Parental basalts in subduction zones: Implications for continental evolution

Abstract

Basaltic liquid is parental to new crust in subduction zones. It occurs as inclusions of glass in crystals of olivine in andesites and dacites as well as basalts. Volcanoes which have never erupted a basaltic rock nevertheless have basaltic glasses included in extruded crystals. The basaltic liquids are rich in A12O3 and Cl and have higher Cl/K2O than associated andesitic liquids. Because decrease in Cl/K20 with increase in K2O and SiO2 implies vapor saturation, most andesitic liquids develop in vapor‐saturated environments. Estimated concentrations of H2O and CO2 in the liquids indicate that vapor saturation is probably restricted to the crust. Consequently, most andesite is a derivative magma generated from basaltic magma within the crust. New crust which is formed in subduction zones is basaltic in composition, not andesitic like the average upper continental crust. Consequently, continental crust is either basaltic in bulk or becoming less silicic. Alternatively, some other processes transport silica‐poor materials from the crust to the mantle, leaving a silicic crustal residue. The chlorine in the parental basaltic liquids probably is supplied by the complete destruction of amphibole and the extraction of about 10% partial melt in subducted oceanic lithosphere or other mantle. The basaltic composition of parental liquids in subduction zones is probably established in hot mantle overlying the Benioff zone, not the Benioff zone itself.