The rheological properties of transition zone minerals have a strong influence on how mantle convection works. There is clear evidence, from seismic tomography, for intense deformation of deep sinking cold materials in the mantle transition zone. However, intense deformation of cold materials is at odds with the conventional view of temperature sensitivity of rheological properties. The main purpose of this project is to provide a new data set, based on newly developed high-pressure deformation techniques, by which we can solve this paradox. One hypothesis to resolve this paradox is that the phase transformations that occur in the transition zone will make materials weak by grain-size reduction. However, so far there is no quantitative data on the sensitivity of rheological properties of transition zone minerals on grain-size.
In this project, the investigator will perform a series of laboratory experiments on wadsleyite (one of the transition zone minerals), using RDA (rotational Drickamer apparatus), to determine the flow laws for grain-size sensitive deformation mechanisms. Samples to be used are synthetic polycrystalline aggregates with controlled grain-size and water content. Specimens will be deformed at constant strain-rates using RDA, and the mechanical data (stress-strain curves) will be obtained in-situ at a synchrotron facility (NSLS at Brookhaven). Deformed samples will be examined ex-situ after the recovery to study the microstructures using a range of techniques including optical microscope, EPMA, SEM (EBSD), FTIR, Raman and TEM. The results will shed light on how mantle convection occurs in Earth and how Earth evolves thermally.
Earth's mantle can be devided into three regions, the upper mantle, the mantle transition zone and the lower mantle. According to various geophysical observations, it is now clear that mantle convection penetrates deep into the mantle and in most regions convection current seems to go through the mantle transition zone. However, geophysical observations particularly seismic tomography (cat scan of Earth's interior using seismic waves) do show that the interaction between amntle convection and the mantle transition zone has a variety of mode. In some cases, convection current (seen from tomographic images) seems to encounter some resistance forces at around the mantle transition zone and the flow is distorted. In other regions, the convective current seems to go through the mantle transition zone without much deformation. Such a variety of interaction between mantle convection and the transition zone is partly controlled by the resistance of materials in the mantle transition zone for properties of transition zone minerals through laboratory studies. It should be emphasized that quantitative studies of plastic properties of transition zone minerals are challenging because no commercially avaialable equipment is present to conduct such studies. In my lab, we have spnet more than 10 years to develop such an equipment, and with continuing effort of its improvement, we have started to obtain quantitative data on plastic properties of transition zone minerals. We have obtained experimental data that can be used to understand the natuer of deformation of materials in the mantle transition zone for the first time. These data can be used in large-scale modelling to understand the nature of materials circulation in the deep interior of Earth. antle
