Experimental modeling of ferropericlase formation under upper mantle conditions

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Ferropericlase, (Mg, Fe)O, is one of the major phases in the lower mantle. It occurs in diamonds from a number of localities worldwide either as single inclusions or together with ultrahigh pressure phases such as Mg-Perovskite (bridgemanite). Ferropericlase on its own or together with olivine is stable even at 1 atm. Diamonds with ferropericlase inclusions alone may therefore also originate in the transition zone or even the upper mantle for appropriate bulk rock compositions. Simultaneous with diamond formation, ferropericlase may precipitate from carbonatitic melts or grow by decomposition of subducted carbonates induced by redox reactions. In order to explore the possibility of ferropericlase crystallization in the upper mantle, we will conduct equilibrium experiments involving ferropericlase and silico-carbonate melt at ~6-10 GPa and temperatures up to 1600°C. An experimental configuration will be employed where carbonate and metal phases are separated by an olivine layer. Thus, the process of oxidation (Fe+1/2O2=FeO) and reduction (MgCO3=MgO+C+O2) will be spatially separated, and conditions will be investigated where ferropericlase and graphite/diamond crystallize directly from Mg-rich silico-carbonate melt. Experiments will be conducted in the high-mg# portion of the FeO-MgO-CaO-SiO2-CO2-C system with the goals of: (1) determining the composition of silico-carbonate melt in equilibrium with ferropericlase and olivine as a function of pressure and temperature, and (2) simulating ferropericlase crystallization via reduction of silico-carbonate melt with simultaneous formation of graphite or diamond.