1. Mineralogisches-Petrologisches Institut und Museum, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany
2. Institut für Mineralogie, Universität Münster, Corrensstrasse 24, 48149 Münster, Germany
3. Institut für Mineralogie und Petrographie, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
4. ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia
5. Max–Planck–Institut für Chemie, Abt. Kosmochemie, 55128 Mainz, Germany
6. Institute of Geology and Mineralogy SB RAS, Novosibirsk 630090, Russia

Correspondence to: Arno Rohrbach^1,2Chris Ballhaus¹ Correspondence and requests for materials should be addressed to A.R. (Email: rohrbaa@web.de) or C.B. (Email: ballhaus@uni-bonn.de).

The oxygen fugacity f_O2of the Earth's mantle is one of the fundamental variables in mantle petrology. Through ferric–ferrous iron and carbon–hydrogen–oxygen equilibria, f_O2 influences the pressure–temperature positions of mantle solidi and compositions of small-degree mantle melts^{1, 2, 3}. Among other parameters, f_O2 affects the water storage capacity and rheology of the mantle^{4, 5}. The uppermost mantle, as represented by samples and partial melts, is sufficiently oxidized to sustain volatiles, such as H₂O and CO₂, as well as carbonatitic melts^{6, 7}, but it is not known whether the shallow mantle is representative of the entire upper mantle. Using high-pressure experiments, we show here that large parts of the asthenosphere are likely to be metal-saturated. We found that pyroxene and garnet synthesized at >7 GPa in equilibrium with metallic Fe can incorporate sufficient ferric iron that the mantle at >250 km depth is so reduced that an (Fe,Ni)-metal phase may be stable. Our results indicate that the oxidized nature of the upper mantle can no longer be regarded as being representative for the Earth's upper mantle as a whole and instead that oxidation is a shallow phenomenon restricted to an upper veneer only about 250 km in thickness.

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