The mantle transition zone, a potential water reservoir in the Earth’s interior, is suggested to contain more than 1 wt.% of H2O, at least locally, by the natural water-rich ringwoodite inclusion (Pearson et al., 2014) and the mineral viscosity (Fei et al., 2017). However, laboratory experiments suggested that the H2O solubility of ringwoodite significantly decreases with increasing temperature. With a typical mantle geotherm, i.e., ~2000 K at the lower part of mantle transition zone, the H2O solubility of ringwoodite is only 0.3 wt.% (Ohtani et al., 2000), which is against the idea of water-rich mantle transition zone.
Here we revisited the H2O solubility in ringwoodite at 23 GPa, 1600-2000 K, corresponding to mantle transition zone conditions. We found that when the water content in the starting material is low (<15%) and without SiO2 buffer, the water content in ringwoodite is starting-material-dependent and undersaturated. In contrast, with higher bulk water content (≥ 15%) or with SiO2 buffer, the water content in ringwoodite is independent from starting material.
The experimental results show that both iron-free and iron-bearing ringwoodite can store about 0.8-1.2 wt.% of H2O under mantle transition zone conditions. Temperature has a relatively small effect on the H2O solubility in iron-bearing system, but much larger in iron-free samples. The high H2O solubility of ringwoodite is compatible with the water-rich mantle transition zone. The relatively low values reported previously is probably due to insufficient H2O source or unbuffered by SiO2.
Temperature dependence of H2O content or solubility in ringwoodite. The data sets labeled with Fo100+5%H2O (Mg2Si1.1O4.2), Fo100+15%H2O , Fo90+22%H2O , Fo90+15%H2O , and Fo75+15%H2O (starting material and it’s water content) should represent the H2O solubility in ringwoodite. The percentages next to the data points are the initial H2O contents in the starting materials.
Fei, H., Katsura, T., High water solubility of ringwoodite at mantle transition zone temperature. Earth Planetary Science Letters, 531, 115987, 2020.