Effect of oxygen fugacity on the H2O storage capacity of forsterite in the carbon-saturated systems

High pressure experiments have been performed in the systems Mg2SiO4–C–O–H and Mg2SiO4–K2CO3–C at 6.3 GPa and 1200 to 1600 °C using a split-sphere multi-anvil apparatus. In the Mg2SiO4–C–O–H system the composition of fluid was modeled by adding different amounts of water and stearic acid. The fO2 was controlled by the Mo–MoO2 or Fe–FeO oxygen buffers. Several experiments in the Mg2SiO4–C–O–H system and all experiments in the Mg2SiO4–K2CO3–C system have been conducted without buffering the fO2. Forsterite in the system Mg2SiO4–K2CO3–C does not reveal OH absorption bands in the IR spectra, while forsterite coexisting with carbon-bearing fluid and silicate melt at logfO2 from FMQ-2 to FMQ-5 (from 2 to 5 log units below fayalite-magnetite-quartz oxygen buffer) contains 800–1850 wt. ppm H2O. The maximum concentrations were detected at 1400 °C and FMQ-3.5. We observed an increase in the solidus temperature in the system Mg2SiO4–C–O–H from 1200 to above 1600 °C with log fO2 decreasing from FMQ-2 to FMQ-5. The increase of the solidus temperature and the broadening of the stability field of the H2O–H2–CH4 subsolidus fluid phase at 1400–1600 °C explain the high H2O storage capacity of forsterite relative to that crystallized from carbon-free, oxidized, hydrous, silicic melt. At temperatures above 1400 °C liquidus forsterite precipitated along with diamond from oxidized (FMQ-1) carbonate–silicate melt and from silicate melt dissolving the moderately reduced C–O–H fluid (from FMQ-2 to FMQ-3.5). Formation of diamond was not detected under ultra-reduced conditions (FMQ-5) at 1200–1600 °C. Olivine co-precipitating with diamond from dry carbonate–silicate or hydrous–silicic fluid/melt can provide information on the H2O contents and speciation of the diamond-forming media in the mantle. The conditions for minimum post-crystallization alteration of olivine and its hydrogen content are discussed.