Deep Mantle/Inner Core: Long-term Goals and Motivations

Geodynamic motivation

Recent studies indicate that the dynamics and evolution of this planet (and other terrestrial planets) is strongly influenced by processes in the deep interior of these planets. The topography of Earth's surface is controlled to a large extent by the mass and viscosity distribution in the deep mantle, and the dynamics and evolution of continents are determined by the rheological properties and the density of materials in the deep continental roots. Chemical evolution associated with large-scale material circulation also controls the distribution of key elements, particularly hydrogen (water), which in turn influences the style of material circulation through its effects on rheological properties. The core dynamics that controls the generation of the geomagnetic field is also affected by the process in the solid inner core, such as the plastic flow that determines the degree of core-mantle coupling.  In these studies, understanding the physical and chemical properties of materials under deep Earth (planets) conditions is key.

Mineral physics research topics

1. Whole Earth (planet) rheology (rheological properties and deformation microstructures)
2. Processes controlling the distribution of hydrogen and grain-size
3. Relationships between geophysical observables (e.g., seismic velocity, attenuation, anisotropy, electrical conductivity) and water content.
4. Microstructural and chemical characteristics of deep mantle rocks to infer rheological and chemical evolution in these regions
5. Density of silicate melts (influence of pressure, temperature and chemical composition)
6. Melt transport process, melt-solid wetting angles

High-Pressure Rheology

We have developed a new apparatus for experimental study of rheology under the deep Earth conditions. This apparatus (RDA: rotational Drickamer apparatus) can be operated at a synchrotron X-ray facility and allows us to quantitatively investigate the rheology and microstructural development at pressure >16 GPa (temperature>1800 K) at large shear strains.

Grain-growth Kinetics in Wadsleyite

Grain-growth kinetics in wadsleyite control the grain-size and hence the rheology of wadsleyite-rich regions (transition zone). We have found that the grain-growth kinetics of wadsleyite is highly sensitive to water content and oxygen fugacity. Grain-size (and rheology) of wadsleyite in the subducted slabs will be highly sensitive to water content as well as temperature.

Effects of Hydrogen on Electrical Conductivity

Hydrogen enhances electrical conductivity (Karato, 1990). We have tested this hypothesis for wadsleyite and ringwoodite, and obtained the relationships between hydrogen content and electrical conductivity. The hydrogen content in the transition zone can be estimated from the comparison of these laboratory results with geophysical measurements of electrical conductivity.

Density of Silicate Melts

Density of silicate melts controls the chemical evolution of Earth and other terrestrial planets. We focus on the role of water on silicate density. With a small addition of water (~5 wt%) ultramafic melt is still denser than the surrounding solid minerals. We also investigate the water speciation in silicate melts. [OH]/[H2O] ratio is predicted to increase with pressure.

Dihedral Angle between Melts and Solids under Transition-zone Conditions (Collaboration with T. Yoshino of RPI)

The dihedral angle (the contact angle between liquid and solids) controls the geometry of liquid and hence its mobility. We investigate the dihedral angle between silicate melts (with and without water) with minerals such as olivine, wadsleyite under the conditions of ~400-450 km depth.

Upper Mantle: Long-term Goals

Deformation Fabrics of Upper Mantle Rocks

We investigate the deformation fabrics (LPO: lattice-preferred orientation) of olivine over a broad range of physical and chemical conditions. The LPO of olivine is a function of water content, stress and temperature. A diagram showing such relationships (a fabric diagram) has been constructed and is being applied to interpret deformation fabrics of naturally deformed olivine and seismic anisotropy in the upper mantle.

A new measure of fabric strength has also been developed. We have demonstrated that there are technical problems with the currently used method, the J-index, and propose a new measure of fabric strength based on the misorientation angles between each pair of grains (the M-index: misorientation index).

Orthopyroxenes in naturally deformed peridotites often show microstructures and microstructural develpment that contrasts strongly with that of olivine. Applying the M-index method, we characterize the LPO development in naturally deformed opx from kimberlite xenoliths. The mechanism of dynamic recrystallization in opx is different from that of olivine leading to a smaller grain-size and deformation by grain-size sensitive creep. This may have important implications for the rheology of the upper mantle. 

Chemical evolution of upper mantle materials involves complicated changes in chemical compositions. However, under some conditions, chemical evolution can be described by a small number of parameters. Chemical evolution controlled by partial melting at mid-ocean ridges can be characterized by a single parameter (Mg#). Characterization of chemical evolution of peridotites from deep continents, however, requires at least two parameters: Mg# and opx#, implying that more than two processes (partial melting at mid-ocean ridges and addition of silica-rich component) are involved in the evolution of these rocks.

We have developed a new method for inverting seismic tomographic results in terms of anomalies in water content, temperature and major element chemistry. We use tomographic results on Vp, Vs and Qp to map water content, temperature and major element chemistry. Current resolution does not allow us to map out major element chemistry, but anomalies in water content and temperature are well resolved. A region of high water content is found in the deep upper mantle that extends far away from the subducted slabs.