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Solid–liquid iron partitioning in Earth’s deep mantle

Abstract

Melting processes in the deep mantle have important implications for the origin of the deep-derived plumes believed to feed hotspot volcanoes such as those in Hawaii1. They also provide insight into how the mantle has evolved, geochemically and dynamically, since the formation of Earth2. Melt production in the shallow mantle is quite well understood, but deeper melting near the core–mantle boundary remains controversial. Modelling the dynamic behaviour of deep, partially molten mantle requires knowledge of the density contrast between solid and melt fractions. Although both positive and negative melt buoyancies can produce major chemical segregation between different geochemical reservoirs, each type of buoyancy yields drastically different geodynamical models. Ascent or descent of liquids in a partially molten deep mantle should contribute to surface volcanism or production of a deep magma ocean, respectively. We investigated phase relations in a partially molten chondritic-type material under deep-mantle conditions. Here we show that the iron partition coefficient between aluminium-bearing (Mg,Fe)SiO3 perovskite and liquid is between 0.45 and 0.6, so iron is not as incompatible with deep-mantle minerals as has been reported previously3. Calculated solid and melt density contrasts suggest that melt generated at the core–mantle boundary should be buoyant, and hence should segregate upwards. In the framework of the magma oceans induced by large meteoritic impacts on early Earth, our results imply that the magma crystallization should push the liquids towards the surface and form a deep solid residue depleted in incompatible elements.

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Acknowledgements

We thank N. Bolfan-Casanova, M. A. Bouhifd, T. Druitt, T. Hammouda and J.-M. Hénot for help and discussions. This work is supported by the French National Centre for Scientific Research’s National Institute for Earth Sciences and Astronomy, the ESRF and the European C2C programme. This is Laboratory of Excellence ClerVolc contribution no. 26.

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D.A., S.P., G.L.N., G.G. and M.M. synthesized the sample and took the XRD and XRF measurements at the ID27 beamline. S.P. and G.V. took the XRF measurements at the ID21 beamline. D.A. and J.-L.D. performed the electron-probe micro-analyses at the Laboratoire Magmas et Volcans. D.A., S.P. and G.L.N. performed the data treatment and wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Denis Andrault.

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Andrault, D., Petitgirard, S., Lo Nigro, G. et al. Solid–liquid iron partitioning in Earth’s deep mantle. Nature 487, 354–357 (2012). https://doi.org/10.1038/nature11294

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