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.

Fei, H., Katsura, T., High water solubility of ringwoodite at mantle transition zone temperature. Earth Planetary Science Letters, 531, 115987, 2020.