Intellectual merit. Previous studies proposed that the global carbon cycle involved Fe3C or Fe7C3 as a dominant component in the solid portion of Earth?s core, even though critical data to test the hypothesis of carbide-rich inner core are still missing. Experimental results on the phase relation of the Fe-C binary system are limited to 70 GPa. Sound velocity data are only available for Fe3C, under conditions up to 68 GPa at 300 K and up to 47 GPa at high temperatures. Density measurements on Fe3C have reached ~ 200 GPa in pressure, but high temperature data are scarce. In particular, experiments are needed to assess the effects of magnetic transition near 70 GPa on the thermoelastic and vibrational properties of the iron carbides, and to quantify the influence of temperature on their magnetism, density and sound velocities. It is proposed to investigate the compression and melting behavior of iron carbides up to the core pressure regime and ~ 3000 K using multi-anvil and diamond anvil cell pressure devices. Specific goals include: [1] determine the partial phonon density of Fe3C and Fe7C3 using the nuclear resonant scattering method; [2] establish the thermal equation of state of Fe3C and Fe7C3 using the synchrotron X-ray diffraction methods; [3] determine the eutectic composition of the Fe-C binary system using a combination of in situ X-ray diffraction and focused ion beam FIB-based quench analysis methods. The proposed activities will provide new data on the phase relations, densities, and sound velocities of candidate iron carbides to core pressures and high temperatures, thus enabling a stringent test of inner core models involving carbon as a major component. The expected results will advance our understanding of Earth?s deep carbon cycle, a central and timely issue in solid Earth research. The investigations will establish stepping-stones toward an ultimate model of core chemistry by elucidating the role of magneto-volume and anharmonicity in the thermodynamics and lattice vibration of potential core materials. Key constraints on the carbon content of the core will complement geochemical, seismological, and geodynamic studies to expand the frontier of deep Earth research. This work will also contribute to astrophysics and planetary science on the pathways of carbon from star birth to planet differentiation, and to materials science and condensed-matter physics on the behavior of carbon-based materials and strongly correlated systems under extreme environments.
Broader impact. The lead PI is committed to teaching and training at high school to postdoctoral levels through course offerings, lectures, and tutorial sessions (e.g., for the Michigan Math and Science Scholar program and at the Cooperative Institute for Dynamic Earth Research Summer Schools). The proposed experiments will train a graduate student to tackle fundamental scientific problems using the state-of-the-art experimental techniques in the high-pressure research laboratories at U. Michigan and the Advanced Photon Source (APS) at the Argonne National Laboratory (ANL). The proposed activity will enhance the infrastructure for research and education through collaborative projects at regional and national facilities including the Geophysical Laboratory, APS, and National Nanotechnology Infrastructure Network (NNIN). The project will produce calibration standards for the community to support carbon-related research. The proposal will involve female researchers and provide partial support for Dr. Chen as a new PI. The PIs will continue to actively disseminate research results broadly through publications, national and international meetings, the Internet, and news media.
Previous studies proposed that the global carbon cycle involved Fe3C or Fe7C3 as a dominant component in the solid portion of Earth’s core, even though critical data to test the hypothesis of carbide-rich inner core are still missing. Experimental results on the phase relation of the Fe-C binary system are limited to 70 GPa. Sound velocity data are only available for Fe3C, under conditions up to 68 GPa at 300 K and up to 47 GPa at high temperatures. Density measurements on Fe3C have reached ~ 200 GPa in pressure, but high temperature data are scarce. In particular, experiments are needed to assess the effects of magnetic transition near 70 GPa on the thermoelastic and vibrational properties of the iron carbides, and to quantify the influence of temperature on their magnetism, density and sound velocities. Together with students and collaborators, the PI Jie (Jackie) Li has investigated the compression and melting behavior of iron carbides up to the core pressure regime and ~ 3000 K using the multi-anvil apparatus and diamond anvil cell. Specifically, they have established a Mie-Gruneisen-Debye equation of state and provided a Kunc-Einstein equation of state, which allows for calculations of any thermodynamic functions of Fe3C versus T and V or versus T and P. They investigated nonstoichiometry and growth of some Fe carbides and found that cohenite may contain 31 to 17 atomic % carbon and that the Eckstrom-Adcock carbide, nominally Fe7C3, showed compositions from 29 to 36 atomic % C at 7 GPa. They conducted experiments to study magneto-elastic coupling in compressed Fe7C3 and detected two discontinuities in the compression curve up to 167 gigapascals (GPa), the first of which corresponds to a magnetic collapse between 5.5 and 7.5 GPa and is attributed to a ferromagnetic to paramagnetic transition. At the second discontinuity near 53 GPa, Fe7C3 softens and exhibits Invar behavior, presumably caused by a high-spin to low-spin transition. Furthermore, the PI Jie (Jackie) Li and collaborators reviewed Experimental constraints on core composition and provided an up-to-date summary of constraints on the light element composition of the core. Their research on carbon in the core and its influence on the properties of core and mantle concluded that as a component of the "light" element in the core the upper likely concentration of carbon is ~1 wt%. The results obtained from this project have provided new data on the phase relations, densities, and sound velocities of candidate iron carbides to core pressures and high temperatures, thus enabling a stringent test of inner core models involving carbon as a major component. The expected results will advance our understanding of Earth’s deep carbon cycle, a central and timely issue in solid Earth research. The investigations have established stepping-stones toward an ultimate model of core chemistry by elucidating the role of magneto-volume and anharmonicity in the thermodynamics and lattice vibration of potential core materials. Key constraints on the carbon content of the core will complement geochemical, seismological, and geodynamic studies to expand the frontier of deep Earth research.These results will contribute to astrophysics and planetary science on understanding the pathways of carbon from star birth to planet differentiation, and to materials science and condensed-matter physics on the behavior of carbon-based materials and strongly correlated systems under extreme environments. Li has contributed teaching and training at undergraduate to postdoctoral levels through course offerings, lectures, and tutorial sessions (e.g., at the Cooperative Institute for Dynamic Earth Research Summer Schools). The proposed experiments have trained the future generation of researchers to tackle fundamental scientific problems using the state-of-the-art experimental techniques at multi-scale facilities ranging from the high-pressure research laboratories at Umich to the Advanced Photon Source (APS) at the Argonne National Laboratory (ANL). The research activities have enhanced the infrastructure for research and education through collaborative projects at regional and national facilities including the Geophysical Laboratory, APS, and National Nanotechnology Infrastructure Network (NNIN). The PI and associates have disseminated research results broadly through publications, national and international meetings, the internet, and news media. The proposed activity will benefit the society by helping to address the world's energy and environmental concerns.
