Hydration of the lithospheric mantle above big mantle wedges indicated by sapphire deposits

The deep water cycle is pivotal in shaping Earth’s habitable environments. A fundamental process of this cycle is upward migration of water from Earth’s deep interior. A significant inquiry concerns how water released from the deep mantle hydrates the lithosphere. Here we report hydrothermal experiments of the “phlogopite + H ₂ O” system, showing that the incongruent dissolution of phlogopite in water produces sapphire (Al ₂ O ₃ ) under lithospheric mantle P-T conditions. Our results suggest that sapphire can be leached from phlogopite in the lithospheric mantle by excess water, and subsequently transported to the surface by basaltic magmas. We propose that the magmatic sapphire deposits in eastern China, Southeastern Asia, and eastern Australia provide evidence of lithospheric mantle hydration. The water that leached the lithospheric mantle mainly originated from the mantle transition zone or subducted slabs, while in eastern Australia and Hainan Island, mantle plumes may also contribute. The occurrence of sapphire deposits indicates extensive hydration of lithospheric mantle in big mantle wedges.