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

Abstract:

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.