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Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging1. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform2. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc3, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine—that is, hydrated mantle rather than crust or sediments—is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards. Serpentine subducted below the Lesser Antilles volcanic arc supplies water to the arc, controlling the location of seismicity, volcanic productivity and thickness of crust.

In the North Atlantic Ocean, a mid-oceanic ridge bisects the Icelandic mantle plume, providing a window into the temporal evolution of this major convective upwelling 1–3 . It is generally accepted that this plume’s transient behavior is indirectly recorded within the fabric \ud of oceanic floor south of Iceland 4–7 . Despite its significance, the structure of this region is \ud poorly known. Here, we present long seismic reflection profiles that traverse the oceanic basin between northwest Europe and Greenland. A diachronous pattern of V-shaped ridges is imaged beneath a thickening blanket of sediment, revealing a complete record of transient periodicity that can be traced continuously back to ∼ 55 Myrs— the longest record of its kind. This periodicity increases from ∼ 3 to ∼ 8 Myr with clear evidence for minor, but systematic, asymmetric crustal accretion. V-shaped ridges grow with time and reflect small (e.g. 5–30◦\ud C) fluctuations of mantle temperature, consistent with quasi-periodic generation of hot solitary waves triggered by growth of thermal boundary layer instabilities within the mantle . Our continuous record of convective activity predicts a history of regional elevation change which moderated overflow of the Neogene precursor of North Atlantic Deep Water and which controlled the growth and decay of multiple Paleogene buried landscapes.