H 2 O subduction beyond arcs

The amount of H 2 O subducted to postarc depths dictates such disparate factors as the generation of arc and back‐arc magmas, the rheology of the mantle wedge and slab, and the global circulation of H 2 O. Perple_X was used to calculate phase diagrams and rock physical properties for pressures of 0.5–4.0 GPa and temperatures of 300–900°C for a range of bulk compositions appropriate to subduction zones. These data were merged with global subduction zone rock fluxes to generate a model for global H 2 O flux to postarc depths. For metasomatized igneous rocks, subducted H 2 O scales with bulk rock K 2 O in hot slabs. Metasomatized ultramafic rocks behave similarly in cold slabs, but in hot slabs carry no H 2 O to magma generation depths because they lack K 2 O. Chert and carbonate are responsible for minimal H 2 O subduction, whereas clay‐rich and terrigenous sediments stabilize several hydrous phases at low temperature, resulting in significant postarc slab H 2 O flux in cold and hot slabs. Continental crust also subducts much H 2 O in cold slabs because of the stability of lawsonite and phengite; in hot slabs it is phengite that carries the bulk of this H 2 O to postarc depth. All told, the postarc flux of H 2 O in cold slabs is dominated by terrigenous sediment and the igneous lower crust and mantle and is proportional to bulk rock H 2 O. In contrast, in hot slabs the major contributors of postarc slab H 2 O are metasomatized volcanic rocks and subducted continental crust, with the amount of postarc slab H 2 O scaling with K 2 O. The Andes and Java‐Sumatra‐Andaman slabs are the principal suppliers of pelagic and terrigenous sediment hosted H 2 O to postarc depths, respectively. The Chile and Solomon arcs contribute the greatest H 2 O flux from subducted continental and oceanic forearc, respectively. The Andean arc has the greatest H 2 O flux provided through subduction of hydrated ocean crust and mantle. No correlation was observed between postarc slab H 2 O flux and slab seismicity.