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 degrees C using a split-sphere multi-anvil apparatus. In the Mg2SiO4C-O-H system the composition of fluid was modeled by adding different amounts of water and stearic acid. The fO(2) 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 fO(2). 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 log/O-2 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 degrees 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 degrees C with log fO(2), decreasing from FMQ-2 to FMQ-5. The increase of the solidus temperature and the broadening of the stability field of the H2O-H-2-CH4 subsolidus fluid phase at 1400-1600 degrees C explain the high H2O storage capacity of forsterite relative to that crystallized from carbon-free, oxidized, hydrous, silicic melt. At temperatures above 1400 degrees C liquidus forsterite precipitated along with diamond from oxidized (FMQ-1) carbonate silicate melt and from silicate melt dissolving the moderately reduced CO H fluid (from FMQ-2 to FMQ-3.5). Formation of diamond was not detected under ultra-reduced conditions (FMQ-5) at 1200-1600 degrees 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. (C) 2010 Elsevier Ltd. All rights reserved.