title: Physicochemical properties of Fe-bearing minerals and metal alloys at deep Earth conditions
Creators: name: Dominijanni, Serena
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Oxygen fugacity
subject: Fe oxidation state
subject: Redox sensors
subject: diamond inclusions
subject: Earth's inner core
subject: light elements
subject: sound velocity
subject: elastic properties of Fe-bearing alloys
subject: 550

description: Understanding the physicochemical properties such as oxidation state, composition, and elastic behavior of candidate materials at the conditions of the Earth's interior is essential for comprehending the evolution and dynamics of our planet. Knowledge of Earth's structure and composition mainly relies on geophysical and cosmochemical evidence, theoretical predictions and laboratory studies. Specifically, experimental works simulate extreme pressure and temperature conditions of the deep Earth using large-volume presses and diamond anvil cells combined with the laser heating system. However, although numerous advancements have been made in experimental techniques during the past decades, we still have an incomplete picture of many processes occurring in the Earth's interior. For example, the oxygen fugacity fo2 of Earth's mantle is a fundamental issue in geoscience since it influences numerous processes such as the mobility of volatile species, the depth of partial melting, and the speciation of magmatic gasses. Nevertheless, this parameter has only been poorly controlled or measured during diamond anvil cell (DAC) experiments, which represent the experimental gateway to the deepest region of Earth's mantle. To address this issue, we developed a novel approach to monitor the oxygen fugacity in diamond anvil cell based on the redox sensor method. Ferropericlase and iridium metal were mixed as starting powder and reacted in a DAC after laser heating to obtain a Fe-Ir alloy and use the activity-composition relation to calculate the fo2. We monitored the variation of redox reactions through Synchrotron Mössbauer Source, X-ray absorption near-edge structure, and electron energy loss spectroscopy measurements and chemical analyses down to the nanometer scale and compared the results with complementary multi anvil experiments. Moreover, we updated the existing thermodynamic model for the Fe-Ir redox sensor, currently available at room pressure, up to 100 GPa including the volume dependence on the activity composition-relations. The newly calibrated model for the Fe-Ir system was tested in experimental systems designed to simulate the interaction between a carbonated subducting slab and a Fe-saturated lower mantle to vary the local fo2 from the iron-wüstite buffer up to conditions where carbonate and diamond coexist, i.e. the lowest fo2 at which carbonate can survive and the highest fo2 at which diamonds are stable in the lower mantle. We investigated the local oxygen fugacity variation induced by the influx of carbonates as recorded by the FePP3+PP/ΣFe ratio in ferropericlase using in-house point source Mössbauer spectroscopy. Furthermore, we confirmed the feasibility of using ferropericlase as an oxybarometer for the lower mantle, observing a positive correlation between oxidation state and oxygen fugacity. Our experimental results are then integrated with measurements of FePP3+PP/ΣFe on ferropericlase inclusions in sub-lithospheric diamonds from Rio Soriso (Brazil) conducted with the Synchrotron Mössbauer Source to address the role of pressure, temperature and oxygen fugacity. We argue about the compositional and redox heterogeneity recorded by both sets of samples with implications for diamond formation via redox freezing of subducted carbonates at lower mantle conditions. Our results provide unique information on the maximum ferric iron content that ferropericlase can incorporate at the redox conditions where carbonate and diamond coexist to explain processes such as redox-driven diamond formation and redox melting. Another aspect that is still under debate is the Earth's inner core composition. It is widely accepted that some light elements, such as Si, are predicted to be in the inner core by compositional models to explain the density deficit between pure Fe and seismological observation of the inner core. However, the effect of Si and Ni on the elastic properties, i.e., density and sound wave velocities, of pure Fe is still poorly constrained. Based on the Fe-Si system's phase relations, the inner core might be composed of a Si-enriched B2 and Si-depleted hcp phase mixture. In this study, we tested this hypothesis by performing high-pressure measurements of the sound velocities and density of polycrystalline B2-Fe-(7 wt. %)Ni-(15 wt. %)Si using inelastic X-ray scattering and powder X-ray diffraction up to 100 GPa. We compared our results with previous experimental and ab initio studies conducted on Fe-Si, Fe-Ni-Si, and Fe-Ni alloys to explore the combined effect of Si and Ni on the bulk sound velocities and density. Moreover, using a thermodynamic model we extrapolated our results to the inner core boundary conditions and matches geophysical observations of density, compressional and shear wave velocities of the inner core. Our main findings provide new constraints on the role of Si as a light element in the inner core and propose a compositional model that accounts for seismological observations of Earth’s inner core.