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

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(1-11). Pressure-induced electronic spin transitions of iron occur in magnesiowustite, silicate perovskite and post-perovskite(1-4,8,10,11). Here we have studied the spin states of iron in magnesiowustite and the isolated effects of the electronic transitions on the elasticity of magnesiowustite 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 magnesiowustite 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 similar to 35 per cent and bulk sound velocity increases by similar to 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(12-21). 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).