MELT GENERATION BENEATH PLATE INTERIORS

Melt generation in the mantle results from convective motions, either associated with plate boundaries of with upwelling beneath plate interiors. The second provides an important constraint on the planform of mantle convection, which is otherwise hard to map. Numerical models of convection in a layer of fluid driven by heating from below show that the planform of high Rayleigh number convection (~10^6) is spoke pattern, with approximately circular upwellings and downwellings, the hubs, joined by hot and cold sheets, the spokes. Such models also show that the melting rates are strongly controlled by the lithospheric thickness, and vary by about three orders of magnitude between the hubs and the spokes. In the hubs melting occurs only at depths of less than about 150 km unless the water content exceeds 0.01%. The much smaller melt generation rate in the spokes occurs only at depths of less than about 80 km. These results, together with those from satellite gravity fields and surface wave tomography, provide the framework required to relate the distribution of volcanism in the western US and the Middle East to the underlying mantle circulation. The spoke pattern is particularly clear beneath North Africa, Arabia and Turkey.