Synthetic Systems for Modeling Hybridization between Hydrous Siliceous Magmas and Peridotite in Subduction Zones

Abstract

The system KalSiO_4-Mg_2SiO_4-SiO_2-H_2O includes model representatives of (1) hydrous siliceous magma from subducted oceanic crust (the eutectic liquid in KalSi_3O_8-SiO_2-H_2O, and (2) the overlying mantle peridotite (forsterite+enstatite). Isothermal sections at 20 kbar to 30 kbar illustrate that hybridization causes precipitation of phlogopite, quartz, and enstatite, with little change in content of the liquid. The effects of CaO, Na_2O, and Al_2O_3 are examined by reviewing available data on phase relationships with additional components CaMgSi_2O_6, NaAlSiO_4, Al_2O_3 , and CaAl_2Si_2O_8. Addition of Na_2O and CaO changes the hybridization reactions by reducing the amount of phlogopite and by adding jadeitic clinopyroxene to the precipitation products, Addition of CaO and Al_2O_3 includes garnet among the precipitation products. The synthetic model systems appear to represent a good basis for extrapolation to the complex natural rock system. If minerals precipitating during hybridization are separated from melt by physical processes, discrete masses of phlogopite rocks, and phlogopite-garnet-websterites, may form a heterogeneous layer above subducted oceanic crust, which is transported deeper than the main, -fluxed sites of magma generation. Values of Mg/(Mg+Fe) of the hybridized layer are probably similar to that of the mantle.