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

Abstract

doi: 10.7185/geochemlet.1918 Carbon is a strongly siderophile element and current models assume that in a magma ocean, most of the carbon is sequestered into the core. Here we show that (i) for an initially highly reduced magma ocean, most of the carbon will be reduced to graphite, which is less dense than a peridotite melt; (ii) the graphite can be dynamically stable at the surface of a magma ocean; (iii) the equilibrium of the primordial atmosphere with graphite buffers CO and CO2 fugacity to such low values, that only traces of carbon dissolve in the magma ocean. Therefore, under very reducing conditions, most of the carbon may remain near the surface during core formation of a terrestrial planet and the extent of carbon sequestration into the core may be limited. We suggest that the ureilite meteorites may be the remnants of such a graphite-rich surface layer on a partially or completely molten planetesimal. A similar, graphite-enriched surface may also exist on Mercury. Received 21 March 2019 | Accepted 7 June 2019 | Published 9 July 2019 1. Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany * Corresponding author (email: [email protected]) Introduction The initial distribution of carbon in terrestrial planets was likely affected by processes occurring during the formation and solidification of a magma ocean (Elkins-Tanton, 2012). Carbon is known to be siderophile and a number of studies have therefore investigated the partitioning of carbon between a silicate melt and iron metal (Dasgupta and Walker 2008; Dasgupta et al., 2013; Chi et al., 2014). Measured partition coefficients DC typically range from 500 to 5000. Accordingly, current models assume that during the evolution and solidification of the magma ocean, most of the carbon is sequestered into the core, leaving a carbon-depleted silicate mantle and primordial atmosphere behind (Dasgupta, 2013; Hirschmann, 2016). The redox state of a magma ocean depends largely on the type of material accreted. Heterogeneous accretion models are required to meet constraints imposed by trace element concentrations in the present day mantle and experimentally determined trace element partition coefficients. The models of Rubie et al. (2011, 2015) suggest that Earth ́s formation started with the accretion of highly reduced material, while more oxidised material was added later. This corresponds to an initial oxygen fugacity 5.5 log units below the iron-wustite buffer (IW-5.5), while during the late stages of accretion, oxygen fugacity increased to IW-2. During the later stages of accretion, most mass was delivered by large impactors and full equilibrium between metal and silicate melt was not achieved. Therefore, the partitioning of carbon into the core mostly occurred early in a very reducing environment. Carbon in chondritic meteorites is mostly present in the form of some carbonaceous, graphite-like material. Figure 1 illustrates the consequences of chemical equilibrium between a reduced magma ocean and graphite when a gas phase is present. The equilibrium between a CO-CO2 gas phase and graphite is given by the CCO buffer. The oxygen fugacity imposed by this buffer depends strongly on the CO + CO2 gas pressure. In equilibrium, the oxygen fugacity of the magma ocean, which is controlled by the concentration and redox state of iron, has to be the same as the oxygen fugacity of the CCO buffer. Figure 1 shows that for example at 2000 K, a magma ocean with an oxygen fugacity 4 log units below the Fe-FeO (iron-wustite, IW) buffer would limit the CO + CO2 pressure coexisting with graphite to 100 bars; at 1600 K, the corresponding value would be just 10 bars. The carbon content in carbonaceous chondrites ranges from 0.1 to >5 wt. %, with an average near 2.9 wt. % (Kerridge, 1985). If the bulk Earth included just 2 % of such volatile-rich material (Marty, 2012) and if all this carbon were converted into a CO-CO2 gas phase, the resulting gas pressure would be in the range of 1600 to 2500 bar, depending on the CO2/CO ratio. Obviously, if oxygen fugacity is controlled by a reduced magma ocean, the resulting low CO + CO2 pressures imply that most of the carbon must be reduced to graphite.