The stability of carbonate bearing minerals and melts with respect to diamond in the Mantle

Dr. Daniel Frost

Bayerisches Geoinstitut

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 have studied the redox conditions under which this oxidation process occurs, thus mapping out the stability fields of carbon-bearing species in the mantle. We have studied this oxidation process for carbon-bearing peridotitic bulk compositions both above and below the solidus. 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 have examined this “redox melting” process, which may mark the onset of melting in the mantle and control the melt fraction at which melting commences. In addition we have examined the ferric/ferrous ratios of mantle minerals at oxygen fugacities buffered by the coexistence of carbonate phases and graphite/diamond. Similarly we have studied 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.