Archives of experimental conditions in solid media apparatus: A new method of determining pressure and temperature applied to a re-evaluation of piston-cylinder techniques and the spinel to garnet phase transition.

Abstract

Considerable uncertainty persists as to the conditions undergone by samples during high pressure, high temperature solid media experiments, despite decades of research using piston-cylinder and multi-anvil apparatus. Pressure- and temperature-composition relations in the Zn2SiO4-Mg2SiO4 and ZnO-MgO and binary systems offer a new means for recording sample conditions in situ during experiments with unprecedented precision in pressure. These P,T-X relations, and phase relations have been investigated experimentally, and a thermodynamic model of the ZnO-MgO-SiO2 system has been developed, which not only allows the pressure and temperature at which experimental samples have been equilibrated to be calculated, but also sheds new light on the properties of rocksalt-structure ZnO, a material of interest due its semiconductor properties. These new pressure and temperature standards are applied to a quantitative evaluation of pressure calibration in the piston-cylinder, showing that previous assumptions of the behaviour of high pressure cells are inaccurate, which has important implications for the entire body of work conducted using piston-cylinder apparatus. A new method has also been developed to measure temperature gradients in situ during piston-cylinder experiments. This understanding of the sample conditions in solid media apparatus is applied to the spinel/garnet transition, a phase transition that is crucially important for geochemical and geophysical models of the upper mantle. The pressure and temperature of the spinel/garnet transition is determined in MAS (MgO-Al2O3-SiO2) and CMAS (CaO-MgO-Al2O3-SiO2) systems, and a long-standing discrepancy between experimental data in these two simplified chemical systems is resolved. These new results are used to evaluate the performance of the most commonly used thermodynamic model covering phase relations in the upper mantle. Show more