The system granite-peridotite-H_2O at 30 kbar, with applications to hybridization in subduction zone magmatism

Abstract

Experiments with mixtures of granite, peridotite and H_2O at 30 kbar were designed as a first step to test the hypothesis that the calc-alkaline igneous rocks of subduction zones are formed by differentiation of magmas derived by partial melting of hybrid rocks generated in the mantle wedge, by reaction between hydrous siliceous magma rising from subducted oceanic crust, and the overlying mantle peridotite. Experiments were conducted in gold capsules in half-inch diameter piston-cylinder apparatus. Results are presented in a 900° C isotherm, and in a projection of vapor-present phase fields onto T-granite-peridotite. Isobaric solution of peridotite in hydrous, H_2O-undersaturated granite liquid at 900° C causes only small changes in liquid composition, followed by precipitation of orthopyroxene until about half of the liquid has solidified; then orthopyroxene is joined by jadeitic clinopyroxene, garnet, and phlogopite. Phlogopite-garnet-websterite continues to be precipitated, with evolution of aqueous vapor, until all of the liquid is used up. The product of hybridization is a pyroxenite without olivine. The products of partial melting of this material would differ from products derived from peridotite because there is no olivine control, and the clinopyroxenes contain up to 7% Na_2O, compared with less than 1% Na_2O in peridotite clinopyroxenes. The reaction products are directly analogous to those in the model system KalSiO_4-Mg_2SiO_4-SiO_2-H_2O, where, with decreasing SiO_2 in the hydrous siliceous liquid, the field for phlogopite expands, and phlogopite instead of orthopyroxene becomes the primary mineral. If this occurs with less siliceous magmas from the subducted oceanic crust, there is a prospect for separation of discrete bodies of phlogopite-rock as well as phlogopite-garnet-websterite. We need to know the products of hybridization, and the products of partial melting of the hybrid rocks through a range of conditions.