2.7 Why? Evidence against slab melting
As was explained in Figure 2.04, the mantle is not molten. Something must cause melting in a convergent margin setting. To evaluate the possibilities, we need to know the temperature distribution in subduction zones. Temperatures cannot be directly measured below the depth of the deepest drill holes (a few kilometers), so we must rely on geophysical models of the thermal budget of subduction zones. Many geologists have published results of numerical models of subduction zone temperatures with a considerable range of outcomes that vary with subduction angle, convergence rate, viscosity, friction, and thermal conductivity of the rocks (see for example, Grove and others, 2009). All of the models show that temperatures are lower in subduction zones than in the surrounding mantle because "cold" lithosphere is being carried down by convection into the otherwise "hot" mantle (see Figure 2.08 above left).
Figure 2.08 is based on the results of Kelemen and others (2003, figure 6B). Temperatures in the subducted lithosphere do not rise above about 600°C at the approximately 100 km depth-to-slab that is observed beneath many volcanic arcs (see for example, Grove and others, 2009). These temperatures are not high enough to melt the basalt/gabbro of the ocean crust or the underlying peridotite of the slab. Even though the melting temperatures of both gabbro and peridotite are lowered by the presence of water (see Figure 2.09 above right), downgoing slab temperatures do not cross the gabbro liquidus or the peridotite liquidus in the region below volcanic arcs.
If the ocean crust or mantle in the subducting lithosphere is not melted, there must be another reason for magma production in subduction zones. That reason turns out to be H2O.
Figure 2.08. Subduction Zone Tempera-tures. Temperature contours based on a subduction zone model by Kelemen and others (2003). Convection moves cold oceanic lithosphere into the hot mantle too fast for temperature equilibration at shallow depths. Click on the diagram for more information, including a link to the data source.
Figure 2.09. Dry and Wet Gabbro Melting. Beginning of melting curves (solidus) for dry gabbro and for wet gabbro. Notice that in the presence of water under pressure (P=PH2O), gabbro melts at much lower temperatures than dry gabbro. Click on the diagram for more information, including the data sources.
