In this section an overview of the most important on-going projects is given. Information concerning recently-completed projects can be obtained from the publication lists of sections 4.1 and 4.2. Please note that these contributions should not be cited.
3.1 Phase transformations and deformation of mantle minerals: investigating mantle dynamics
Transformations of upper-mantle minerals (olivine, pyroxene) to higher density phases are responsible for the seismic discontinuities that define the Earth's transition zone (410 - 660 km deep). These phase transformations are important for understanding mantle dynamics because of the large changes in physical properties that accompany the changes in mineral structure. Under conditions of a normal mantle geotherm, (Mg,Fe)2SiO4 olivine transforms to the mineral wadsleyite (ß-phase) at 410 km which then transforms to ringwoodite, with the spinel structure (γ-phase), at 550 km. At the top of the lower mantle (660 km) ringwoodite transforms to a mixture of magnesiowüstite and a mineral with the perovskite structure. Similarly, (Mg,Fe)SiO3 enstatite transforms first to a high-pressure clinoenstatite and then into a garnet known as majorite in the transition zone. Under lower temperature conditions that prevail in subduction zones, the high-pressure form of enstatite is expected to transform to wadsleyite plus stishovite, ringwoodite plus stishovite, and then to the ilmenite structure with increasing depth, respectively. The mechanisms of these transformations and the resulting microstructures strongly effect physical properties and anisotropy within the Earth's mantle. We are therefore combining high-pressure experimental techniques with microstructural characterization to investigate the kinetics and mechanisms of these mantle phase transformations.
Phase transformations are especially important when applied to subduction dynamics because of the large lateral variations in temperature associated with down-going slabs of relatively cold oceanic lithosphere. The low temperatures in down-going slabs have two possible effects on phase transformations. Assuming equilibrium conditions, transformation of olivine to wadsleyite would occur at shallower depths than 410 km in cold slab interiors. However, at the low temperatures, reaction kinetics are expected to be sluggish which may cause olivine and pyroxene to persist metastably to depths as great as 700 km. The depth at which phase transformations occur in subducting slabs affects the density of the slab and therefore the buoyancy forces acting on the slab. Metastable persistence of olivine has also been used to explain the origin of deep-focus earthquakes associated with subduction zones. In the transformational faulting model of deep-focus earthquakes, faulting deep in subducting slabs is caused by the metastable transformation of olivine to spinel. To test the hypotheses of metastable peridotite in subduction zones, the kinetics and mechanisms of olivine and pyroxene transformations are being investigated at high pressure.
The rheology and viscosity of the mantle is dependent on the strength of the high-pressure phases as well as the distribution of these phases in the mantle and the microstructures that develop from phase transformations. With the development of high-pressure deformation techniques in the multi-anvil apparatus, we are able to deform mantle minerals, including the high-pressure phases, under pressure and temperature conditions of the upper mantle and transition zone. The relative strengths of the minerals are being investigated as well as the effects of phase transformations on rheology. Microstructural characterization of samples deformed at high-pressure using the transmission electron microscope allows us to investigate deformation mechanisms likely to occur in the Earth's mantle and the effects of stress on the mechanisms of phase transformations.