Redox state of the upper mantle

The oxygen fugacity condition of equilibration has been carefully determined from a spinel lherzolite from Mongolia, olivine xenocrysts from chrome pyrope-bearing peridotite nodules from kimberlites of Yakutia, and basaltic samples from ocean floor, iron arcs and the continental areas. These indicate that the spinel lherzolites occurring within alkali basalts from Mongolia, equilibrated under an \(f_{O_2 } \) condition similar to that of WM buffer. The diamond and chrome pyrope-bearing peridotites from the kimberlite pipes equilibrated between IW and WM buffers. Some of the ilmenite-bearing peridotite crystallized under \(f_{O_2 } \) conditions similar to that between WM and QFM buffers and chondrites equilibrated below the QFI buffer. It is concluded that during geochemical processes in the upper mantle the \(f_{O_2 } \) conditions vary broadly, and are similar to that between FMQ and IW buffers. There is a dramatic change in the composition of the kimberlitic fluid, which is CH₄-bearing at an early stage, but is in equilibrium with H₂O and CO₂ at a later stage. This is related to mass transfer of fluids from the lower part of the mantle with a low oxidation state to the upper part having a higher \(f_{O_2 } \) condition.