DIAMONDS, ECLOGITES, AND THE OXIDATION-STATE OF THE EARTHS MANTLE

The reaction dolomite + 2 coesite reversible diopside + 2 diamond + 2O 2 defines the coexistence of diamond and carbonate in mantle eclogites . The oxygen fugacity of this reaction is approximately 1 log unit hig her at a given temperature and pressure than the oxygen fugacities of the analogous reactions that govern the stability of diamond in perido tite. This difference allows diamond-bearing eclogite to coexist with peridotite containing carbonate or carbonate + diamond. This potential coexistence of diamond-bearing eclogite and carbonate-bearing peridot ite can explain the presence of carbon-free peridotite interlayered wi th garnet pyroxenites that contain graphitized diamond in the Moroccan Beni Bousera massif at the Earth's surface and the preferential prese rvation of diamond-bearing eclogitic relative to peridotitic xenoliths in the Roberts Victor kimberlite.