Equations of state for postperovskite phases in the MgSiO3-FeSiO3-FeAlO3 system

Equations of state and axial compressions are investigated for the Mg0.9Fe0.1SiO3 and Mg0.85Fe0.15Al0.15Si0.85O3 postperovskite phases in the MgSiO3-FeSiO3-FeAlO3 system by high-pressure in situ X-ray diffraction measurements using a laser-heated diamond anvil cell. The b-axis is found to be the most compressible axis for both phases, while the c-axis is slightly less compressible. The incompressible behavior of the c-axis is likely to be attributable to repulsion among oxygen atoms arranged along the c-axis. Using the Tsuchiya gold pressure scale, the Birch-Murnaghan equation of state yields isothermal bulk moduli of 224(11) GPa and 220(13) GPa, and zero-pressure unit-cell volumes of 164.1(15) Å3 and 166.5(20) Å3 for these FeSiO3- and FeAlO3-bearing postperovskites, respectively. The incorporation of FeSiO3 and FeAlO3 into the MgSiO3 postperovskite results in a marked decrease in the isothermal bulk modulus. The bulk sound velocity is also confirmed to decrease considerably at the perovskite-postperovskite transition, even in the FeSiO3- and FeAlO3-bearing systems. The velocity thus decreases not only by the postperovskite transition but also by enrichment in the FeSiO3 and FeAlO3 components. Chemical heterogeneity of the FeSiO3 and FeAlO3 components may therefore represent an additional mechanism of velocity variation in the D″ layer of the lower mantle.