Disproportionation of (Mg,Fe)SiO3 Perovskite and its Implications to the Deep Earth

Models of the Earth’s deep interior have been built upon the basic assumption that the lower mantle down to the top of the D" layer mainly consists of orthorhombic perovskite (pv) with nominally 10 mol% Fe. However, seismic observations show enigmatic features in the deep lower mantle, such as, the sharp boundary on the top of the D″ layer, the anticorrelations between V_S and V_P, the seismic anisotropy, and the existence of large low-shear-velocity provinces. The mineralogical constitution of the Earth’s mantle is fundamental for understanding the geophysical and geochemical properties of this region. Our recent study using laser-heated diamond anvil cell technology coupled with synchrotron x-ray diffraction in-situ at high pressure-temperature and TEM studies of the quenched sample has demonstrated the disproportionation reaction of (Mg,Fe)SiO₃ pv to a nearly Fe-free pv and an Fe-rich H-phase with a previously unknown hexagonal structure at 95 to 101 GPa and 2200 to 2400 K, thus fundamentally changing the geochemistry and geophysics of the bottom half of the lower mantle. We suggest that (Mg,Fe)SiO₃ pv may not be the major silicate throughout the lower mantle down to the top of the D" layer. Instead, interpretations for key enigmatic features in the deep lower mantle require the knowledge of the three-phase P-T-x relationship of pv, ppv, and the H-phase.