Experimental Simulation of Mantle Hybridization in Subduction Zones

Abstract

Experiments conducted at 30 kbar, 850-1050°C, in piston-cylinder apparatus simulate hybridization between hydrous siliceous magma rising from subducted oceanic crust into overlying peridotite. Gold capsules containing granite and peridotite powders separated by sharp boundaries, and H_2O, were run in vertical and horizontal positions. The aqueous vapor caused minor metasomatic changes in the peridotite until it was withdrawn into the melting granite, producing H_2O-undersaturated granite liquid adjacent sintered, anhydrous peridotite. Minerals developed in the hybrid reaction zones between liquid and peridotite are orthopyroxene and jadeitic clinopyroxene in all runs, and one or more of garnet, phlogopite, and quartz in some runs. (The granite liquid moves upwards relative to the peridotite even in runs of 27 hours.) In the longest runs of 120 hours the hybridization zone remains narrow, the body of granite liquid remains crystal-free and only slightly changed in composition (lower SiO_2, MgO increased from 0.1% to about 1.5%). The products are consistent with phase equilibria in synthetic model systems, and the system peridotite-granite-H_2O (determined using mixtures). The hybridization process in subduction zones would produce discrete rock bodies dominated by pyroxenites without olivine. Na is fixed in jadeitic clinopyroxene. K is fixed in phlogopite dispersed through the pyroxenite, but there is a prospect that it may become concentrated into phlogopite-rich rocks by crystal fractionation. Partial melting of these source rocks would generate magmas different from those generated in peridotite or subducted oceanic crust.

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