Melting Studies of Core-forming Alloys

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The results expected from this project will improve our understanding of the temperature and chemical composition of Earth's core. Earth's core is known to be an iron-rich alloy, mostly molten but with a solid inner core. In addition to iron, it contains approximately 5% nickel and approximately 10% of a lighter element, whose identity is uncertain but is likely a combination of oxygen, sulfur, silicon, and/or carbon. A more precise understanding of the composition of Earth's core is important to deduce better the chemical and physical processes that led to the formation of our planet. An underutilized constraint on this problem is the density contrast at the solid inner core to liquid outer core boundary, where solidification has taken place over time. This density difference, determined by seismological studies to be approximately 7%, implies a subtantial compositional difference between the solid and liquid, and is not easily satisfied by existing phase diagrams of the relevant iron-rich systems. These phase diagrams are poorly known, however, and accordingly, the aims of this project are to obtain them by measuring the compositions of coexisting solid and liquid iron alloys under the extreme pressure, temperature conditions that exist in Earth's core. The experimental results will be applied to place more precise bounds on the composition of Earth's core. This research project will also serve as a platform for the training of graduate and undergraduate students in advanced laboratory research and scientific pursuit.The specific experimental research goals of this project are to measure the solid/melt phase relations in the Fe-O-S system up to pressures and temperatures comparable to those in Earth's deep interior (i.e. exceeding 200 GPa and 4000 K), using laser heated diamond anvil cell technology. The experiments will be performed in the Laboratory for Mineral Physics at the University of Chicago. Sample extraction and microanalysis will be performed using focused ion beam (FIB) and scanning electron microscopy (SEM). Selected samples may also be examined by transmission electron microscopy (TEM) as needed. The measured compositions of coexisting liquid and solid phases will be used to construct phase diagrams for the Fe-rich alloy systems. To augment the experiments, the results will be used to guide a thermodynamic description of the alloy systems; this will minimize the number of experiments needed as well as provide a framework for extrapolating / interpolating the results to pressure-temperature-composition points that are not directly studied by experiment.