Iron Oxidation State and Compressional Behaviors of Al,Fe-rich Mantle Silicate Perovskite up to 90 GPa

Magnesium silicate perovskite, the dominant lower-mantle phase, may contain much more Al and Fe in the subducting oceanic crust than in the normal mantle. Therefore, it is important to understand the physical properties of Al,Fe-rich mantle silicate perovskite. We have synthesized Fe,Al-rich mantle silicate perovskites from two different starting materials [Mg₂FeAl₂Si₃O₁₂ (Alm33) and Mg_2.5Fe_0.5Al₂Si₃O₁₂ (Alm17)] and conducted in-situ synchrotron X-ray diffraction (GSECARS and HPCAT) and Mössbauer spectroscopy (Sector 3) in the laser heated diamond-anvil cell at Advanced Photon Source. The garnet starting materials were synthesized in the multi-anvil press at Arizona State University. The starting materials were mixed with 10 wt% Au powder (internal pressure standard) and loaded with an Ar or Ne pressure medium in the diamond-anvil cell for X-ray diffraction measurements. Pure starting materials are loaded in the diamond-anvil cell with an Ne or KCl pressure medium for synchrotron Mössbauer measurements. In synchrotron Mössbauer spectroscopy, we obtained pressure from the unit-cell volumes of pressure media combined with their equations of state. During the laser heating of the Alm33 starting material at ~66 GPa and ~2300 K, perovskite was synthesized with a small amount of stishovite. For the same starting material, pure perovskite was synthesized at 90 GPa and ~2300 K. For the Alm17 starting material, perovskite was synthesized with a small amount of stishovite at 65-92 GPa and ~2200K. Between 60 and 90 GPa, the unit-cell volumes of the Alm33 and Alm17 perovskites are only slightly higher (0.3-0.6%) than that of Mg-end member perovskite and are comparable to that of (Mg_0.85Fe_0.15)SiO₃ perovskite, indicating higher density of the Al,Fe-rich perovskite in subducting oceanic crust than surrounding mantle. Both Al,Fe-rich perovskites show a steep increase in unit-cell volume during decompression between 60 and 40 GPa, suggesting either spin transition or metastability at the pressure range. We successfully quenched both Al,Fe-rich perovskites and measured the unit-cell volumes at 1 bar: 166.3×0.1 ų for Alm17 perovskite, and 165.0×0.2 ų for Alm33 perovskite. Our direct measurements indicate that the combined effects of Fe and Al on the unit-cell volume is much greater at lower pressures (~2%) than those observed at higher pressure (0.3-0.6%). The preliminary spectral fitting of synchrotron Mössbauer data indicates that 50~80% of iron is Fe³⁺ in both Al,Fe-rich pervoskites, suggesting that similar degree of charge disproportionation reaction occurs in Al-rich mantle silicate perovskite. This result is in disagreement with a recent thermodynamic prediction by Nakajima et al. (2012, JGR) that Fe³⁺ concentration may be very low (near zero) in Al-rich perovskite.