"X-ray Diffraction and Nuclear Forward Scattering Measurements of Mantle Phases up to 150 GPa"

Sang-Heon Shim, Krystle Catalli, Justin Hustoft, Vitali Prakapenka, Wolfgang Sturhahn, and Martin Kunz, Massachusetts Institute Technology

Developments at third-generation synchrotron facilities have fostered many new discoveries in Earth and planetary sciences. The recent discovery of the post-perovskite transition has provided new interpretations for the enigmatic seismic structures at Earth’s core-mantle boundary region. Furthermore, measurements on the property changes across the post-perovskite boundary allow us to extract new geodynamic and geochemical information from the seismic observations. We have recently confirmed the theoretically predicted bulk sound speed decrease at the post-perovskite transition. In order to use this unusual property change for probing the chemical heterogeneities at the lowermost mantle, we have measured the equations of state of mantle silicate perovskite and post-perovskite with different chemical compositions (e.g., Fe2+ and Fe3+) using the same experimental setup, including pressure standard and pressure medium at the GSECARS sector of the Advanced Photon Source and beamline 12.2.2 of the Advanced Light Source. We found that the magnitudes of changes in density and bulk sound speed across the post-perovskite transition are greater by a factor of two in Fe3+ bearing silicates than in Fe2+ bearing silicates, which will make the post-perovskite transition much more visible in Fe3+ enriched regions of the lowermost mantle. From the fact that Fe3+ is associated with Al in the mantle, our study provides a new probe to search for chemical heterogeneities (Fe3+ and/orAl) in the lowermost mantle.

We have measured nuclear forward scattering of Fe2O3 post-perovskite at Sector 3 of the Advanced Photon Source. We found that Fe2O3 undergoes high spin ® low spin ® high (or mixed) spin transitions and magnetic ® nonmagnetic ® magnetic transitions during the hematite ® orthorhombic ® postperovskite transitions at high pressure. This suggests that structural transitions existing in the deep planetary interior strongly perturb pressure-induced magnetic collapse and spin pairing, which would result in complex depth dependence of some physical parameters sensitive to the electronic and magnetic structures of Fe.

Jupiter-sized Earth-like extrasolar planets, super-Earths, have recently been discovered. Understanding of the inner workings of these planets requires measurements at even higher pressures and temperatures. Therefore, a nanometer sized bright X-ray source will play a critical role in understanding the evolution and dynamics of these planets.