Abstract
The oxygen fugacity of the Earth’s mantle is one of the fundamental variables in mantle petrology. Through ferric–ferrous iron and carbon–hydrogen–oxygen equilibria, influences the pressure–temperature positions of mantle solidi and compositions of small-degree mantle melts1,2,3. Among other parameters, affects the water storage capacity and rheology of the mantle4,5. 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 melts6,7, 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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Acknowledgements
We thank the Museum of Natural History in London for providing Karoo picrite samples from Malawi. Comments on the manuscript by R. Frost and D. Canil and D. Frost improved the paper. Financial support by the German Research Council to C.B. is gratefully acknowledged.
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The file contains Supplementary Discussion with additional references; Supplementary Figure S1 with Legend and Supplementary Table S1. Supplementary Discussion uses Karoo picrites from southern Malawi to discuss the possibility that melts from metal–saturated sources can be recognized by their olivine compositions. Supplementary Fig. S1 shows the NiO contents and Ni/Co atomic ratios of olivines in the Karoo picrite sample. The Supplementary Table S1 lists the starting composition, run conditions and representative electron microprobe analyses of the run products. Details about electron microprobe and EELS measurements are given in the legend. (PDF 295 kb)
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Rohrbach, A., Ballhaus, C., Golla–Schindler, U. et al. Metal saturation in the upper mantle. Nature 449, 456–458 (2007). https://doi.org/10.1038/nature06183
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DOI: https://doi.org/10.1038/nature06183
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