Electronic Spin and Valence States of Iron in the Al-rich NAL and CF Phases in the Subducted Slabs
Abstract:The New hexagonal aluminous (NAL) phase and orthorhombic calcium-ferrite (CF) type phase are believed to be the main hosts of aluminum (with ~ 20 wt% proportion) in the MORB composition in the Earth’s lower mantle. It has been proposed that the NAL and CF phases contains approximately 12 mol% Fe. Most of previous studies on the NAL and CF phases have focused on their crystal structural stability and transitions1,2, while, their spin and valence states of Fe as well as the Fe effects on the behavior of Al-rich phases remain unclear. The spin transition of Fe-bearing phase in the lower mantle has significant effects on density, compressibility, elasticity and sound velocity of the host mineral, and that is important to understand the geophysics and geodynamics of the Earth’s mantle3. Here we have investigated the iron spin and valence states of the NAL and CF phases up to 85 GPa via high-pressure nuclear forward scattering and X-ray diffraction measurements. Our results show that both Fe2+ and Fe3+ valence states exist in the NAL phase structure, and that Fe3+ in the octhahedral site undergoes a high-spin to low-spin transition at approximately 30 GPa. X-ray diffraction results further reveal the effects of the spin and valence states on the equation of state parameters of the NAL phase. These results are applied to understand how the spin transition affects the behavior of the NAL and CF phases in the subducted slabs in the Earth’s lower mantle. And the spin transition of Al-rich phases may play a significant role on the subduction behavior of the slabs in the Earth’s lower mantle.
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