Carbon solubility in mantle minerals

The solubility of carbon in olivine, enstatite, diopside, pyrope, MgAl2O4 spinel, wadsleyite, ringwoodite, MgSiO3-ilmenite and MgSiO3-perovskite has been quantified. Carbon-saturated crystals were grown from carbonatite melts at 900-1400 °C and 1.5 to ∼ 26 GPa in piston cylinder or multi-anvil presses using carbon enriched to > 99% in the 13C isotope. In upper mantle silicates, carbon solubility increases as a function of pressure to a maximum of ∼ 12 ppm by weight in olivine at 11 GPa. No clear dependence of carbon solubility on temperature, oxygen fugacity or iron content was observed. The observation that carbon solubility in olivine is insensitive to oxygen fugacity implies that the oxidation state of carbon in the carbonatite melt and in olivine is the same, i.e., carbon dissolves as C4+ in olivine. Carbon solubility in spinel MgAl2O4, transition zone minerals (wadsleyite and ringwoodite), MgSiO3-ilmenite and MgSiO3-perovskite are below the limit of detection of our SIMS-based analytical technique (i.e., below 30-200 ppb by weight). The differences in carbon solubilities between the various minerals studied appear to correlate with the polyhedral volume of the Si4+ site, consistent with a direct substitution of C4+ for Si4+. These results show that other, minor carbon-rich phases, rather than major, nominally volatile-free minerals, dominate the carbon budget within the bulk Earth's mantle. A significant fraction of total carbon could only be stored in silicates in a thin zone in the lowermost upper mantle, just above the transition zone, and only if the bulk carbon content is at the lower limit of published estimates. The carbon budget of the remaining mantle is dominated by carbonates and possibly diamond. The low melting point of carbonates and the high mobility of carbonate melts suggest that carbon distribution in the mantle may be highly heterogeneous, including the possibility of massive carbon enrichments on a local scale, particularly in the shallow subcontinental mantle.