OXYGEN FUGACITY REGIME IN THE UPPER MANTLE AS A REFLECTION OF THE CHEMICAL DIFFERENTIATION OF PLANETARY MATERIALS

Abstract

Oxygen fugacity (fO2) in the Earth's mantle has a bearing on the problems of the chemical differentiation of the Earth's materials and formation of the chemical and phase state of its shells. This paper addresses some problems concerning changes in the redox state of the upper mantle over geologic time and through its depth and the possible influence of fO2 stratification in the interiors on geochemical processes. Among these problems are the formation of fluids enriched in H2O, CO2, CH4,and H2; the possible influence of reduced fluid migration from mantle zones with low fO2 values on reactions in the lithosphere; and the formation of films of silicate liquids with high H2O and CO2 contents, which could be responsible for metasomatic transformations in rocks. The formation of a metallic core and accompanying large-scale melting of the silicate part of the Earth are the early mechanisms of the chemical differentiation of the mantle that must have had an effect on the redox state and the composition of volatile components in planetary materials. The molten metallic and silicate phases were prone to gravitational migration, which affected the formation of the metallic core. Volatile components had to be simultaneously formed in the zones of large-scale melting of the early Earth. The composition of these volatiles was largely controlled by the interaction of hydrogen and carbon, the two major gas-forming elements in the mantle, with melt under low fO2 values. A remarkable feature is that, despite fairly low fO2 values imposed by the presence of a metallic phase, both reduced (CH4 and H2) and oxidized species of hydrogen and carbon (H2O, OH- and CO32-) are stable in the melt. This peculiarity of carbon and hydrogen dissolution in reduced melts may be crucial for the elucidation of mechanisms for the formation of initial amounts of CO2 and H2O connected with incipient melting in the reduced mantle. © Pleiades Publishing, Inc. 2006.