Effects of the Fe 3+ spin transition on the equation of state of bridgmanite

We have investigated the equation of state of Fe-bearing bridgmanite, (Mg0.9Fe0.1)SiO3, using synchrotron X-ray diffraction in diamond anvil cells up to 125 GPa and 300 K. Combined with previous synchrotron Mossbauer spectroscopy results, we have found that the occurrence of the low-spin Fe3+ in the octahedral sites (B site) of bridgmanite has produced a 0.5(±0.1)% reduction in the unit cell volume at 18–25 GPa and has increased the isothermal bulk modulus to 284(±4) GPa, consistent with recent theoretical calculations. Together with literature results, we note that the addition of Fe can cause an increase in the density, bulk modulus, and bulk sound velocity in both Al-free and Al-bearing bridgmanite at lower mantle pressures. The presence of Fe3+ in the B site of bridgmanite can further enhance this increase. The observed spin transition of B site Fe3+ in bridgmanite is thus important for understanding the density and velocity structures of the lower mantle.