Boron isotope systematics of vent fluids from the Mid-Atlantic Ridge (MAR) and the faster spreading East Pacific Rise (EPR) reveal that at the MAR ~50% of seawater boron is removed from circulating fluids on the downwelling limb of the hydrothermal convection cell, whereas little boron is removed at EPR sites. Geophysical and petrographic studies suggest that downwelling fluids on the MAR penetrate deep in the oceanic crust and upper mantle where boron is removed during serpentinization of peridotite. The associated hydration of the oceanic crust reduces its density and modelling of the data suggests that this results in uplift, leading to significant elevation of the median valley walls in the vicinity of active hydrothermal systems. ¿ American Geophysical Union 1996