Melting and subsolidus phase relations in peridotite and eclogite systems with reduced COH fluid at 3-16 GPa

- Peridotite and eclogite coexisting with reduced C-O-H fluid studied at 3-16 GPa and 1200-1600 °C.
- The double-capsule technique with fO2 control by Mo-MoO2 and Fe-FeO buffer was used.
- The solidus temperatures were higher (300-500 °C) than in the systems with H2O and CO2.
- The partial melt from peridotite and eclogite has basaltic composition.
- We argue that redox melting can be important melting process in the deep Earth's interior.

Melting phase relations of peridotite and eclogite coexisting with reduced C-O-H fluid have been studied at 3-16 GPa and 1200-1600 °C. In order to perform these experiments the double-capsule technique with fO2 control by outer Mo-MoO2 or Fe-FeO buffer capsule was designed and developed for multianvil experiments at pressures 3-21 GPa. Silicate phase assemblages resemble those in volatile-free lithologies, i.e. olivine/wadsleyite-orthopyroxene-clinopyroxene-garnet in peridotite and garnet-omphacite in eclogite. Melting was detected by the appearance of quenched crystals of pyroxene, feldspar and glassy silica. Estimated solidus temperatures for peridotite + C-O-H fluid with fO2=Fe-FeO are 1200 °C at 3 GPa and 1700 °C at 16 GPa. The solidus of the system with fO2=Mo-MoO2 was about 100 °C lower. Estimated solidus temperatures for eclogite + C-O-H fluid with fO2=Fe-FeO are 1100 °C at 3 GPa and 1600 °C at 16 GPa, and for eclogite at fO2=Mo-MoO2 solidus temperatures were 20-50 °C lower. These solidus temperatures are much higher (300-500 °C) than those for peridotite and eclogite systems with H2O and/or CO2, but are still 300-400 °C lower than the solidi of volatile-free peridotite and eclogite at studied pressures. The compositions of partial melt were estimated from mass-balance calculations: partial melts of peridotite have CaO-poor (6-9 wt.%) basaltic compositions with 44-47 wt.% SiO2 and 1.1-1.6 wt.% Na2O. Melts of eclogite contain more SiO2 (47-49 wt.%) and are enriched in CaO (9-15 wt.%), Na2O (9-14 wt.%), and K2O (1.3-2.2 wt.%). All runs contained graphite or diamond crystals along with porous carbon aggregate with micro-inclusions of silicates indicating that reduced fluid may dissolve significant amounts of silicate components. Analyses of carbon aggregates using a defocused electron microprobe beam reveal compositions similar to estimated partial melts. The diamonds formed from reduced C-O-H fluid may have natural analogues as polycrystalline diamonds. The oxygen fugacity in the Earth's mantle decreases with pressure from about fayalite-magnetite-quartz at shallow depths of 20-50 km to about iron-wustite at 250-300 km according to fO2 estimations from cratonic peridotite. We show significant increase of solidus temperatures in peridotite and eclogite coexisting with reduced CH4-H2O fluid relative to the systems with oxidized H2O-CO2 fluid. We emphasize that redox melting by change of oxidation state across a mantle section, a phase transition, or the lithosphere-asthenosphere boundary can be the dominant melting process in the deep Earth's interior.

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