Metamorphic chemical geodynamics of subduction zones

Study of metamorphic suites directly representing the deep subduction of altered oceanic crust and sediments can help elucidate the geochemical evolution of the forearc-to-subarc slab–mantle interface, the nature of slab-derived fluids added to arc lava source regions, and the chemical changes in subducting rocks potentially contributing to the geochemical heterogeneity of the deeper mantle. The stage is set for more focused geochemical study of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic suites incorporating knowledge of mineral chemistry and reactions, kinetics and disequilibrium, prograde and exhumation-related P–T paths, fluid flow and fluid–rock interactions, and experimental evidence for the physical and chemical properties of fluids at the pressures and temperatures experienced by subducting slabs. Detailed studies of HP and UHP metamorphism published thus far provide some insight regarding the geochemical effects of devolatilization and the element transport capabilities of metamorphic fluids in forearc regions of subduction zones (< 100 km). Future research in this rapidly developing field should prioritize study of UHP suites metamorphosed at depths > 100 km, which have potentially experienced partial melting, with the goal of better characterizing fluid additions to arcs from subducting slabs and sediments and the compositions of dehydration and melting residues that could impact geochemical compositions of the mantle including source regions for ocean island basalts.