Formation of an interconnected network of iron melt at Earth’s lower mantle conditions

Abstract

Core formation represents the most significant differentiation event in Earth’s history. Our planet’s present layered structure with a metallic core and an overlying mantle implies that there must be a mechanism to separate iron alloy from silicates in the initially accreted material1,2. At upper mantle conditions, percolation has been ruled out as an efficient mechanism because of the tendency of molten iron to form isolated pockets at these pressures and temperatures3,4,5,6. Here we present experimental evidence of a liquid iron alloy forming an interconnected melt network within a silicate perovskite matrix under pressure and temperature conditions of the Earth’s lower mantle. Using nanoscale synchrotron X-ray computed tomography, we image a marked transition in the shape of the iron-rich melt in three-dimensional reconstructions of samples prepared at varying pressures and temperatures using a laser-heated diamond-anvil cell. We find that, as the pressure increases from 25 to 64 GPa, the iron distribution changes from isolated pockets to an interconnected network. Our results indicate that percolation could be a viable mechanism of core formation at Earth’s lower mantle conditions.

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Figure 1: 3D distribution of iron alloy melt in silicate perovskite.
Figure 2
Figure 3: Distribution of apparent dihedral angles for contacts between iron alloy melt and silicate perovskite.
Figure 4: 3D renderings of the tomographic reconstruction of the iron alloy melt prepared at 64 GPa.
Figure 5: Schematic diagram showing possible Earth core formation mechanisms.

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Acknowledgements

W.L.M. and C.Y.S. are supported by NSF-EAR-1055454. Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF MRI-1126249. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. Portions of this research were carried out at the SSRL, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the DOE Office of Science by Stanford University. HPSynC is supported by EFree, an Energy Frontier Research Center funded by US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES) under award number DE-SC0001057.

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W.L.M. proposed this project. C.Y.S. prepared and made measurements on all samples and reconstructed the TXM data. Y.L., J.W., W.Y. and J.C.A. assisted in the TXM data collection. L.Z. synthesized the starting material and assisted with the laser heating experiments. Y.M. assisted with the laser heating experiments. W.L.M. and C.Y.S. analysed the results and wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Wendy L. Mao.

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The authors declare no competing financial interests.

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Shi, C., Zhang, L., Yang, W. et al. Formation of an interconnected network of iron melt at Earth’s lower mantle conditions. Nature Geosci 6, 971–975 (2013). https://doi.org/10.1038/ngeo1956

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