Mineral Physics Studies of Elasticity, Phonon, and Rheology at Pressure-Temperature Conditions of the Earth's Deep Interior

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

The mineralogy of the Earth's lower mantle, which comprises the largest fraction of our planet (>55% by volume), was previously regarded as simple and monotonous. An iron-magnesium silicate with the perovskite structure was thought to be the stable mineral unchanging over the enormous pressure and depth range some 400 to 1,800 miles (660 to 2,900 kilometers) below the Earth's surface. With prior NSF grant support, the PI's team made a surprising discovery: at the conditions of the deep lower mantle, the iron-magnesium silicate decomposes into a pure magnesium silicate and a previously unknown Fe-rich silicate mineral with entirely new properties. The next challenge in the present grant is, therefore, to investigate how the new minerals respond to seismic waves and how they deform under compression. The team will use a battery of novel probes for characterization of these properties in-situ under the high pressures and temperatures generated in their laboratory. The results will provide the key for deciphering the seismological information and gain fundamental understanding of the deep Earth interior. Focused research on the frontier of the deep lower mantle is proposed. Very recently, the perovskite phase of (Mg,Fe)SiO3 was found to disproportionate to an Fe-free MgSiO3 pv and a new Fe-rich hexagonal silicate (H-phase) at the pressure-temperature (P-T) conditions corresponding to the bottom ~1000 km, representing an important, surprising discovery. In the present proposal, the observation of disproportionation and H phase will be further explored and their exact P-T boundaries and physical properties will be investigated. The newly developed multigrain crystallography technique will be applied to P-V-T-x studies in the multiphase region that perovskite, H-phase, and/or post-perovskite coexist. Synchrotron Mössbauer spectroscopy, x-ray emission spectroscopy, and nuclear resonant inelastic x-ray scattering will be used to probe the Fe2+/Fe3+ ratio, spin state, and phonon density of state, respectively, for the H-phase in-situ under high P-T. Radial x-ray diffraction and the newly developed Bragg coherent diffraction imaging method will be developed to study lattice preferred orientation and shape-preferred orientation as possible sources of seismic anisotropy in the D" layer. Results from the present study will contribute to fundamental understanding the seismic complexities of the deep lower mantle.