Effect of oxygen fugacity on the H sub(2)O storage capacity of forsterite in the carbon-saturated systems

High pressure experiments have been performed in the systems Mg sub(2)SiO sub(4)-C-O-H and Mg sub(2)SiO sub(4)-K sub(2)CO sub(3)-C at 6.3 GPa and 1200 to 1600 C using a split-sphere multi-anvil apparatus. In the Mg sub(2)SiO sub(4)-C-O-H system the composition of fluid was modeled by adding different amounts of water and stearic acid. The fO sub(2) was controlled by the Mo-MoO sub(2) or Fe-FeO oxygen buffers. Several experiments in the Mg sub(2)SiO sub(4)-C-O-H system and all experiments in the Mg sub(2)SiO sub(4)-K sub(2)CO sub(3)-C system have been conducted without buffering the fO sub(2). Forsterite in the system Mg sub(2)SiO sub(4)-K sub(2)CO sub(3)-C does not reveal OH absorption bands in the IR spectra, while forsterite coexisting with carbon-bearing fluid and silicate melt at logfO sub(2) from FMQ-2 to FMQ-5 (from 2 to 5 log units below fayalite-magnetite-quartz oxygen buffer) contains 800-1850 wt. ppm H sub(2)O. The maximum concentrations were detected at 1400 C and FMQ-3.5. We observed an increase in the solidus temperature in the system Mg sub(2)SiO sub(4)-C-O-H from 1200 to above 1600 C with log fO sub(2) decreasing from FMQ-2 to FMQ-5. The increase of the solidus temperature and the broadening of the stability field of the H sub(2)O-H sub(2)-CH sub(4) subsolidus fluid phase at 1400-1600 C explain the high H sub(2)O 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 H sub(2)O 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.