Synchrotron Radiation Study of the Reaction-Deformation Coupling Processes in Earth's Mantle Minerals at High Pressures

The reaction-deformation coupling in silicate minerals is a critical issue to understand the dynamics of subducting oceanic plates in deep Earth. We conducted high-pressure eutectoid transformation experiments with constant strain-rate deformation. High-energy monochromatic X-rays were used to measure reaction kinetics and creep behavior simultaneously at high pressures. These quantitative measurements combined with microstructural observations of recovered samples suggest sequential variation of creep mechanisms from dislocation creep of the eutectoid colonies to grain-size sensitive creep in the deformation-induced degenerated colonies. These reaction-deformation coupling processes are very important to understand the large deformation and the cessation of deep earthquakes in the lower mantle. Oceanic plates are subducting into deep Earth as cold currents of mantle convection. When the subducted plates (slabs) pass through the mantle transition zone (between the two major seismic discontinuities at 410 and 660 km depths), most of the constituent minerals undergo high-pressure transformations, resulting in significant changes of rock density, microstructures, and viscosity. This has been thought to be responsible for deep slab deformation and deep earthquakes. Seismological and mineral physics studies have revealed that non-equilibrium phase transformations control density and viscosity structures of subducting oceanic slabs [1]. Especially, the processes of reaction-enhanced ductility and reaction-induced faulting are crucial for understanding deep slab deformation and deep earthquakes. In order to reveal such reaction-deformation coupling processes, we have started to conduct simultaneous and quantitative measurements of creep behavior and reaction kinetics at high pressures by combining high-pressure deformation apparatus with synchrotron monochromatic X-ray measurements. Here we report experimental results on creep behavior during eutectoid transformation [2].