Volatile elements, such as carbon (C) and nitrogen (N), play an essential role on Earth in forming living organisms, maintaining a habitable climate, tracing geological processes, and powering the core dynamo. Constraining the distribution and budgets of volatile elements on terrestrial planets, such as the Earth and Mars, holds the key to understanding their formation, evolution, dynamics, and habitability. In this dissertation, I performed a series of experiments to investigate the physical properties and chemical behavior of carbonates and iron nitrides at high pressure and temperature conditions to decipher the role of carbon and nitrogen in the Earth and planetary deep interior.The circulation of carbon between reservoirs on Earth’s surface and interior is the key to governing long-term atmospheric CO2 budget. Carbonates, including calcite (CaCO3), magnesite (MgCO3), and dolomite (CaMg(CO3)2, are believed to be the major carriers to transport surface carbon to the Earth’s mantle at subduction zones. However, the global carbon flux to the convecting lower mantle and the stable format of carbon at mantle conditions remain largely uncertain, due to lack of constraints on thermodynamic properties of subducted carbonates and limited understanding of the fate of carbonates through subduction. In chapter 2, I measured the thermal equation of state of CaCO3-Pmmn, a stable polymorph of CaCO3 through much of the lower mantle, using synchrotron X‐ray diffraction in a laser-heated diamond-anvil cell up to 75 GPa and 2200 K. Using the newly determined thermodynamic parameters, I modeled the physical properties of CaCO3-Pmmn and (Ca,Mg)-carbonate-bearing eclogite, showing the presence of carbonates in the subducted slab is unlikely to be detected by seismic observations, and the buoyancy provided by carbonates has a negligible effect on slab dynamics. In chapter 3, I examined the stability of MgCO3 and CaCO3 coexisting with the mantle silicates along mantle geotherm. With in-situ X-ray diffractions and ex-situ electron microscopic analysis, I showed if CaCO3 can be transported to the deep lower mantle and even the core-mantle boundary, it can remain stable and coexist with the mantle silicates, while MgCO3 can only be stable at depth above ~1850 km. The observations indicate CaCO3 the dominant host of oxidized carbon at the core-mantle boundary.The presence of light elements in the core are inferred by seismic and cosmochemical observations, and possible light elements are narrowed to Si, O, S, C, H. Recently, nitrogen has been added to the candidates’ list, and thus iron nitrides are possible constituents in the Earth’s and other terrestrial planet cores. However, the physical properties, especially pressure-induced magnetic changes and effects on compressibility of iron nitrides remain poorly understood. In chapter 4, I constrained the magnetic transition pressure and the equation of state of ε-Fe7N3 and γ’-Fe4N up to 60 GPa at 300 K, indicating the completion of magnetic transition induces elastic stiffening in ε-Fe7N3 by 22% at ~40 GPa, but has no resolvable effect on the compression behavior of γ’-Fe4N. I re-examined evidence for magnetic transition and effects on compressibility of other candidate components of terrestrial planet cores, Fe3S, Fe3P, Fe7C3, and Fe3C, showing the completion of magnetic transition of Fe3S, Fe3P and Fe3C induces elastic stiffening, whereas that of Fe7C3 induces elastic softening.To sum up, this dissertation expands our understanding on the role of carbon and nitrogen in the properties of Earth and planetary interiors, by revealing the stability and fate of carbonate subducted to the lower mantle and the physical properties iron nitrides.
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