Phase stability and thermoelastic property constraints of manganese metal, oxide, and silicate at mid-mantle conditions

Abstract:

Manganese is an important minor transition element in the Earth's crust and mantle, and a potentially powerful geochemical indicator of core/mantle formation and evolution; especially if analyzed in conjunction with Fe and Ni. Yet compared with Fe and Ni, significantly less is known about the structure, stability, and thermoelastic properties of Mn metal and its oxides, carbonates, and silicates high pressures and high temperatures. Here we present the results of an experimental study of the high pressure temperature phase diagram of Mn2SiO4-olivine, measured during laser heating at 20 GPa, 26.6 GPa, and 36.6 GPa at beamline 12.2.2 at the Advanced Light Source. Our results show that at the low end of this pressure range, perovskite-structured MnSiO3 is formed; and at the higher end of this pressure range, MnO + SiO2 is stabilized. We introduce two new approaches to extracting high precision information from poor or "spotty" X-ray diffraction patterns from polycrystalline material in the diamond cell. We show a method to extract the set of positions, intensities, and widths of individual scatterers at each two-theta positions. This provides information complementary to the integrated intensity versus two-theta data, including more precise measurements of the positions of each diffraction peak, and additional information about the diffracted peak intensities. The coexistence of phases in the diamond cell at high pressure permits precise measurements of the lattice parameters of MnSiO3 and MnO, with respect to SiO2 stishovite as an internal standard. Our new measured constraints on the thermodynamic properties of MnSiO3 and MnO are combined with existing measurements on other Mn-bearing species such as Mn metal and MnCO3 rhodochrosite, to help inform models of the thermodynamic behavior of Mn at high pressures and temperatures, and in a variety of different species relevant to Earth's core and mantle.