCO) for different CO + CO2 pressures (see Methods in Supplementary Information). Plausible oxygen fugacities of a magma ocean range from IW-2 to IW-5.5 (Rubie et al., 2011

Rubie, D.C., Frost, D.J., Mann, U., Asahara, Y., Nimmo, F., Tsuno, K., Kegler, P., Holzheid, A., Palme, H. (2011) Heterogeneous accretion, composition and coremantle differentiation of the Earth. Earth and Planetary Science Letters 301, 31–42.

). The diagram shows that equilibrium between such a reducing magma ocean and graphite buffers the CO + CO2 pressure in a primordial atmosphere to very low values.
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Figure 2 Two estimates for the diameter of the largest graphite particles that can be entrained in both a partially (ϕ = 0.4) and a fully molten (ϕ = 1.0) convecting magma ocean for planetary objects ranging from 25 km radius up to Earth’s radius. The grey band indicates typical diameters of graphite particles found in meteorites.
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Figure 3 Partitioning of carbon between atmosphere, silicate melt, and metal in the presence of graphite. (a) CO fugacity, CO2 fugacity, and total carbon in a silicate melt in equilibrium with graphite for a temperature of 1800 and 2000 K at the surface of a magma ocean. (b) Carbon content in the silicate melt as expected from the equilibrium with graphite at 2000 K at the surface of a magma ocean, predicted metal/silicate melt partition coefficient of carbon inside the magma ocean (after Chi et al., 2014

Chi, H., Dasgupta, R., Duncan, M.S., Shimizu, N. (2014) Partitioning of carbon between Fe-rich alloy melt and silicate melt in a magma ocean - Implications for the abundance and origin of volatiles in Earth, Mars, and the Moon. Geochimica et Cosmochimica Acta 139, 447–471.

, NBO/T = 3, 2000 K and 3 GPa) and predicted carbon content of metal in equilibrium with the silicate melt. In the presence of hydrogen, methane could be an additional carbon species; however, as we show in the Supplementary Information, the effect of CH4 on the behaviour of carbon is likely negligible.
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Figure 4 The fate of carbon during core formation of a terrestrial planet. Maximum possible carbon content in the core as a function of the number of impact events by planetary embryos, for oxygen fugacities from IW-6 to IW-2 and a surface temperature of 2000 K (left scale); predicted maximum fraction of total carbon sequestered in the core (right scale) for a terrestrial planet with a bulk carbon content of 500 ppm (Marty, 2012

Marty, B. (2012) The origins and concentrations of water, carbon, nitrogen and noble gases on Earth. Earth and Planetary Science Letters 313–314, 56-66.

).
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