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.