Experimental investigation of reduction in the CaO-SiO2-CO2 system with application to the formation of Breyite inclusions in diamonds

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Ca-rich minerals can occur as inclusions diamond and these have attracted special attention since their parageneses suggest a sub-lithospheric or “superdeep” origin, potentially from the transition zone or even lower mantle. One mineral of particular interest is CaSiO3 with the walstromite-type structure, which has now been named breyite. This phase occasionally occurs along with larnite or titanite-structured CaSi2O5, or both phases and has often been considered to represent a re-equilibration product from a Ca-perovskite precursor. However, this interpretation has recently been questioned. Alternatively, breyite could form by reaction of CaCO3 with SiO2 in former sediments of subducted slabs. This would involve reduction via a reaction like: CaCO3+SiO2 = CaSiO3+C+O2, where breyite and diamond crystallize simultaneously. This study aims to experimentally determine the possible conditions of breyite formation under reducing conditions in equilibrium with elemental carbon in order to explore the feasibility of trapping this Ca-silicate as an inclusion during diamond growth. Experiments will be performed at 6-10 GPa and 1000-1500°C in the simple CaO-SiO2-CO2±H2O system. Two scenarios simulating reactions in subducting sediment will be investigated: 1) interaction between CaCO3 and SiO2 layers under reducing conditions to model the reaction mentioned above; and 2) reaction of CaCO3 and SiO2 mixtures in the presence of a reduced COH fluid that has a low CO2 activity. Breyite could form via the reaction CaCO3 + SiO2 = CaSiO3 + CO2, except that melting occurs in the CO2-endmember system. Fluids with a lower CO2 activity could prevent melting, allow breyite to crystallize, and potentially stabilize diamond as well. In both experiment series, the oxygen buffering assemblage will be physically separated from the sample material to prevent unwanted reaction, but yet impose the desired oxygen fugacity.