This paper reports on an experimental study of water solubility in majorite garnet under high pressure and temperature conditions relevant to the Earth's mantle transition zone and uppermost lower mantle. The main findings are:

• Water solubility in majorite decreases with increasing temperature, from about 2900 wt ppm at 1670 K to 400 wt ppm at 2270 K at 20 GPa pressure.
• Water solubility does not depend strongly on majorite composition (Al₂O₃ and SiO₂ content).
• Majorite can store more water than bridgmanite in the lower mantle, making it an important water reservoir in the topmost lower mantle (660-800 km depth).
• The results suggest hydrous melting may occur over a broader depth range (660-800 km) during slab subduction into the lower mantle, due to the contrast in water solubility between majorite and bridgmanite.

The study provides new constraints on water storage and transport in the deep mantle, with implications for mantle dynamics and melting processes at the top of the lower mantle.

Figure. Logarithmic H₂O contents in majorite as a function of resiprocal temperature.

Liu, D., Purevjav, N., Fei, H., Withers, A.C., Ye, Y., Katsura, T., 2024. Temperature and compositional dependences of H_2,O solubility in majorite. Am. Miner. 109, 1646-1652. 10.2138/am-2023-9130