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 interior^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite^{1, 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 (Mg_0.83,Fe_0.17)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 mantle^{12, 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 transition^{5, 6, 22, 23}.

1. Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington DC 20015, USA
2. HPCAT, Carnegie Institution of Washington, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
3. The Mineral Physics Institute, University of New York at Stony Brook, Stony Brook, New York 11794, USA
4. †Present address: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA

Correspondence to: Jung-Fu Lin^1,4 Correspondence and requests for materials should be addressed to J.-F.L. (Email: j.lin@gl.ciw.edu).

Received 17 January 2005; Accepted 13 May 2005

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