Is the upper mantle metal-saturated?

The equilibria controlling silica activity and oxygen fugacity (fO2) in the upper mantle are related and all sensitive to pressure. With increasing pressure, both silica activity andfO2 will fall. Provided there is no systematic change in mantle bulk chemistry, the rate of reduction per unit GPa pressure increase may be of the order of −0.6 log bars relative to the fayalite-magnetite-quartz buffer. At that rate, one may predict (1) that C-H-O volatiles in the convecting upper mantle are dominated by methane and hydrogen, and (2) that Fe metal saturation may occur at pressures around 9 GPa.fO2 may be a key variable to explain the fractionation of platinum-group elements between a mantle residue and a partial melt.