Fe-bearing redox reactions do control the deep carbon cycle in the interior of the Earth

The redox conditions of the Earth’s upper mantle are generally constrained through the use of oxy-thermobarometers, i.e. the chemical equilibrium involving iron bearing silicate/oxide end-member minerals that are representative of peridotite and eclogite mantle rocks [1,2]. The use of these oxy-thermobarometers requires the determination of the ferric/ferrous iron ratio in minerals like spinel and garnet equilibrated with minerals such as olivine, orthopyroxene and clinopyroxene. The knowledge of the iron speciation in minerals allows to model the variation of the mantle redox state, which likely control the speciation of carbon in the form of grapite (diamond) and carbonate (either solid or fluid) as function of pressure and temperature. Similarly, other mantle minerals such as wadsleyite, ringwoodite, bridgmanite, ferropericlase, have the potential to incorporate ferric iron in their structure, implying their possible role in controlling the carbon speciation also at transition zone and lower mantle depths. The aim of this study was to investigate the iron oxidation state of spinel, garnet, cpx, wadsleyite, ringwoodite, ferropericlase and bridgmanite and their textural features when coexisting with both carbonate and graphite/diamond. Experiments were performed using synthetic mixtures of Fe-bearing minerals and carbon/carbonate bearing assemblages, between 3 and 50 GPa and temperature of 900-1700 °C. Additional runs were performed by loading the capsules with monolayers of Fe-bearing minerals in contact with graphite or carbonate monolayers. The quenched products were analyzed by electron probe and Moessbauer spectroscopy. Preliminary results from these experiments allow to better understand processes such as redox melting, redox freezing and diamond formation that significantly affect the deep carbon cycle.