A numerical approach to melting in warm subduction zones

- A thermomechanical/petrological model of subduction zone is presented.
- We constrain dehydration, re-hydration and melting in a warm subduction zone.
- Warm slab crust devolatilizes completely at shallow depth.
- Crust melting is only achieved if H2O is advected from the deeper part of the slab.
- Warm subduction zone leads to extensive mantle wedge melting at shallow depth.

The complex feedback between dehydration and melting in hot subduction zones is quantitatively addressed in this study. We present an integrated numerical tool that combines a high-resolution thermo-mechanical subduction model with a thermodynamic database that allows modeling metamorphic devolatilization, and subsequent re-hydration and melting reactions. We apply this tool to quantify how the hydration state of a lithologically layered subducting slab varies during interaction with the hot mantle wedge and how this affects any melting taking place in the subducting crust or the overlying mantle wedge. Total crustal dehydration is achieved before any crustal melting can occur, even in very young subducting slabs. Significant oceanic crust melting is only achieved if the metamorphic fluids from the dehydrating underlying subducting slab mantle are fluxed through the dry eclogites. But our models further demonstrate that even if the oceanic crust can melt in these specific conditions, the preceding crustal dehydration will simultaneously result in extensive mantle wedge melting at lower pressures than for colder slabs. The significant mantle wedge melting implies that also for hot subduction zones, most of the melt feeding the overriding plate is of mantle origin.