THE fate of pelagic sediments in subduction zones has an important bearing on global geochemical cycles and the thermal structure of subduction zones. Recent mass-balance calculations(1) have indicated that about 20% of subducted sediments are recycled to volcanic arcs. Trace-element studies(2) of these volcanic arcs imply that the subducted sediment melts while the gabbroic crust that underlies it dehydrates. This requires a rather specific thermal structure in subduction zones. Here we report laboratory melting experiments that allow us to derive melting curves for pelagic red clay at pressures of up to 40 kbar, equivalent to depths of 120 km within the mantle. The melting behaviour of wet red clay is similar to that for wet gabbro, but melting occurs at a slightly tower temperature, showing that sediment melting and gabbro dehydration can occur at the same temperature. The combination of trace-element data and phase diagrams such as that derived here mag thus be used to constrain the temperature of the slab-mantle boundary.