Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems(1). Serpentinized mantle rocks are found beneath the sea floor at slow-to ultraslow-spreading mid-ocean ridges(1) and are thought to be present at about half the world's rifted margins(2,3). Serpentinite is also inferred to exist in the downgoing plate at subduction zones(4), where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults(3,4). Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that seawater reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth.
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