The Effect of Fe3+ on the Equations of State of Mg-Silicate Perovskite and Post-Perovskite

Krystle Catalli, Massachusetts Institute of Technology

The Earth’s lower mantle is made up primarily of (Mg,Fe)SiO3 perovskite which transforms to post-perovskite 200-400 km above the core-mantle boundary. A significant fraction of iron in perovskite (Pv) and post-perovskite (PPv) has been found to be ferric. However, the effect of ferric iron on the physical properties of these dominant lower-mantle phases has not been assessed. Here we investigate the effect of ferric iron on the equations of state of Mg-silicate Pv and PPv. Synchrotron X-ray diffraction measurements were performed in the diamond-anvil cell in a quasi-hydrostatic Ar medium. Pv was synthesized at 50 GPa and 2000 K, and volume measurements were made up to 100 GPa. PPv was synthesized at 135 GPa and 2500 K, and volume measurements were performed between 40 and 140 GPa on decompression. At 125 GPa (2700-km depth), volume and bulk modulus of ferric PPv are 3.5(2)% and 17(4)% smaller than those of ferric Pv, respectively, resulting in a 10(1)% decrease in bulk sound speed at the PPv transition. This magnitude of decrease in bulk sound speed is a factor of four greater than that found in the ferrous system. At 125 GPa, the volume of ferric Pv (18 mol% Fe3+) is 0.9(2)% greater than ferrous Pv (9 mol% Fe2+), however, ferric PPv has a 0.4(2)% lower volume than ferrous PPv. This leads to a factor of two larger density jump at the PPv transition in the ferric system over the ferrous. Our results suggest that an enrichment in Fe3+ increases the difference in density and bulk sound speed between Pv and PPv, which has important implications for mantle flow and interpretations of the seismic wave variations at the lowermost mantle, respectively.

Abstract Author(s): Krystle Catalli, Sang-Heon Shim, Atsushi Kubo, and Vital B. Prakapenka