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Oxidized iron in garnets from the mantle transition zone

Abstract

The oxidation state of iron in Earth’s mantle is well known to depths of approximately 200 km, but has not been characterized in samples from the lowermost upper mantle (200–410 km depth) or the transition zone (410–660 km depth). Natural samples from the deep (>200 km) mantle are extremely rare, and are usually only found as inclusions in diamonds. Here we use synchrotron Mössbauer source spectroscopy complemented by single-crystal X-ray diffraction to measure the oxidation state of Fe in inclusions of ultra-high pressure majoritic garnet in diamond. The garnets show a pronounced increase in oxidation state with depth, with Fe3+/(Fe3++ Fe2+) increasing from 0.08 at approximately 240 km depth to 0.30 at approximately 500 km depth. The latter majorites, which come from pyroxenitic bulk compositions, are twice as rich in Fe3+ as the most oxidized garnets from the shallow mantle. Corresponding oxygen fugacities are above the upper stability limit of Fe metal. This implies that the increase in oxidation state is unconnected to disproportionation of Fe2+ to Fe3+ plus Fe0. Instead, the Fe3+ increase with depth is consistent with the hypothesis that carbonated fluids or melts are the oxidizing agents responsible for the high Fe3+ contents of the inclusions.

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Fig. 1: Chemical composition of majoritic garnet inclusions in diamonds worldwide.
Fig. 2: Ferric iron contents of majoritic garnets from Jagersfontein diamonds compared with lithospheric garnets from peridotite xenoliths.
Fig. 3: Calculated oxygen fugacities (see Methods) of the majoritic inclusions and of garnets crystallized in coexistence with Fe metal20.

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Acknowledgements

We thank T. Holland for checking some of our calculations and D. Frost for providing his spreadsheet for oxygen fugacity calculations using garnet equilibria, A. Schönleber for discussion of XRD results and D. Simonova for assistance during Mössbauer experiments. We acknowledge support from European Research Council grant 267764 to B.J.W. and NERC grant NE/L010828/1 to E.S.K. Financial support was provided to L.D. and C.M. through DFG grants Mc 3/18-1 and Mc 3/20-1, and through BMBF grants. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities.

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Work was initiated and planned by E.S.K. and L.D. T.S. and J.W.H. provided the samples and their detailed description. M.B., D.M.V., E.B. and L.D. performed the X-ray diffraction measurements. D.M.V., M.B., E.B. and L.D. processed and analysed the diffraction data. D.M.V., V.C., A.C. and C.M. collected, processed and analysed the Mössbauer spectra. E.S.K. and B.J.W. interpreted the data, performed the thermodynamic calculations and prepared the manuscript. All co-authors read, commented and approved of the manuscript.

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Correspondence to Ekaterina S. Kiseeva.

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Kiseeva, E.S., Vasiukov, D.M., Wood, B.J. et al. Oxidized iron in garnets from the mantle transition zone. Nature Geosci 11, 144–147 (2018). https://doi.org/10.1038/s41561-017-0055-7

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