Electronic Spin and Valence States of Iron in the Lower-Mantle Silicate Perovskite
Thursday, 18 December 2014
Magnesium silicate perovskite (Pv) is the most abundant phase in the Earth’s lower mantle and can make up to 75% in volume of the lower mantle in a pyrolitic compositional model. The electronic spin and valence states of iron (Fe) in Pv directly influence its physical and chemical properties, and therefore it is important to study the electronic spin and valence states of Fe in Pv at conditions corresponding to the Earth’s lower mantle. Numerous studies have been dedicated to understanding the electronic spin and valence states of Fe in Pv at high pressures using experimental and theoretical techniques. However, experimental results and possible interpretations remain highly debated due to the multiple valence states and site occupancies of Fe in Pv. Previous studies also showed the incorporation of aluminum (Al) into a Fe-bearing Pv can affect the placement of Fe3+ and further complicate interpretations of experimental results. Here we present the experimental result on electronic spin and valence states of Fe using well-characterized single-crystal Fe-bearing and Al-Fe-bearing Pv to the pressures relevant to the lower mantle by synchrotron X-ray emission spectroscopy (XES) and Mössbauer spectroscopy (SMS). The samples used in this experiment were first characterized by Mössbauer spectroscopy (MS) at ambient conditions to identify the initial Fe electronic spin and valence states before performing high-pressure experiments. The XES spectra were analyzed using a new analytical method, integrate relative difference (IRD) method. The new IRD method had been shown in the previous study to be a more effective method of probing the pressure range of Fe spin transition and can avoid the influence of peak broadening on the interpretation of the spin states of Fe at high pressures. Our study will assist the comprehension and interpretation of the electronic spin and valence states of Fe on the major lower-mantle mineral. The result has implications to deep Earth geophysics and geochemistry, including elasticity, element partitioning, diffusion, and transportation properties.