Hydrous Leaching in the Lithospheric Mantle Beneath East China Indicated by Sapphire Deposits

Hydration of the lithospheric mantle is a critical part of deep water cycling and is essential in shaping Earth's habitable environments. At low water/rock ratios, water is stored in nominally anhydrous minerals or hydrous minerals; however, at high water/rock ratios, the leaching effect of water on mantle minerals remains poorly understood. Here, we present hydrothermal experiments of the ‘phlogopite + H 2 O’ system at 500°C–950°C and 0.9–1.8 GPa, showing that incongruent dissolution of phlogopite in water produces corundum (Al 2 O 3 ) as an insoluble residual phase at pressures above ∼1.5 GPa. Water leaches soluble components (K, Mg and Si) from phlogopite, leaving corundum as the residue under lithospheric mantle P‒T conditions. This provides a mechanistic explanation for sapphire formation in the lithospheric mantle, consistent with natural sapphire gemstones occurring as mantle xenoliths and xenocrysts within alkaline basalts. Sapphires form through hydrous leaching of phlogopite at depths greater than ∼50 km and are subsequently transported to the surface by alkaline basaltic magmas originating from at least ∼70 km depth. The widespread sapphire deposits associated with alkaline basalts in East China indicate extensive hydrous leaching of the lithospheric mantle, with water ultimately sourced from the subducted slab, mantle transition zone or mantle plumes.