The oxygen fugacity at which graphite or diamond forms from carbonate-bearing melts in eclogitic rocks

The oxygen fugacity (fO(2)) at which carbonate-bearing melts are reduced to either graphite or diamond in synthetic eclogite compositions has been measured in multi-anvil experiments performed at pressures between 3 and 7 GPa and temperatures between 800 and 1,300 degrees C using iron iridium and iron platinum alloys as sliding redox sensors. The determined oxygen fugacities buffered by the coexistence of elemental carbon and carbonate-bearing melt are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving only solid phases. The measured oxygen fugacities normalized to the fayalite magnetite quartz oxygen buffer decrease with temperature from similar to-0.8 to similar to-1.7 log units at 3 GPa, most likely as a result of increasing dilution of the carbonate liquid with silicate. The normalized 102 values also decrease with pressure and show a similar decrease with temperature at 6 GPa from similar to-1.5 log units at 1,100 degrees C to similar to-2.4 log units at 1,300 degrees C. In contrast to previous arguments, the stability field of the carbonate-bearing melt extends to lower oxygen fugacity in eclogite rocks than in peridotite rocks, which implies a wider range of conditions over which carbon remains mobile in natural eclogites. The raised prevalence of diamonds in eclogites compared to peridotites may, therefore, reflect more effective scavenging of carbon by melts in these rocks. The ferric iron contents of monomineralic layers of clinopyroxene and garnet contained in the same experiments were also measured using Mossbauer spectroscopy. A preliminary model was derived for determining the fO(2) of eclogitic rocks from the compositions of garnet and clinopyroxene, including the Fe3+/Sigma Fe ratio of garnet, using the equilibrium,