Experimental data for the system MgO-FeO-Fe 2 O 3 -CaAl 2 Si 2 O 8 -SiO 2 and their petrologic implications

Abstract

Liquidus equilibrium relations among olivine, pyroxene, anorthite, magnetite, tridymite, liquid, and a gas phase have been determined at 1 atm total pressure and at oxygen partial pressures ranging from 10 (super -11) to 10 (super -0.7) atm. At PO 2 values as low as 10 (super -9) atm, anorthite, pyroxene, magnetite, and tridymite coexist with liquid which represents the end result of fractional crystallization of basalt-like liquids in the system. The system reported represents addition of anorthite to the four component system MgO-FeO-Fe 2 O 3 -SiO 2 investigated by Osborn, 1959. The general nature of crystallization is not changed by addition of anorthite, but liquid compositions, temperatures, and oxygen partial pressures are much closer to those of basalts and andesites. However, for constant oxygen content, the course of fractional crystallization is toward ferrogabbroic liquids and low values for PO 2