REDOX MECHANISMS FOR THE DIFFERENTIATION OF MANTLE MATERIAL

Abstract

The oxygen fugacity is one of the key factors governing mineral equilibria in the mantle. Of particular importance, it governs the composition of mantle fluids and thus their ability to dissolve minerals or to themselves dissolve in the magma. The stability of diamond also depends on redox equilibria. There now exists a large body of data on the compositions of coexisting minerals in spinel-containing mantle paragenesses. It therefore appears possible to estimate the mantle fO(2) in terms of the constant of a chemical reaction, given the paper involving various iron, oxygen and silicon compounds. Fe2O4 + 3 FeSiOs = 3 Fe2SiO4 + 1/2 O2 To do this, one must know not only the activities for the olivines and orthopyroxenes. which are close to 1, but also the thermodynamic models of the more complex spinel solid solutions that have been published in recent years. In this paper the authors determine the mantle fo2 by their method, in which they evaluate the factors that describe the deviation of spinel solid solutions form ideality. These factors are determined by analyzing the data on the compositions of coexisting natural spinels, one of which is enriched in the magnetite component.