The deep subduction fluids in high and ultrahigh pressure rocks and their interaction with the overlying mantle wedge

The crust-to-mantle mass transfer at subduction zones is mediated by the fluid phases released by the subducting plates. The deep slab fluids may correspond to aqueous solutions, to hydrous silicate melts, or to transitional supercritical fluids, enriched in large ion lithophile (LILE), light (B, Li) and light rare earth (LREE) elements. Despite the recent advancements in understanding the slab agents, yet uncertain is their fate inside the mantle wedge, after departure from the slab source. Antigorite dehydration releases LILE, Li and B rich aqueous fluids which are able to carry several components of arc magmas. The infiltration of such aqueous fluids into sedimentary and/or granitic layers of the slab may enhance partial melting and/or release of transitional supercritical fluids enriched in the whole range of incompatible element detected in arc lavas. However, the long-range migration of such fluid phases inside the mantle is unclear. Constraints can be gained by orogenic UHP ultramafic rocks, which are associated with deeply subducted crust and which are frequently metasomatized. A relevant case study is represented by some garnet orthopyroxenites hosted by garnet–coesite gneisses (Maowu area, Dabie Shan, China). Such pyroxenites contain orthopyroxene (opx2) + garnet (gnt2) ± clinopyroxene. Opx2 replaces olivine from a previous garnet-harzburgite assemblage: its formation is caused by the influx at UHP (4.0–6.0 GPa, 700–750 °C) of a SiO2, Al2O3 LREE (and LILE)-rich agent (likely a hydrous granitic melt), sourced from the associated crustal rocks. Gnt2 contains primary polyphase inclusions, corresponding to a solute-rich aqueous fluid enriched in LILE and LREE. These features point to a crustal origin of the trapped fluid, showing an incompatible element signature quite similar to the one of arc lavas. We suggest that hydrous felsic melts reacted with mantle peridotites to produce a metasomatic garnet–orthopyroxenite layer together with a residual aqueous fluid which concentrates LILE and LREE. This fluid phase is in equilibrium with mantle minerals and is the ultimate transfer agent of the ‘subduction component’ inside the mantle wedge. The trace element fingerprint of wedge-type Alpine amphibole + garnet peridotites (Ulten Zone, Italian Alps) is similar to the one of such residual fluid and indicate that this process represents a general feature and is a valid proxy for the crust-to-mantle exchange processes in these settings.