Spring 2005 - Abstract Jung-Fu Lin, April 4th

Iron as a new window into the Earth's core and lower mantle

Jung-Fu Lin
Geophysical Laboratory, Carnegie Institution of Washington

Recent advances in static high-pressure and high-temperature diamond-anvil cell techniques are providing new windows of unprecedented clarity on the physical properties of materials in the Earth's interior. With advanced X-ray spectroscopic and laser-heated diamond cell techniques, structural, elastic, electronic, magnetic, and vibrational properties of planetary materials have been examined under extreme conditions. Key examples of these new static studies using iron as a new window into the Earth's core and mantle will be given in this talk which illustrates both the state-of-the-art techniques and current scientific findings in high-pressure mineral physics. In particular, sound velocities of hcp-Fe, the most abundant constitute of the Earth's core, have been measured with nuclear inelastic X-ray scattering in a laser-heated diamond cell. The compressional (V_P) and shear wave velocities (V_S) of hcp-Fe decrease significantly with increasing temperature under moderate high pressures, indicating that the linear sound velocity-density relation, Birch's law, should be corrected to lower velocities in extrapolations to inner core conditions. On the other hand, iron is also the most abundant transition-metal element in the Earth's mantle. Structural, electronic, and magnetic phase diagrams of the FeO-MgO binary system under lower mantle conditions will be addressed. The electronic spin states of iron in magnesiowustite [(Mg,Fe)O] and the isolated effects of the electronic transitions on the elasticity of magnesiowustite have been measured by 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. Recently, the X-ray emission spectroscopy has been used in conjunction with laser-heated diamond cell technique for studying the spin states of iron in its lower mantle host phases, magnesiowustite [(Mg,Fe)O] and silicate perovskite [(Mg,Fe)SiO₃], under high pressures and temperatures. X-ray emission spectra of the ferrous iron in magnesiowustite [(Mg_0.76,Fe_0.24)O] show that the electronic spin transition has a positive Clapeyron slope and the transition is likely to occur at the top of the D"-zone, providing an additional explanation for the seismic wave heterogeneity in the lowermost mantle. Synchrotron Mossbauer spectroscopic studies on FeO reveal that the NiAs phase of FeO is in the magnetic state. The existence of the magnetic phase of FeO in the lowermost mantle would affect magnetic properties of the layer and hence affect geomagnetic coupling between the outer core and the low mantle. These new results have implications for a variety of outstanding problems in high-pressure mineral physics.

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