Figure 2.08. Subduction Zone Temperatures. Temperature contours in degrees Celsius based on a subduction zone model by Kelemen and others (2003, figure 6B). Convection moves cold oceanic lithosphere into the hot mantle too fast for temperature equilibration at shallow depths. Temperature contours show that the subducted lithosphere is much cooler than the surrounding mantle. The dark gray band represents ocean crust, which has a basalt/gabbro composition. None of the temperatures and pressures shown for the ocean crust are above the experimentally determined minimum melting temperatures for gabbro shown in Figure 2.09, even in the presence of excess H2O. Therefore, melting of ocean crust is not likely to be the source of magma for volcanic arcs at convergent plate boundaries.