Experimental investigation of the redox conditions at which carbonate minerals and melts transform to graphite or diamond in Earths mantle

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The redox state of the Earth’s interior will influence the speciation of volatile elements both in the mantle and in mantle-derived magmas. Carbon is one of the principal elements to be affected in this way because under reducing conditions it forms graphite or diamond, and under oxidising conditions carbonate (or CO2-bearing) minerals and melts. In a series of high pressure and temperature experiments we will study the redox conditions under which this oxidation process occurs, thus mapping out the stability fields of carbon-bearing species in the mantle. We will study this oxidation process both in peridotitic and eclogitic bulk compositions. Several studies have indicated that the mantle may become more reduced with depth. This means that the oxidation of elemental carbon (graphite or diamond) may occur in up welling rocks where the oxidised product is a carbonate bearing magma. We will focus on this “redox melting” process, which may mark the onset of melting in the mantle and control the melt fraction at which melting commences. Similarly we will study the reduction of carbonate minerals with increasing depth in subduction zones, and particularly within subducted oceanic crust, which may have a direct influence over the residence time of carbon in the mantle.