Carbon partitioning under reducing conditions: implications for Mercury

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Abstract

Mercury is the smallest planet of our Solar system and also the closest to the Sun. It is characterized by a very large core (70 vol.$\percent$ of the planet) and its surface is highly depleted in FeO. These features have been interpreted as an evidence for primary differentiation of Mercury under very reducing conditions (IW$-5\pm 2$; with IW being the iron-wustite equilibrium). The primordial crust of Mercury may have been made up of graphite and would have formed by graphite flotation during solidification of a magma ocean. Recent neutron spectroscopy measurements in deep craters by the MESSENGER spacecraft show abundant C concentration (1-3 wt$\percent$) interpreted as relict from a primordial graphite crust. In order to model the formation of such a crust and to understand how carbon is distributed amongst the various geochemical reservoirs of Mercury, new data of carbon partitioning between silicate and metal is needed. 

In this project, we performed high-temperature (1200-1900°C), high-pressure (0.1-26 GPa) experiments under moderately to highly reducing conditions (IW to IW-9). We will present new, high-precision SIMS data on carbon solubility in silicate melts and new EPMA data on carbon solubility in metal melts. For the silicate, carbon solubility is mainly controlled by the conditions of oxygen fugacity, reaching percentage levels at IW-9.  Pressure also has an extremely large effect on carbon solubility. For the metal phase, oxygen fugacity also controls carbon solubility through its dependence on sulfur and silicon concentrations.

Our results are combined in a new parametrization of carbon partitioning. We estimate that, in contrast to terrestrial planets having formed in more oxidized conditions, Mercury’s core might be depleted in carbon and Mercury’s primary mantle may have contained hundreds to thousands of ppm of carbon.