Redox Freezing and Melting during Peridotite Interaction with Carbonated Metasediments and Metabasics: Experiments at 10 GPa

The hypothesis of redox freezing is based on the assumption that a Fe–Ni metal phase becomes stable in the peridotite mantle at increasing pressure and can serve as a reducer for carbonate–silicate melts. $${\text{CO}}_{3}^{{2 - }}$$ reduction and formation of elementary C (graphite or diamond) result in an increase in the solidus temperature and melt freezing. Thermodynamic calculations show that equilibrium oxygen fugacity in peridotite with carbon and magnesite is significantly lower than the values buffered by the mineral assemblages of metasediments (garnet–kyanite–SiO2–aragonite–elemental carbon) or eclogites (pyroxene–garnet–magnesite–elemental carbon). Hence, redox interaction between carbon-bearing peridotites and metasediments or eclogites may occur in the absence of metal and even in a Fe-free system. To explore this suggestion, we conducted experiments on interaction between forsterite (as a peridotite proxy) with synthetic mixtures simulating carbonatized metasediment (SiO2 + CaCO3 + Al2O3) and carbonatized eclogite (SiO2 + MgCO3 ± Al2O3 ± CaO) at 10 GPa and 1200–1500°C. To reduce the transport of major components, the mixtures were separated by a graphite disc, which also served as a source of C. The interaction resulted in the decarbonation of the carbonate-bearing metasediment or eclogite with diamond formation on the surface of the graphite disc. The graphite disc was dissolved at contact with peridotite, and metasomatic zoning developed. Pyroxene and magnesite with low Ca contents appeared in the distal metasomatic zone. The contents of Ca in the newly formed pyroxene and carbonate increases toward the graphite disc, and high-Ca pyroxene and garnet were observed in the proximal metasomatic zone. The obtained results indicate that coupled redox reactions occur in peridotite and metasediment (or eclogite): Mg2SiO4 + C + O2 = MgSiO3 +MgCO3 and CaCO3 + 1/3Al2S-iO5 + 2/3SiO2 = 1/3Ca3Al2Si3O12 + C + O2, respectively. The reactions occur owing to oxygen diffusion along intergranular melt channels. The interaction also involves the transfer of major cations, which resulted in the formation of carbonatized lherzolite and a diamond-bearing eclogite assemblage. Such a process is possible in nature at a contact of carbonated metasediment or eclogite with peridotite. The obtained results indicate that the presence of Fe–Ni metal is not necessary for redox freezing. The processes modeled in the experiments provide a possible explanation for the existence of diamond-rich eclogites and the scarcity of diamond in peridotite xenoliths.