Experimental Evidence for a Reduced Metal-saturated Upper Mantle

Abstract

The uppermost mantle as sampled by xenoliths, peridotite massifs and primitive basaltic melts appears to be relatively oxidized, with oxygen fugacities between the magnetite-wüstite and fayalite-ferrosilite-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 1650°C 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-wüstite equilibrium. Analysis of the experimental phases for ferric iron using electron energy loss spectroscopy reveals that at pressures higher than 7 GPa, subcalcic pyroxene and majoritic garnet incorporate appreciable amounts of ferric iron, even though at the experimental conditions they were in redox equilibrium with metallic iron. The major ferric iron carrier in the upper mantle is majoritic garnet, followed by subcalcic pyroxene. At around 8 ± 1 GPa, corresponding to ∼250 ± 30 km depth in the upper mantle, sufficient quantities of subcalcic pyroxene and majoritic garnet are stabilized that all the ferric iron thought to be present in fertile upper mantle (i.e. ∼2000 ppm) can be accommodated in solid solution in these phases, even though they were synthesized in redox equilibrium with metallic Fe. Based on the results of the experiments, it can be stated that, on a global scale, an oxidized upper mantle near the fayalite-ferrosilite-magnetite equilibrium is the exception rather than the rule. More than 75 vol. % of the Earth's present-day mantle is likely to be saturated with metallic iron