Determining the water dissociation constant from the relationship between water solubility in ringwoodite and oxygen fugacity

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The recently discovered ringwoodite inclusion in a diamond was found to contain more than 1.4% water, which suggests that the mantle transition zone is hydrous, at least locally. An open question is whether the transition zone is water-rich globally or locally. Oxygen fugacity in the mantle transition zone is considered to be very low: just below the iron-wustite (IW) buffer. Thermodynamic investigation suggests that the fraction of water in fluids decreases with decreasing oxygen fugacity due to the dissociation of water into hydrogen and oxygen. This decrease in the water fraction in fluids should decrease the maximum water content in ringwoodite, and therefore the water-rich and highly reduced conditions could be incompatible. If the majority of the transition zone were under reduced conditions, the transition zone could not be water-rich, making the water-rich condition a local feature. In order to prove this hypothesis, we will determine the maximum water content in ringwoodite as a function of oxygen fugacity under transition-zone conditions.We will measure the maximum water content in ringwoodite in the following three systems: 1) Mg2SiO4 ringwoodite + H2O-H2 fluid, 2) Mg2SiO4 ringwoodite + diamond + H2O-CH4 fluid, and 3) ringwoodite with natural olivine composition + diamond + H2O-CH4. Hydration experiments will be conducted at a pressure of 19.5 GPa and a temperature of 1400 K using a large-volume multi-anvil press. A modified double-capsule technique will be adopted to equilibrate the sample, oxygen buffer, and fluid: the sample and water will be loaded in the inner capsule, and a buffer and water will be loaded between the inner and outer capsules. Eleven oxygen buffers of Pt-PtO2, Fe3O4-Fe2O3, Ni-NiO, Re-ReO2, FeO-Fe3O4, Mg2SiO4-MgSiO3-MgCO3-C, Mo-MoO2, Fe-Fe(x)O, Cr-Cr2O3, Si-SiO2, and Zr-ZrO2 will be used for the first system. Six buffers of Mg2SiO4-MgSiO3-MgCO3-C, Mo-MoO2, Fe-Fe(x)O, Cr-Cr2O3, Si-SiO2, and Zr-ZrO2 will be used for the second system. Five buffers of Fe-FexO, Mo-MoO2, Re-ReO2, Ni-NiO2, and FexO-Fe3O4 will be used for the third system. The water content in the transition zone will be investigated based on the relationship between water content and oxygen fugacity.