This is a computer model of the thermal structure of a subduction zone (convergence at 6 cmm/year) showing how cool rocks of the slab pass to great depths, only warming gradually. If this type of model is correct (and it is consistent with seismic velocity information) it suggests that the ocean crust of the subducted slab will only melt at depths greater than 600 km. In fact the volcanic line of island arcs lies above the slab which has reached depths of just 120-140 km. This modelling shows the temperature there to be just 200-300C - far too cool for the ocean crust too melt - but warm enough to drive off fluid into the overlying mantle wedge.
