Manganese Metal and Manganese Oxide Reactions with CO2 at High-Pressures and Temperatures

The goal of this study is to measure the relative phase stability of end-member transition metal oxides and carbonates at temperatures and pressures equivalent to Earth's mantle. This is accomplished by reactions experiments between mantle-abundant transition metals and CO₂ in the laser-heated diamond anvil cell. Studying these reactions will determine the stability field of carbon-bearing phases, which will lead to a better understanding of carbon storage in the Earth. This requires measurements of the phase stability and thermoelastic properties of metals, oxides, and carbonates at deep-Earth conditions. Transition metals are of interest because they readily form both carbonates and oxides at ambient conditions. Additionally, their varying compatibility in mineral phases allows them to be used as geochemical tracers. Manganese is of particular interest because it is one of the most abundant transition metal geochemical tracers in the mantle.

Here, we present X-ray diffraction data obtained from a series of reaction experiments of manganese metal and manganese oxide with CO₂ at high pressures and temperatures in the laser-heated diamond anvil cell. Three different samples of Mn metal were gas-loaded with CO₂, and compressed to 6, 23, and 44 GPa, and laser-heated at beamline 12.2.2 at the Advanced Light Source. Diffraction patterns obtained after heating showed creation of rhodochrosite (MnCO₃) at 6 GPa and 23 GPa, indicating that the carbonate phase is more stable than the oxide phase at these conditions. This was confirmed via additional experiments in which manganese oxide was loaded with CO₂ and laser heated at 15 and 30 GPa. In both cases, rhodochrosite formed. For the Mn metal plus CO₂ experiment at 44 GPa, the crystal structure of Mn metal has clearly changed after laser heating. In addition, many new diffraction peaks are observed, precluding a clear identification of a manganese-bearing carbonate and/or oxide phase.