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Experimental evidence for a phase transition in magnesium oxide at exoplanet pressures


Magnesium oxide is an important component of the Earth’s mantle and has been extensively studied at pressures and temperatures relevant to Earth1. However, much less is known about the behaviour of this oxide under conditions likely to occur in extrasolar planets with masses up to 10 times that of Earth, termed super-Earths, where pressures can exceed 1,000 GPa (10 million atmospheres). Magnesium oxide is expected to change from a rocksalt crystal structure (B1) to a caesium chloride (B2) structure at pressures of about 400–600 GPa (refs 2, 3). Whereas no structural transformation was observed in static compression experiments up to 250 GPa (ref. 4), evidence for a solid–solid phase transition was obtained in shockwave experiments above 400 GPa and 9,000 K (ref. 5), albeit no structural measurements were made. As a result, the properties and the structure of MgO under conditions relevant to super-Earths and large planets are unknown. Here we present dynamic X-ray diffraction measurements of ramp-compressed magnesium oxide. We show that a solid–solid phase transition, consistent with a transformation to the B2 structure, occurs near 600 GPa. On further compression, this structure remains stable to 900 GPa. Our results provide an experimental benchmark to the equations of state and transition pressure of magnesium oxide, and may help constrain mantle viscosity and convection in the deep mantle of extrasolar super-Earths.

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Figure 1: Experimental set-up for X-ray diffraction and ramp-compression at the OMEGA Laser Facility.
Figure 2: X-ray diffraction patterns of compressed MgO.
Figure 3: d-spacings and density versus stress.


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The authors thank S. Uhlich, W. Unites, T. Uphaus and R. Wallace for their assistance in target preparation and the operation staff at the OMEGA Laser Facility for supporting these experiments. F.C. is grateful to D. C. Swift for providing a tabular equation of state of MgO. The authors thank R. E. Cohen and M. J. Mehl for discussions and for making simulation results available. This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. The research was supported by NNSA/DOE through the National Laser Users’ Facility Program under contracts DE-NA0000856 and DE-FG52-09NA29037. Part of this work was financially supported by the Laboratory Directed Research and Development program at LLNL (project number 12-SI-007).

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F.C. was the primary person who designed and performed the experiment, analysed the data and wrote the manuscript. R.F.S., J.H.E. and T.S.D assisted in the design and performance of the experiment and contributed to the manuscript. J.W. assisted in the design and performance of the experiment. J.R.R., A.L. and J.A.H. participated in the design of the experiment. G.W.C. participated in the design of the experiment and contributed to the manuscript. All of the authors discussed the results together and commented on drafts of the manuscript.

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Correspondence to F. Coppari.

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Coppari, F., Smith, R., Eggert, J. et al. Experimental evidence for a phase transition in magnesium oxide at exoplanet pressures. Nature Geosci 6, 926–929 (2013).

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