Participation of metallic iron in the processes of natural diamond formation has recently been suggested based on experimental studies, as well as the observation of native iron inclusions in natural diamond. The redox conditions of diamond-producing metal-carbon melts in the Earth's mantle are therefore of interest. We calculated the values of oxygen fugacity, corresponding to the equilibrium of Fe-C and FeNi-C melts with wüstite and diamond at a pressure of 60 kbar. Diamond crystallization from Fe-Ni-C-O melts at usual P-T conditions of the catalytic diamond synthesis was shown experimentally. It was also shown that iron-carbon melts are stable at the values ranging from the stability filed of iron to that of wüstite. From metal-carbon melts either iron (iron-nickel) or wüstite can crystallize along with diamond, depending on the redox conditions. The stability of the metal-carbon melts is not limited to such reducing conditions as solid metals, and such conditions could have existed in the ancient Earth's mantle.

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