Experimental evidence for a reduced metal-saturated upper mantle

The uppermost mantle as sampled by xenoliths, peridotite massifs and primitive basaltic melts appears to be relatively oxidized, with oxygen fugacities between the magnetite^wu « stite and fayalite^ferro-silite^magnetite equilibria.Whether this range in oxygen fugacity is a shallow mantle signature or representative of the entire upper mantle still is unclear and a matter of debate because mantle regions deeper than 200 km are not well sampled. To constrain the redox state of the deeper upper mantle, we performed experiments from 1 to 14 GPa and 1220 to 16508C on a model peridotite composition, encompassing the convecting asthenospheric mantle down to the Transition Zone at 410 km depth.The experiments were run in iron metal capsules to buffer fO2 close to an oxygen fugacity about 0·5 log units below the iron^wu « stite equilibrium. Analysis of the experi-mental phases for ferric iron using electron energy loss spectroscopy reveals that at pressures higher than 7 GPa, subcalcic pyroxene an