Yousheng Xu, Yu Nishihara, Zhenting Jiang, Shun-ichiro Karato
Yale University
shun-ichiro.karato@yale.edu
High-pressure and high-temperature deformation experiments are critical for the understanding of deep Earth dynamics but currently the quantitative data on rheology and deformation microstructures are missing above ~1 GPa. Consequently, rheological properties and microstructural development in Earth deeper than ~30-40 km have been virtually unconstrained from mineral physics. To extend the depth range for quantitative study of rheological properties and microstructural development to deeper portions of Earth, we have developed a new apparatus for deformation experiments under high-pressure and high-temperature by modifying the Drickamer-type opposed anvil apparatus. The Drickamer-type apparatus consists of opposed anvils with lateral support from the gasket that allows one to conduct high-pressure (and high-temperature) experiments beyond the limit of the Bridgman apparatus. We have added a torsion actuator to the Drickamer apparatus and conducted large-strain deformation experiments on (Mg,Fe)O and wadsleyite. Our goals are (1) to determine deformation microstructures (fabrics (i.e., lattice preferred orientation), grain geometry such as the connectivity of a weaker phase) under quasi-steady state, large strain conditions and (2) to obtain flow laws for Earth materials under conditions equivalent to the transition zone and deeper. Large strain is essential in investigating deformation microstructures. It is also critical to conduct deformation experiments under well-controlled conditions (i.e., constant strain-rate) as opposed to stress-relaxation tests in which stress changes significantly in a single run. The torsion actuator consists of a dc servo-motor and a harmonic-gear that allows deformation experiments with a constant rate of rotation. A thin disk-shape sample (~1.6 mm diameter, ~0.2 mm thick) is sandwiched between two disk-shaped furnaces (TiC+ diamond) separated by a thin layer of Al2O3. Deformation experiments up to the shear strain of ~4 were conducted at ~1200°C to ~16 GPa. Initial results on (Mg,Fe)O and wadsleyite show significant deformation in samples and corresponding fabric development. Quantitative rheology measurements are planned using the X-ray stress measurements technique at one of the synchrotron beam lines in near future.