That Earth has always been such an active planet is largely due to water having continually being shifted into the mantle by subduction of oceanic lithosphere. Emplaced at temperatures around 1200°C the basaltic crust and ultramafic rocks of the lithospheric mantle become hydrothermally altered by interaction with the ocean, so that they contain a range of hydrous minerals. The mantle is estimated to contain between a quarter and four times the present volume of all ocean water. The vast bulk of the mantle is not undergoing partial melting at any one time. Most magmatic activity is linked to plate tectonics through linear belts such as those along oceanic rift systems and above subduction zones, with a small proportion at ocean islands above isolated mantle plumes and similarly, though more sparsely above hot spots below continents. Such plume-related magmatism has at intervals in the past been vastly bigger than now, forming flood-basalt provinces and ocean-floor plateaus, such as the Deccan Traps and the Ontong Java Plateau; some linked to extinction events. The particular role of water in mantle melting is its reduction in the temperature at which melting begins at depth: ‘dry’ mantle does not melt but remains solid, albeit ductile. Two recent studies have provided important insights into previously unsuspected roles that water can play deep in the mantle.
Jianfeng Yang of the Chinese Academy of Science and colleagues from China and University of Padua, Italy provide evidence that ancient subducted slabs that gather at the mantle transition zone (MTZ) may trigger ocean-island and ocean-plateau volcanism (Yang, J. et al. 2026. Subduction legacies in the mantle transition zone modulate intraplate oceanic volcanism. Nature Communications, in press; DOI: 10.1038/s41467-026-73403-7). In fact there is a multiplicity of modes wherein subducted slabs interact with the MTZ, some are retained within it while, in one way or another, others eventually pass through it to the deeper mantle. Long-dead relics of subduction zones trapped there form ‘reservoirs’ of water 410 to 660 km below the surface at concentrations far higher (1 to 3 %) than does pristine mantle (less than 0.1%). It is stored as OH ions in dense mafic minerals, such as ringwoodite a high-pressure form of olivine (Mg2SiO4) containing up to 2.6 % of OH ions, and bridgmanite (MgSiO3), which forms once subducted slabs pass into the mantle transition zone. If that transformed lithosphere rises above about 410 km, such minerals transform back into anhydrous olivine, thereby liberating their water. At such depths, where temperature in the surrounding dry mantle is about 1800°C the emergence of water triggers a decrease in the temperature at which the ancient slab and also the surrounding mantle can melt. The authors cite evidence that such a process has contributed to the Azores oceanic plateau where the crust is 10 to 20 km thick. It is conceivable that a similar process of deep water ‘recycling’ may have been associated with continental flood basalts. Yang et al.’s new insight may also help unravel hitherto puzzling geochemical anomalies in other kinds of basaltic igneous rocks, such as those which well-up at mid ocean ridges to form modern oceanic crust.
Slabs that descend deeper into the mantle retain their dense mafic minerals and thus the water trapped within them. That water may eventually be involved in transformations at much higher pressures and temperatures, as deep as the core-mantle boundary. One possibility is their retention in mantle plumes that rise from the CMB to facilitate partial melting once they pass through the MTZ
See also: Subduction legacies shape intraplate ocean volcanoes. Scienmag, 20 May 2026
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