Preliminary deformation results to 12 GPa pressure using the Deformation-DIA

High-pressure studies of the rheological behavior of Earth materials under high pressures are essential for understanding the dynamics of Earth's interior. However, outside of shock experiments and those in the diamond anvil cell, the highest working pressure for conventional deformation rigs (e.g., triaxial gas-medium apparatus, Griggs solid-medium apparatus) has been about 4 GPa. We report here first results from a new deformation apparatus called the deformation-DIA (D-DIA), a cubic-anvil, solid-medium apparatus specifically designed for constant-pressure deformation and capable of deforming samples at high temperatures (up to 2000 K) to pressures up to 15 GPa. Based on the cubic DIA apparatus, the D-DIA allows independent motion of the two vertical anvils to impose deformation on samples, and at the same time allows reverse motion on the four side anvils, thus preventing pressure from rising during deformation. Preliminary tests have been conducted on samples of NaCl, MgO, and olivine. The cold-pressed samples (1-2 mm in length and 1.2 mm in diameter), hard alumina pistons, and thermocouples form a 6-mm-long deformation column that is inserted into a graphite resistance heater within a 6-mm edge length cubic pressure medium, either pyrophyllite or boron epoxy. The cell is first squeezed hydrostatically to reach desired pressures and then deformed in compression at constant pressure. Experiments are carried out using synchrotron radiation, which makes it possible to measure sample length change and pressure in-situ to levels of <1 mm and ~10 MPa, respectively. To date we have carried out several constant displacement rate experiments at sample shortening rates of ~1 x 10-5 to 4 x 10-4 s-1 over shortening strains of 10 - 30% at temperatures of 500 - 1200°C and pressures of 5 - 12 GPa. Pressure is monitored by the location of diffraction peaks of a small amount of well-characterized proxy material (often MgO) in the deformation column. Pressure is controlled manually by metering hydraulic fluid from the main ram (which drives the side anvils) at a rate such that diffraction pattern holds constant as the deformation proceeds. In our first tests, we have been able to hold pressure to within < 1 GPa of the desired value. With experience, better use of proxy materials, and eventual use of x-ray transparent anvils, we expect to achieve much finer control of pressure. We are as yet unable to determine the state of deviatoric stress within samples because the tungsten carbide (WC) anvils we now use limit x-ray diffraction to the vertical plane (At this stage, stresses within samples are estimated from quenched dislocation density within deformed samples). Modifications to the D-DIA are being undertaken that will allow us to replace x-ray opaque WC anvils with transparent cubic boron nitride anvils, which will not only to make multi-plane diffraction possible but may increase our pressure limit above 15 GPa. Our goal is to conduct quantitative deformation experiments under high pressures and temperatures at P-T conditions corresponding to or near those of Earth's transition zone.