Hybridization of magmas above subducted oceanic crust

Abstract

Hydrous siliceous melts rising from subducted oceanic crust may experience hybridization with overlying peridotite. Products of hybridization have been explored in the synthetic system KAlSiO_4-MgO-SiO_2-H_2O. The phase diagrams were used to interpret phase relationships determined at 30 kbar for mixtures of granite, peridotite, and H_2O. Results indicate that solution of peridotite in hydrous, H_2O-undersaturated granitic liquid at 900°C, 30 kbar, causes only small changes in liquid composition, and precipitation of phlogopite-garnet-websterite. Additional experiments, set up with H_2O-undersaturated granite liquid in contact with solid peridotite at 30 kbar, 850-1050°C, produced hybrid reaction zones containing orthopyroxene and clinopyroxene in all runs, and one or more of garnet, phlogopite, and quartz in some runs. The hybridization process in subduction zones could produce discrete rock bodies dominated by pyroxenite (without olivine); there is a prospect that potassium could become concentrated into phlogopite-rich rocks. The heterogeneouss rock layer produced above subducted crust provides aqueous fluids expelled during hybridization, which rise to generate in overlying mantle the H_2O-under-saturated basic magnesian magma which is the probable parent of the calc-alkalic rock series erupted at the volcanic front.