Pressure dependence of H solubility in magnesiowüstite up to 25 GPa: Implications for the storage of water in the Earth's lower mantle

Abstract

The solubility of hydrogen in (Mg0.93Fe0.07)O was studied from 5 to 25 GPa at 1200°C under oxidizing conditions, using FTIR spectroscopy on quenched single crystals. OH solubility increases with increasing pressure, concomitant with a decrease in ferric iron content. This result suggests that under lower mantle conditions, the charge neutrality condition could change from [FeṀe] = 2[V″Me] to [(OH)̇o] = 2[V″Me], which would have important implications for the rheology of that region. The OH solubility data are well explained by incorporation of H as isolated hydroxyl groups via reduction of ferric iron. The amount of H incorporated at 25 GPa and 1200°C is ∼100 H/106 Me (Me = Mg, Fe), i.e. ∼20 ppm wt H2O, under water-saturated conditions. When integrated over the entire mass of the lower mantle, these 20 ppm wt H2O amount to about 7.3 × 1018 kg H2O, a storage capacity of ∼0.5% of the ocean mass.