SiO2-SiC Mixtures at High Pressures and Temperatures: Implications for Planetary Bodies Containing SiC

We present results from high-pressure and high-temperature experiments on mixtures of SiC and SiO2 to explore the stability of SiC in the presence of oxygen-rich silicates at planetary mantle conditions. We observe no evidence of the ambient pressure predicted oxidation products, CO or SiO, resulting from oxidation reactions between SiC and SiO2 at pressures up to similar to 40 GPa and temperatures up to similar to 2500 K. We observe the decomposition of SiC through releasing C, resulting in vacancies in the SiC lattice and consequently the contracted SiC ambient volume V-0 observed in the heated regions of sample. The decomposition is further supported by the observations of diamond formation and the expanded SiO2 V-0 in the heated regions of samples indicating the incorporation of C into SiO2 stishovite. We provide a new interpretation of SiC decomposition on laboratory timescales, in which kinetics prevent the reaction from reaching equilibrium. We consider how the equilibrium decomposition reaction of SiC will influence the differentiation of a SiC-containing body on planetary timescales and find that the decomposition products may become isolated during early planetary differentiation. The resulting presence of elemental Si and C within a planetary body may have important consequences for the compositions of the mantles and atmospheres of such planets. Plain Language Summary Silicon carbide (SiC) has been proposed to be a major component in carbon-rich planets. However, SiC is known to oxidize in the presence of oxygen and oxygen-rich phases, at least at ambient pressure. As oxygen is an abundant element in most star systems, assessing the stability of SiC in the presence of oxygen-rich phases at planetary conditions is essential to understanding its presence in planetary bodies. Here we experimentally test the stability of SiC and SiO2 mixtures at high pressures and high temperatures using the laser-heated diamond anvil cell. We observe no oxidation reactions, indicating that SiC may remain stable against oxidation at moderate temperatures in the presence of silica at high pressure. We do, however, observe the decomposition of SiC evidenced by the formation of diamond as well as the corresponding reduced unit cell volume of SiC in the heated areas, resulting from vacancies in the SiC lattice formed due to the release of C. Additionally, we find evidence for the incorporation of C into the SiO2 structure. Based on these results, we present a model for the interior structure of a carbon-rich planet containing SiC and SiO2.