Diamond, Carbide and Carbonate Planets

More than five hundred planets have been discovered outside of our solar system to date, yet very little is known of their internal compositions and subsequent mineralogy. The principal factors in determining planetary mineralogy are nebula composition, pressure, temperature and oxygen fugacity. While work has been done on determining the stable minerals with respect to pressure and temperature in these planets, very little has been done in determining the oxygen fugacity and the resulting geology. Planetary formation models propose a new kind of planet: carbon super-Earths. The planets have very high C/Fe ratios and are unlike any in our solar system. The interplay between carbon, oxygen and iron in these planets provide an end-member test of the effects of oxygen fugacity on carbon mineralogy as well as the potential for carbon entering Earth's core as iron-carbide. We combine experimental diamond anvil cell x-ray diffraction and Raman spectroscopy with thermochemical modeling to determine the oxidation state and relative oxidation potential of the siderite-diamond-wüstite (SDW) buffer relative to the iron-wüstite (IW) buffer over a range of pressures spanning those of Earth's lower mantle to that of a carbon super-Earth. We find that over all pressures along a mantle adiabat, the SDW buffer is above the IW buffer, suggesting that both the Earth and carbon super-Earth mantles contain reduced species of carbon. Experiments to 65 GPa and 2400 K on siderite, iron, and wüstite mixtures show reduction of carbon to diamond via x-ray diffraction, Raman spectroscopy, and STEM-EDX. The reduced carbon present in these planets will therefore be present as iron carbide with excess diamond. In a carbon super-Earth, differentiation processes will sequester iron carbide into a core, leaving a significant inventory of diamond in the mantle. We present mass-radius relationships for such planets and implications for the dynamical evolution of diamond-rich mantles.