Investigation of the properties of iron-bearing alloys and silicates and their implications for the Earth's lower mantle and core

Zusammenfassung

(1) (Mg,Fe)(Si,Al)O3 perovskite comprises about 80% of the Earth's lower mantle by volume, leaving ~ 15% to (Mg,Fe)O ferropericlase and ~ 5% to CaSiO3 perovskite. Therefore characteristics of the lower mantle would be determined mostly by the properties of the silicate perovskite and ferropericlase. While high-spin low-spin crossover in (Mg,Fe)O is nicely described in literature, the electronic state of iron in silicate perovskite at high pressures and temperatures remains controversial. Conclusions derived from the results of X-ray emission (XES) and nuclear forward scattering (NFS) spectroscopic studies of Fe-bearing silicate perovskite are not in agreement on the pressure and temperature conditions of the transition and on whether Fe2+ or Fe3+ or both iron cations are involved. We undertook an alternative study of (Mg,Fe)(Si,Al)O3 perovskite at a wide pressure and temperature range using a number of different spectroscopic techniques (conventional Mössbauer, X-ray absorption near edge structure (XANES), NFS and X-ray diffraction (XRD) spectroscopies), in order to get a rather complete picture regarding the spin state of iron in this compound. Desirable pressures relevant to those in the Earth’s lower mantle were achieved by means of diamond anvil cells, equipped with miniature external resistive heaters, providing homogeneous heating up to 1000 K, which enables us to estimate the effect of temperature as well. Our Mössbauer and XANES data, collected at pressures to 110 GPa and temperatures to 1000 K for silicate perovskite, revealed a gradual transition involving Fe2+, which at room temperature occurs over a rather wide pressure range, 35-70 GPa, but becomes narrow at high temperatures. This observation coincides with the previously reported drop in spin number revealed by XES. Taking this into account and based on the fact that our XRD measurements, performed at the corresponding pressure-temperature conditions, do not suggest any appreciable structural change in perovskite, we conclude that the origin of the observed transition is fully electronic ...
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