Thermodynamic Measurements and Phase Equilibria of High-Pressure Silicates

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Funding is requested to measure the low-temperature heat capacities of selected high-pressure mantle silicates. Heat capacity is a fundamental thermodynamic parameter that until recently has been impossible to measure at low temperatures on small volumes of synthetic material. Synthesis of the desired mineral phases will be conducted in a multi-anvil apparatus and low-temperature heat capacity measurements will be conducted with a Physical Properties Measurement System. Heat capacity measurements will be carried out on several mantle phases, including high-pressure polymorphs of olivine and feldspar. These measurements will allow generation of the Gibbs free energies of these and associated phases from the extant enthalpy measurements and/or experimental phase equilibria. Results of this study are expected to shed light on the stability of assemblages in the mantle and will allow calculation of new phase diagrams for high-pressure rocks that are metamorphosed during subduction and continental collision. A deeper understanding of the increasing number of rocks known to have reached ultrahigh pressures and returned to the surface during mountain-building events or explosive volcanic action requires the evaluation of new reactions, to which this study will contribute materially. This work contributes to a broader understanding of fundamental tectonic processes in the deep Earth. Previous studies of mantle assemblages have been based on many simplifications, including relatively simple compositions. Systems of expanded composition more closely represent the real mantle and the volcanic rocks that are generated there. Future study of such complex systems is dependent on a thorough knowledge of the thermodynamics and phase equilibria of important mantle minerals including those explored in this proposal. The phase equilibria provide important constraints on the structure and dynamics of the mantle, which is the source for the great majority of volcanic rocks and for many major earthquakes.