Oxidation Status of the Mantle: Past and Present

It is important for a number of reasons that we obtain a thorough understanding of the present oxidation status of the mantle. Suband supersolidus phase relationships, the nature of degassed volatile species, electrical conductivity, diffusivity, and mechanical behavior arc some of the properties that are a function of this parameter. Of further interest is the fact that the present oxidation state must in some way reflect the complex processes that have occurred since the formation of the proto-mantle some 4.5 Gyr ago. These processes include the redox characteristics of accreted material, heterogeneous equilibria prevailing during the growth of the Earth (such as those involved in core formation), convective cycling in the mantle, subduction of oxidized surface layers of the Earth and mixing in the mantle, incorporation of impacted planetesimals, infiltration by melts and fluids, and selective volatile losses and gains. It would be fair to characterize the current range of opinions on the oxidation status of the upper mantle as diverse, partly as a result of discrepancies between theoretical and experimental approaches to the problem, and partly because of differences in philosophical viewpoints concerning the major redox controls. Granted this divergence of opinion for the upper mantle, it must also be recognized that knowledge of the redox state(s) of the greater part of the mantle to the boundary with the core is rudimcntary. There does appear, however, to be some intriguing evidence not only that the present upper mantle is heterogeneous with respect to oxidation state, but also that a secular trend toward oxidation of the subcontinental mantle portion of the lithosphere has taken place.