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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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An experimental study of serpentine decomposition at high pressure (4.5 GPa) was carried out to elucidate if water can be preserved in the system in the form other than structural admixtures in minerals. This problem is of interest because it is water that plays a leading role in the melting in a subducted slab and a mantle wedge. To estimate the possible content of an aqueous fluid in deep-seated rocks, a BARS pressless split-sphere apparatus was used in complex with thermobarogeochemistry and gas chromatography. It has been established that the serpentine decomposition is accompanied by the release of water, which concentrates in inclusions in the produced minerals (olivine and orthopyroxene) and their interstices. Chromatographic analysis with a stepwise heating of samples showed that most of the released water is localized in the interstices, and the rest is conserved in fluid inclusions in the minerals. The produced solid phases conserve 0.13 to 2.43 wt.% fluids as inclusions, with water amounting to 0.1–2.06 wt.%. The content of inclusions determined by microscopic examination falls in this region. Since the mobility of the fluid conserved as inclusions in the olivine and orthopyroxene is significantly lower than that in the interstices, this fluid might be better preserved in olivine-containing rocks subsided to depth.