Mineral Physics of Earth and Planetary Interiors
Inner core anisotropy
Located over 5000 km below the Earth’s surface, the inner core represents the most remote portion of our planet. Seismology has provided a wealth of information on the elastic behavior of this region. It has been established that the inner core exhibits elastic anisotropy with compression waves traveling 3% faster in the polar than in the equatorial direction. There may be additional complexities in the structure, such as variations in anisotropy with depth and differences between the eastern and western hemispheres. Recently we demonstrated that by coupling a number of newly developed in-situ x-ray synchrotron techniques – hydrostatic axial x-ray diffraction, radial x-ray diffraction, nuclear resonant inelastic x-ray spectroscopy, and phonon inelastic x-ray spectroscopy – the elastic anisotropy of iron can be characterized at high pressure (Mao et al, GRL 2008). We plan to extend these integrated studies to inner core conditions, which we hope will produce the accurate results critical for interpreting the observed inner core anisotropy and its implications for the structure, composition, and evolution of Earth’s core.
Earth’s D” Layer
The D” layer, the lowermost portion of the mantle, sits just above the molten iron-rich outer core. Seismic observations have revealed a region with an intriguingly complex signature. This relatively thin layer, varying around 250 km in thickness, may hold the key to understanding how the core and mantle interact. The D” layer may also be where deep mantle plumes originate and where subducting slabs terminate. Some of the most puzzling seismic features include the splitting of shear-waves travelling through this layer and the presence of ultralow-velocity zones (ULVZ). ULVZs are thin (5- 40 km thick) patches in which the compressional and shear wave velocities are depressed by 5-10% and 10-30%, respectively, relative to the neighboring region.
We found that silicate post-perovskite (ppv), the major constituent in the D” layer, can be highly enriched in iron (Mao et al, PNAS 2004b, Mao et al, PNAS 2005), which has a dramatic effect on its physical and chemical properties (Mao et al, GRL 2006; Mao et al, AGU Monograph 2007). The ability of post-perovskite to absorb iron may explain the anomalously low velocities in ULVZs (Mao et al, Science 2006). More recently we studied the elastic anisotropy of this phase and found that it can exhibit large shear-wave splitting (Mao et al, PEPI 2009), consistent with seismic observations and theoretical calculations. We have been working on transport measurements of the thermal conductivity which is a key property for studying Earth’s thermal evolution and internal dynamics (Goncharov et al, PEPI 2010), and investigating how iron enrichment affects Fe-Mg partitioning and structural variation among a number of possible post-perovskite phases. The goal of these studies is to compile a comprehensive set of materials properties for each individual phase as well as for mineral assemblages, in order to deepen our understanding of how this boundary layer plays such a central role in global dynamics and the evolution of Earth.
Planetary Core Formation
Early-stage core-mantle differentiation and core formation represent pivotal geological events that defined the major geochemical and geophysical signatures of Earth. In order to test potential mechanisms and hypotheses of core formation, we need textural information on the interaction and separation of the solid and liquid phases. Previous experiments have been mostly limited to upper mantle conditions, though the actual events may have occurred at much higher pressures and temperatures. DAC can achieve the necessary pressure and temperature conditions of this region, but textural study of a 100 µm-sized sample requires nanoscale in-situ probes that do not currently exist.
We have been working todevelop nanoscale x-ray computed tomography (nanoXCT) within a laser-heated diamond anvil cell. Pilot studies we conducted this year have revealed the exciting potential for using nanoXCT as a powerful 3D petrographic probe for non-destructive, nanoscale (<40nm) resolution of multiple minerals and amorphous phases (including melts). This will extend measurements of the texture, shape, porosity, dihedral angle, and other characteristics of molten iron-rich alloys coexisting with silicates and oxides to the relevant high pressure and high temperature conditions. NanoXCT can also be used to investigate grain shape, intergrowth, orientation, and foliation -- as well as mineral chemistry and crystallography -- to understand whether shape-preferred orientation is a primary source of the observed seismic anisotropy in Earth’s D” layer and to determine the textures and shapes of the melt pockets and channels formed by the putative partial melt which may exist in ULVZs.
Novel DAC-Synchrotron X-ray Technology
Our ability to address questions about planetary interiors is often limited by what properties can be measured and what conditions can be accessed (Mao and Mao, Treat. Geophys. 2007). We have been developing promising, new applications for x-ray synchrotron probes at extreme conditions. These include using partial fluorescence yield x-ray absorption spectroscopy to probe the pressure-induced changes in the 3d electron band in Fe2O3 and collaborating with Tom Devereaux’s theory group in Photon Science to interpret the spectra (Wang et al., PRB 2010). We have also used energy dispersive EXAFS to study element-specific local structural changes in amorphous materials under compression (Baldini et al, PRB 2010). Additionally, we have used nanodiffraction to study the structure of crystalline Fe (Wang et al., PNAS 2010), Fe-rich ppv (Yamanaka et al., J. Phys. 2010), and Ca (Mao et al., PNAS 2010), demonstrating that this technique can be used to select individual submicron crystals within a heterogeneous sample and resolve extremely steep pressure, temperature, and compositional gradients. These technological developments have opened up new directions and fulfilled specific needs in my high-pressure research plan.