The amount of water and volatiles stored in the downgoing plate and the depth at which they are released into the mantle wedge are key variables in the mass balance of earth’s mantle, but are difficult to constrain. The Lesser Antilles subduction zone is a global end member for slow subduction of slow spread, highly tectonized, hydrated oceanic lithosphere, which could result in variable rehydration and volatilization of the mantle. Here we present a 3D shear velocity model from Rayleigh wave tomography and a series of numerical experiments of mantle flow to constrain the degree of hydration and partial melting of the back arc mantle wedge. We find the average shear velocity of the upper 20 km of the mantle of the incoming plate varies from 4.74 ±0.08 km/s in the south to 4.54 ±0.08 km/s in the north suggesting an addition of 2-3 weight % water to the upper mantle in the north due to serpentinization and a relatively dry mantle in the south. Beneath the arc/back arc behind the largest northern islands in the arc, Guadeloupe/Martinique, we image low shear velocities (4.20±0.05 km/s) at 70-90 km depth. Based on comparisons with geodynamic models of the thermal structure of the Lesser Antilles we infer an equivalent of 3000 H/106Si in the mantle as either a free fluid phase or melt/fluid-the equivalent of ~ 1% melt fraction, with 0.02 weight % water at 2 GPa and 1200 °C. The seismic observations and thermal modelling suggest a cold slab that releases a significant amount of water 140-160 km depth to create the anomalies in the back arc. The melt may pond beneath the fast lid and move further into the back arc in the north. The north-south variation in the incoming plate structure is likely related to a change from mid-Atlantic slow spread lithosphere in the north and lithosphere related to continental break up in the south. Stark variations the hydration incoming plate result in stark variations in mantle wedge hydration and volcanic arc production.