Earth is a very complex and dynamic system in which global geologic phenomena such as volcanism, earthquakes or plate tectonics have major impacts on human civilization. Proper understanding of these global phenomena requires microscopic models of chemical and physical processes in which rocks, minerals and fluids are involved, as well as understanding of changes in the physical properties of the Earth forming materials at high pressure and temperature. Changes in the structure of materials (phase transitions) and chemical reactions between the major stable mineral components of the Earth's interior have been convincingly linked with observed seismic velocity changes (discontinuities). At the same time, however, geophysicists gather more and more convincing evidence that the Earth is heterogeneous in composition, temperature and density throughout the Earth. Dynamic geologic environments, such as subduction zones can also produce conditions that are quite far from the normal mantle. All these facts fuel new motivation for exploring the lower-temperature metastable regime in high pressure experiments in search for previously unknown metastable transformations, which may impact our interpretation of observations from seismology.
Pyroxene minerals constitute a significant portion of the Earth's crust and upper mantle. The general layout of the pyroxene stable phase diagram has been well established, however, until recently, the limitations of experimental techniques prevented systematic investigations of the metastability effects at conditions corresponding to depths higher than 300 km. Equipped with a novel experimental method, synchrotron single-crystal micro-diffraction, the investigators recently discovered a number of intriguing, previously unknown phase transformations in the pyroxene family, occurring in the metastable regime. Based on these preliminary results they hypothesize that metastable phase diagrams of pyroxenes are far more complex than previously assumed, and that the previously unknown polymorphic transformations may affect buoyancy of the subducting slab and could be seismically detectable. In this project they propose to use a combination of advanced experimental and computational methods, including synchrotron single-crystal micro-diffraction, Raman spectroscopy, first principles quantum mechanics calculations and thermokinetic modeling to conduct a systematic investigation of the pyroxene system and covering range of pressures from ambient to 60 GPa and temperature from ambient to 1200°C, representing estimated conditions within a subduction zone. Crystallographic investigations, such as synchrotron single-crystal micro-diffraction experiments provide fundamental information about the structural parameters such as density, atom coordination geometry, bond lengths, etc., which govern the physical and chemical properties of minerals and are indispensible for building reliable geophysical and geochemical models. Computational quantum mechanics models, on the other hand, allow utilization of these crystallographic results to calculate thermodynamic properties of minerals, which are difficult or impossible to measure experimentally at high pressure and temperature. Thermokinetic modeling will enable scientists to put the crystallographic and thermodynamic information in the geophysical context and determine the implications of the newly discovered transformations. The project is expected to contribute to improvement and optimization of experimental methodology for synchrotron single-crystal micro-diffraction experiments at the national synchrotron user facilities, create unique research and education opportunities for graduate and undergraduate students from the University of Hawaii to participate in state-of-the-art experiments at these facilities, and to significantly improve in house experimental infrastructure in Hawaii for student training and research.
