Complex Effects of Alumina/Silica on Ferric/Ferrous Iron in Earth's Lower Mantle

The electronic states of Fe in silicates are key to the chemistry, physical properties and dynamics of Earth’s mantle. In the lower mantle’s dominant phase, perovskite-structured (Mg,Fe)SiO₃, recently named bridgmanite, and its deep lower mantle polymorph, post-perovskite, Fe can occupy either cation site and multiple valence and spin states. In addition, studies of Fe²⁺-bearing starting materials at lower mantle conditions have observed oxidation to Fe³⁺ in the synthesized silicate, either by a disproportionation, Fe²⁺ = Fe³⁺ + Fe metal, or possibly reduction of the high-pressure cell. The incorporation of Al in lower mantle silicates has been observed to promote higher Fe³⁺/ΣFe. Due to this complexity, electronic states of Fe in lower mantle silicates are controversial. We used energy-domain synchrotron Mössbauer spectroscopy at ESRF beamline ID18 to examine spin and valence states of bridgmanite and post-perovskite synthesized from Fe²⁺-rich compositions with and without Al. ⁵⁷Fe-enriched starting materials (Mg_0.5Fe_0.5)SiO₃ pyroxene and Fe_2.8Al_2.2Si_3.0O₁₂ almandine-composition glass were pressurized in an NaCl medium in the laser-heated diamond anvil cell to up to 170 GPa. Bridgmanite was synthesized at 75-99 GPa and 2000-2500 K. Post-perovskite was synthesized at 149-160 GPa and 2500-3000 K. The observed quadrupole splitting (QS) and center shift (CS) are consistent with dominant Fe²⁺ for all compositions and do not show higher Fe³⁺/ΣFe with higher Al-content. The dominant doublet at lower mantle pressures exhibits QS=3.6-4.2 mm/s and CS=0.9-1.1 mm/s, similar to previous observations of high or intermediate spin Fe²⁺. A second high-spin Fe²⁺ doublet is observed at QS=2.2-3.3 mm/s and CS=0.8-1.2 mm/s. A minor high-spin Fe³⁺ doublet is fit to QS~1.2 mm/s and CS=0.3-0.5 mm/s. For the Al-bearing bridgmanite, ambient spectra before and after synthesis contain no more than ~10% Fe³⁺/ΣFe, indicating no disproportionation or oxidation of Fe²⁺. In contrast, previous studies of Al-bearing bridgmanite observed ~50% Fe³⁺/ΣFe. This difference may reflect the high Si-content of the starting material and balance of Al between the Si-site and Mg-site. The effects of Al on oxidation potential of Fe in bridgmanite and the lower mantle oxygen fugacity must also be assessed as a function of Si-content and Si/Al ratio.