P.E. van Keken, B. Kiefer, and S. Peacock,
High resolution models of subduction zones: Implications for mineral
dehydration reactions and the transport of water into the deep mantle
Geochemistry, Geophysics, Geosystems, in press, 2002.
Arc volcanism is intimately linked to mineral dehydration reactions in the subducting oceanic mantle, crust, and sediments. The location of slab dehydration reactions depends strongly on the temperature and pressure conditions at the top of the subducting plate and hence on the detailed thermal structure of subduction zones. A particularly important physical property of these models is the viscosity of the mantle wedge. We constructed a new high resolution finite-element model to re-evaluate the thermal structure of the Honshu and Cascadia subduction zonesassuming a more appropriate olivine rheology for the mantle wedge. Induced corner flow in the mantle wedge generates high temperatures at the slab-wedge interface that are consistent with Th and Be evidence for sediment melting. In the Honshu subduction zone, deeper portions of the subducting oceanic crust and upper mantle remain relatively cool due to the age of the Pacific plate and the rapid convergence. This is consistent with B data suggesting low temperature dehydration of the oceanic crust. Low temperatures persist to great depth in the shallow upper mantle of the subducting plate, which allows for water transport in the form of hydrous minerals to the deep mantle. The strong temperature gradient at the top of the subducting plate may thus reconcile conflicting estimates of slab temperature in subduction zones that are characterized by rapid subduction of mature oceanic lithosphere.