Phase Transformations, Microstructures, and their Seismic Signals from the Earth's mantle

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Our knowledge of the solid Earth is built upon concerted research in different fields: For example, phase transformations in minerals induce physical boundaries in the Earth's interior. The analysis of seismic signals arising from these regions brings key information for our knowledge of the structure, composition, and dynamics of the planet. Due to the extreme conditions of pressure and temperature in Earth's interior, minerals undergo drastic transformations and their properties must be investigated in laboratories under realistic conditions. In parallel, seismology is one of the few means of direct observation of deep Earth structures as seismic waveforms are constrained by the present-day state of matter along their propagation path. The transition into the lower mantle at 660 km depth, for instance, has been characterized through seismology. Mineral physics demonstrated that it is mostly due to the decomposition of the mineral, ringwoodite, into ferropericlase and bridgmanite. Can we move our Earth model beyond simple comparison between seismic discontinuities and mineral reaction depths? Can we use seismic signals from boundary layers to characterize processes deep inside the Earth? Will this change our current view of the Earth? These are the questions the TIMEleSS project aims to answer. Phase transformations induce changes in the material's structure, density, elastic properties, but also microstructure, i.e. the arrangement of mineral phases, grain sizes, grain orientations, and strains. Boundaries with discontinuous physical properties in the Earth induce signatures in the seismic signals. But part of the signals measured in seismology and their connection to deep Earth processes are not fully understood. This is especially true for the regions lying between 600 and 1700 km depth, with a complex structure of reflections at 660 km, small scale-structures at mid-mantle depth, and an elusive supplementary discontinuity at ~1000 km. By the end of this project, we intend to constrain and model the effect of phase transformations and microstructures on such observations and use this new knowledge to interpret physical processes in this depth range. This project requires high pressure/temperature experimental studies and state-of-the art in-situ methods for understanding microstructures induced by phase transformations in relevant mineral compositions. In parallel, we will conduct seismological studies to analyze new combinations of waves, that, when used together, offer stronger possibilities to decipher physical parameters of structures in the mantle. Combining these two fields allows to better understand connections between phase transformation, microstructures and their associated seismic signals. TIMEleSS' goal is to develop new approaches and to address questions which cannot be explained by a simple analysis of the sequence of thermodynamic phase transitions as they involve microstructural and dynamic processes deep inside the Earth.