4.5 STAGNO Vincenzo

Fe speciation in ferropericlase and bridgmanite as indicator of diamond-forming processes in the Earth’s lower mantle

Vincenzo Stagno

The knowledge of the oxidation state of the Earth’s interior is needed to understand conditions at which volatiles (carbon, hydrogen, sulfur, and nitrogen) are mobilized, as well as their role in melting processes and diamond formation. Under oxidizing conditions, for instance, carbon will be in the mobile form of carbonate (solid or melt), while under reducing conditions immobile phases such as diamonds and carbides will be stored in the interior of the Earth. In the lower mantle, iron-nickel metal alloy is predicted to be stable along with Fe-periclase (Fe-pc) and bridgmanite (Bgm) that can incorporate volatiles such as carbon or hydrogen. However, superdeep diamonds brought to the surface from the transition zone and lower mantle by kimberlitic eruptions often show inclusions of carbonates either solid or liquid suggesting that deep portions of Earth might be more oxidized than previously thought. The main goal of this study was to investigate the iron speciation (Fe3+/Fe2+) between Bgm and Fe-pc, the rock-forming minerals of the lower mantle, as function of pressure, temperature and oxygen fugacity. Such data are essential to verify whether these minerals, also found trapped in natural diamonds, can be taken as indicators of the redox state of the Earth’s lower mantle. In addition, results from these studies allow to develop an oxybarometer for the lower mantle based on which the release of volatiles and diamond formation can be modeled over time.