Mantle oxidation state (redox state) applies the concept of oxidation state in chemistry to the study of the Earth's mantle. The chemical concept of oxidation state mainly refers to the valence state of one element, while mantle oxidation state provides the degree of decreasing of increasing valence states of all polyvalent elements in mantle materials confined in a closed system. The mantle oxidation state is controlled by oxygen fugacity and can be benchmarked by specific groups of redox buffers.  
Mantle oxidation state changes because of the existence of polyvalent elements (elements with more than one valence state, e.g. Fe, Cr, V, Ti, Ce, Eu, C and others). Among them, Fe is the most abundant (~8 wt% of the mantle) and its oxidation state largely reflects the oxidation state of mantle. Examining the valence state of other polyvalent elements could also provide the information of mantle oxidation state.  
It is well known that the oxidation state can influence the partitioning behavior of elements and liquid water between melts and minerals, the speciation of C-O-H-bearing fluids and melts, as well as transport properties like electrical conductivity and creep.
The formation of diamond requires both reaching high pressures and high temperatures and a carbon source. The most common carbon source in deep Earth is not elemental carbon and redox reactions need to be involved in diamond formation. Examining the oxidation state can help us predict the P-T conditions of diamond formation and elucidate the origin of deep diamonds.