Elasticity and Spin Transitions of Iron in the Earth's Lower Mantle

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Earth's mantle is a silicate rocky shell about 2900 km thick that constitutes approximately 84% of the planet's volume. It is further separated into three layers, upper mantle, transition zone, and lower mantle, as a result of the structural phase transitions of the olivine polymorphs. The lower mantle ranging from 670 km to 2900 km in depth is mainly made of bridgmanite (silicate perovskite) and ferropericlase in which iron is the most abundant transition metal. Deep-Earth scientists have recently discovered that the spin transitions of iron in these lower-mantle minerals can have a series of effects on physical, chemical, and transport properties of the host minerals. Studying the sound velocity profiles of the lower-mantle minerals across the spin transition of iron can thus help us understand the chemistry and physics of the region. With an emphasis on the effects of the spin transitions, this project aims broadly to understand thermal elasticity of the lower-mantle minerals at relevant pressure-temperature-composition conditions, providing direct mineral physics results and thermodynamic database to be integrated with seismic tomographic images, geochemical analyses, and geodynamic models of the mantle. The geophysical and geochemical consequences of the spin transition in the lower mantle will be examined through modelling the thermal elasticity and the partitioning coefficient of iron in ferropericlase and bridgmanite along an expected lower-mantle geotherm. These results will help test hypotheses on seismic wave profiles, elucidate the chemistry of iron and its compositional model, and determine the dynamic behavior of the Earth's deep mantle.The proposed mineral physics research uses synchrotron X-ray and in-house laser spectroscopies in a diamond anvil cell designed to probe structural and elastic properties of the mantle minerals at pressure-temperature conditions of the Earth's mantle. Under the initiatives of the project, students and postdoctoral researchers will have unique research opportunities to use advanced synchrotron X-ray facilities at the Advanced Photon Source and laser spectroscopies at the University of Texas at Austin to obtain laboratory results needed to decipher seismic and geochemical observations of the planet's lower mantle. These activities will contribute to the education of the next generation of independent researchers with a thorough knowledge of the Earth's deep interior. Outreach activities in this project focus on exposing underrepresented K-12th graders to deep-Earth research by involving them in the outreach summer programs. Research results from this award will be disseminated broadly through teaching, seminars, conferences, and peer-reviewed publications. The proposed work enhances infrastructure for research and education by using shared facilities and the development of new collaborations among researchers in different fields. The project will provide students and postdoc researchers with a great opportunity to collaborate with scientists in the multidisciplinary fields of seismology, geodynamics, and geochemistry.