Spin transition of iron in magnesiowüstite in the Earth's lower mantle

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Abstract

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earth's interior1,2,3,4,5,6,7,8,9,10,11. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite1,2,3,4,8,10,11. Here we have studied the spin states of iron in magnesiowüstite and the isolated effects of the electronic transitions on the elasticity of magnesiowüstite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowüstite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by 35 per cent and bulk sound velocity increases by 15 per cent across the transition in (Mg0.83,Fe0.17)O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle12,13,14,15,16,17,18,19,20,21. The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition5,6,22,23.

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Figure 1: Representative X-ray emission spectra of Fe-Kβ collected from a single-crystal magnesiowüstite, (Mg 0.75 ,Fe 0.25 )O, in 〈110〉 orientation at high pressures.
Figure 2: Normalized volume of magnesiowüstite, (Mg 0.83 ,Fe 0.17 )O, as a function of pressure at 300 K.
Figure 3: Calculated isothermal bulk modulus and bulk sound velocity as a function of pressure for the high-spin and low-spin states.

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Acknowledgements

We thank R. Caracas, R. Cohen, G. Shen, V. Prakapenka, W. Sturhahn, J. M. Jackson, P. Silver, B. Militzer, S. Hardy, C. Prewitt, M. Somayazulu, P. Dera, and Y. Fei for discussions, S. J. Mackwell for help with sample synthesis, HPCAT for the use of the X-ray facilities, and GSECARS, APS, for the use of the Raman system. This work and use of the APS are supported by the US Department of Energy, Basic Energy Sciences, Office of Science and the State of Illinois under HECA. Work at Carnegie was supported by DOE/BES, DOE/NNSA (CDAC), NSF, and the W. M. Keck Foundation.

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Correspondence to Jung-Fu Lin.

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Supplementary information

Supplementary Notes

This contains legends for Supplementary Figures S1 and S2, Supplementary Methods and Supplementary References. (DOC 37 kb)

Supplementary Figure S1

Representative X-ray emission spectra of Fe-Kβ in magnesiowüstite [(Mg0.40,Fe0.60)O] at high pressures. (PDF 40 kb)

Supplementary Figure S2

Relative volume of (Mg0.40,Fe0.60)O as a function of pressure at 300 K. (PDF 36 kb)

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Lin, J., Struzhkin, V., Jacobsen, S. et al. Spin transition of iron in magnesiowüstite in the Earth's lower mantle. Nature 436, 377–380 (2005) doi:10.1038/nature03825

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