The speciation of carbon within eclogite rocks as function of pressure, temperature and oxygen fugacity

The speciation of volatile elements within mantle rocks is strongly controlled by the local redox conditions buffered by the surrounding abundant Fe-bearing minerals. The application of an oxy-thermobarometer proposed for eclogite rocks (Stagno et al., 2015) allows thermodynamic predictions of the redox state for a subducting oceanic crust as function of pressure and temperature. In contrast, the speciation of carbon during subduction can only be inferred if the fo2 at which carbon (graphite or diamond) and carbonate (solid/melt) coexist is known. This fo2 can be calculated using a thermodynamic dataset for the subsolidus end-member mineral equilibrium as dolomite + coesite = diopside + carbon + oxygen (DCDG/D; Luth et al. 2003). However, an experimental calibration of this oxygen buffer at temperatures and pressures at which CO2-rich melts can form is still missing. We performed experiments at pressures between 3 and 6 GPa and temperatures between 900 and 1300 °C using the Voggenreiter 840 t, Walker-type multi anvil press available at HP/HT Lab at National Institute of Geophysics and Volcanology (INGV, Rome). The starting material employed for all the experiments is a mixture of synthetic omphacitic glass, quartz, dolomite and graphite representative of the Dolomite-Coesite-Diopside-Graphite buffering assemblage [DCDG/D] loaded in a graphite capsule wrapped by a metal foil. The synthetic eclogite mixture is, then, sandwiched between layers of ilmenite + rutile (1:1 ratio) and ~3 wt% iridium used as redox sensor (Taylor et al., 1992). The recovered quenched samples were polished for textural and chemical characterization of the mineral phases using FE-SEM and the electron microprobe. Preliminary results shows that with increasing temperature a carbonatitic melt forms with ~5 wt% SiO2 at 900 °C. At 1100 °C, the melt is carbonate-silicate with 25 wt% SiO2, that increases to ~32 wt% SiO2 at 1200 °C. The experimentally measured fo2s plotted as function of pressure and temperature appears lower than thermodynamic predictions, and show a gradual decrease as the silica content of the melt increases. The results from this study show that magmas with compositions from carbonatitic to carbonate-silicate (hybrid) melts can form in eclogite rocks within less than 1 log unit of fo2 interval. Our results are, then, integrated with additional experiments performed at lower mantle conditions to model the origin of diamonds from CO2-bearing fluids by redox reactions involving Fe-bearing minerals.