The join forsterite-diopside-iron oxide and its bearing on the crystallization of basaltic and ultramafic magmas

Abstract

At oxygen fugacities from 10 (super -0.68) to (super -6) atm minimum liquidus temperature on the above join remains nearly constant in temperature and composition at 1300 degrees C and 79 percent diopside, 21 percent iron oxide. Comparison with other data indicates: Skaergaard intrusion is a result of a close approach to fractional crystallization of an anhydrous magma at constant total composition; if similar basaltic magmas have crystallized at relatively low pressures and derived liquids have been produced at the surface as lavas, oxygen fugacities were controlled by oxidizing buffers; reaction rims of hypersthene and magnetite on olivine phenocrysts result from partial resorption of olivine; at low pressures peridotites exist while partly liquid at temperatures as low as 1250 degrees C, and fractional crystallization of ultramafic magma with buffered oxygen fugacity explains origin of the Duke Island, Alaska, ultramafic complex.