Effect of Hydrogen on the Properties of Fe alloys in the Earth's Core

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For more than half a century, scientists have known that the Earth's core - the 3500-km large ball of mostly iron at the center of the planet - has a density 5-10% lower than that of pure iron. This is because the core contains light elements, such as Si, O, S, C, and H. While the core cools down forming the solid inner core, light elements segregate preferentially in the liquid outer core. This process powers in part the magnetic field which shields us from the solar wind. Furthermore, knowing the composition of the core is critical to constrain its formation and evolution. Light elements in metallic iron have, thus, been extensively studied at the extreme pressure and temperature conditions of the deep Earth. However, because of experimental limitations, the effect of hydrogen on core materials is still largely unknown. Here, the researchers investigate how hydrogen affect the properties of the core. Taking advantage of recent technical developments, they study the iron-hydrogen system at the high pressure and temperature of Earth's interior. The experiments, carried out at national synchrotron facilities, quantify the melting temperature and density of iron-rich alloys. Theoretical calculations at the atomic level guide the experimental approach and the data analysis. The results improve current models of the core with implications for the understanding of its formation, evolution and present-day magnetic field, with numerous ramifications in Earth Sciences. This two-year project provides support for an early-career female scientist and a graduate student, and training opportunities for undergraduate summer interns in state-of-the art Mineral Physics. It also increases the public awareness of the important role of hydrogen in the Earth through presentations during open-house events at Arizona State University.Here, the team investigates Fe-H, Fe-Ni-H and Fe-Si-H alloys in the laser-heated diamond anvil cell (DAC), where specimens are compressed at the tips of two opposing diamonds and heated by focused laser beams. Target pressures and temperatures are up to 150 GPa (~1.5 million atm) and in excess of 2000 K. Quantifying hydrogen-iron alloys at these conditions in the DAC has been challenging because H2 tends to break the anvils on heating. The team has recently demonstrated that this issue can be overcome by pulsed laser heating. This technique, coupled with an improved X-ray detection system at synchrotron facilities, allows the researchers to quantify in situ the effect of hydrogen on the structure, equation of state, bulk modulus and melting temperatures of iron-rich alloys. Run products are investigated by electron microscopy to study their texture and the partitioning of Si and Ni among the phases in presence. Ab initio calculations guide the experimental approach and the data analysis. The key questions leading the research are: (1) Does hydrogen change the crystal structures of iron metal and alloys in the Earth's core? What is the effect of hydrogen on (2) the equations of state of iron metal and alloys and (3) their melting temperatures at core conditions?This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.