Capturing the oxidation of silicon carbide in rocky exoplanetary interiors

Abstract: Theoretical models predict the condensation of silicon carbide around host stars with C/O ratios higher than 0.65 (cf. C/O$_{\mathrm{Sun}}$ = 0.54), in addition to its observations in meteorites, interstellar medium and protoplanetary disks. Consequently, the interiors of rocky exoplanets born from carbon-enriched refractory material are often assumed to contain large amounts of silicon carbide. Here we aim to investigate the stability of silicon carbide in the interior of carbon-enriched rocky exoplanets and to derive the reaction leading to its transformation. We performed a high-pressure high-temperature experiment to investigate the reaction between a silicon carbide layer and a layer representative of the bulk composition of a carbon-enriched rocky exoplanet. We report the reaction leading to oxidation of silicon carbide producing quartz, graphite, and molten iron silicide. Combined with previous studies, we show that in order to stabilize silicon carbide, carbon saturation is not sufficient, and a complete reduction of Fe$^{2+}$ to Fe$^{0}$ in a planetary mantle is required, suggesting that future spectroscopic detection of Fe$^{2+}$ or Fe$^{3+}$ on the surface of rocky exoplanets would imply the absence of silicon carbide in their interiors.