The oxygen fugacity within carbonated eclogites

The subduction of carbon back into the mantle is an important natural form of carbon sequestration. Eclogitic nodules and CO2-rich metasomatic agents show evidence for the passage of carbon and carbonate (solid or melts) in the mantle. Further, diamonds from kimberlites occur in eclogite xenoliths more frequently than in peridotite xenoliths but the reason for this is unclear. The redox conditions at which both carbon and carbonate are stable in eclogitic settings are still relatively uncertain. Possible control factors may include silicate-carbonate-diamond reactions and ferrous/ferric bearing minerals such as garnet and clinopyroxene. A comparison between the oxygen fugacities defined by carbon/carbonate equilibria in peridotite and eclogite assemblages indicates that diamond-bearing eclogites might be stable at conditions where only carbonates would be stable in peridotite rocks. Although these observations are suggested by thermodynamic calculations involving typical equilibria in eclogitic rocks, an experimentally calibrated oxy barometer is still not available. The aim of this study is to determine the oxygen fugacity at which elemental carbon coexists with carbonate minerals and melts in eclogitic rocks. Therefore, we performed experiments at both above and below the solidus of a carbonated eclogite in the Na-Ca-Mg-Al-Si-Fe-O-C system at pressures between 3 and 7 GPa and temperature of 800-1250 °C. A MORB-like eclogite bulk composition was saturated in kyanite. Iridium powder was added to the starting mixture to act as redox sensor. Experiments were run in piston cylinder and multi anvil devices. Further, the ferric iron of synthetic omphacite and garnet equilibrated with carbon and carbonate in the eclogitic assemblages was measured using Mössbauer spectroscopy. The results are used to understand the oxygen fugacity at which carbonate melts become stable in eclogitic regions of the mantle and to determine the ferric iron contents of eclogitic minerals as a function of pressure and temperature in the presence of carbonate. The results also have implications for the formation of diamonds in eclogitic rocks and are used to develop an oxygen thermobarometer for eclogitic rocks.