Melting and subsolidus phase relations in peridotite and eclogite systems with reduced C O H fluid at 3–16 GPa

Abstract 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 f O 2 control by outer Mo–MoO 2 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 f O 2 = Fe–FeO are 1200 °C at 3 GPa and 1700 °C at 16 GPa. The solidus of the system with f O 2 = Mo–MoO 2 was about 100 °C lower. Estimated solidus temperatures for eclogite + C–O–H fluid with f O 2 = Fe–FeO are 1100 °C at 3 GPa and 1600 °C at 16 GPa, and for eclogite at f O 2 = Mo–MoO 2 solidus temperatures were 20–50 °C lower. These solidus temperatures are much higher (300–500 °C) than those for peridotite and eclogite systems with H 2 O and/or CO 2 , 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.% SiO 2 and 1.1–1.6 wt.% Na 2 O. Melts of eclogite contain more SiO 2 (47–49 wt.%) and are enriched in CaO (9–15 wt.%), Na 2 O (9–14 wt.%), and K 2 O (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 f O 2 estimations from cratonic peridotite. We show significant increase of solidus temperatures in peridotite and eclogite coexisting with reduced CH 4 –H 2 O fluid relative to the systems with oxidized H 2 O–CO 2 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.