On the thermal evolution of the mantle wedge at subduction zones

We use a 2-dimensional numerical geodynamic model to investigate the evolution of the mantle wedge at subduction zones over a period of 300 Myr following the onset of subduction. A variety of parameterizations of the overriding plate, as well as plate subduction rates and mantle potential temperatures, are considered systematically. The average temperature in the mantle wedge within 120 km of the trench is found to decrease substantially (up to ∼100 °C) over this period, with the most rapid cooling occurring during the first 100 Myr. The observed cooling is not spatially uniform, but rather is concentrated in the uppermost 50 km of the mantle wedge, immediately beneath the overriding plate. Temperatures in the mantle wedge in the vicinity of arc volcanism decrease by as much as >200 °C over the 300 Myr period. This decrease in temperature results in a decrease in the portion of mantle wedge beneath the arc in which hydrous melting can occur, impacting both the distribution of melting within the wedge and likely the composition of erupted melts. The observed variations in temperatures in the subarc portion of the mantle wedge with time are significantly larger than the changes in temperature resulting from inclusion of radiogenic heat production in the overriding plate or a zone of partial coupling between the slab and mantle. The results suggest that the mantle wedge is warmer than predicted by steady state thermal models, which may account in part for the differences between the petrologically determined pressure and temperature of equilibration for arc magmas and predictions of the thermal structure of the mantle wedge obtained from steady state thermal models.

► Mantle wedge cools with time.
► Slab surface cools with time.
► Mantle wedge not in steady state.