Determining the water dissociation constant from the relationship between water solubility in ringwoodite and oxygen fugacity
Final Report Abstract
The primary aim of this project was to determine the H2O dissociation coefficient of fluids and melts under mantle-transition-zone conditions by measuring the H2O contents in Mg2SiO4 ringwoodite with a wide range of the oxygen fugacity and investigate whether the H2O-rich conditions indicated by the hydrous ringwoodite inclusion in diamond is compatible with the reducing conditions. We measured the H2O contents of wadselyite instead of ringwoodite at a pressure of 17 GPa, a temperature of 1670 K and oxygen fugacity of -22 to +12 log bars and demonstrated that the Mg2SiO4 wadsleyite H2O content is independent of the oxygen fugacity. This is because that H2O is stored not as H2O molecules but OH- groups in silicate melts, in which the OH- groups are strongly bonded. Since it was found that the H2O dissociation does not occur in silicate melts in the mantle transition zone, we measured the maximum H2O solubility in transition zone minerals, wadsleyite, ringwoodite, and majorite. We determined that the water solubility of iron-bearing wadsleyite is 0.6~2.4 wt.% in the transition zone temperature. We demonstrated that both iron-free and iron-bearing ringwoodite can store 0.8-1.2 wt.% of H2O under mantle transition zone conditions. The solubility of H2O in majorite decreases from 0.73 to 0.13 wt. % H2O with increasing temperature from 1670 to 2270 K, and is independent of the chemical composition. We also demonstrated that protons are incorporated in the Si site in olvine.
Publications
- High water solubility of ringwoodite at mantle transition zone temperature. Earth Planet. Sci. Lett., 531, 115987, 2020
Fei, H., Katsura, T.
(See online at https://doi.org/10.1016/j.epsl.2019.115987) - Pressure dependence of proton incorporation and water solubility in olivine. J. Geophys. Res.: Solid Earth, 125, e2019JB018813, 2020
Fei, H., Katsura, T.
(See online at https://doi.org/10.1029/2019JB018813) - Independent hydrogen incorporation in wadsleyite from oxygen fugacity and non-dissociation of H2O in the reducing mantle transition zone, Earth Planet. Sci. Lett., 557, 116755, 2021
Druzhbin, D., Fei, H., Katsura, T.
(See online at https://doi.org/10.1016/j.epsl.2021.116755) - Water solubility in Fe-bearing wadsleyite at mantle transition zone temperatures, Geophys. Res. Lett., 48, e2021GL092836, 2021
Fei, H., Katsura, T.
(See online at https://doi.org/10.1029/2021GL092836)
