The source rocks for basaltic lavas that form ocean islands are often inferred to have risen as part of a thermal plume from the lower mantle. These rocks are water-rich compared with average upper-mantle rocks. However, experiments indicate that the solubility of water in the dominant lower-mantle phases is very low, prompting suggestions that plumes may be sourced from as-yet unidentified reservoirs of water-rich primordial material in the deep mantle. Here we perform high-pressure experiments to show that Al2 SiO4 (OH)2-the aluminium-rich endmember of dense, hydrous magnesium silicate phase D-is stable at temperatures extending to over 2,000 °C at 26 GPa. We find that under these conditions, Al-rich phase D is stable within mafic rocks, which implies that subducted oceanic crust could be a significant long-term water reservoir in the convecting lower mantle. We suggest that melts formed in the lower mantle by the dehydration of hydrous minerals in dense ultramafic rocks will migrate into mafic lithologies and crystallize to form Al-rich phase D. When mantle rocks upwell, water will be locally redistributed into nominally anhydrous minerals. This upwelling material provides a potential source for ocean-island basalts without requiring reservoirs of water-rich primordial material in the deep mantle.