Phase relations and partial melting of C-bearing sediment under mantle conditions

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Subduction of sedimentary rocks is believed to be an important process for volatile and trace element recycling in the Earth's mantle. Experimental and theoretical investigations show that carbonates and hydrous minerals may survive subduction. They can produce deep metasomatizing fluids which are important for the formation of enriched domains in the mantle. Available experimental data were obtained in systems with either CO2 or H2O or both CO2 and H2O. In both water- and CO2-bearing experiments, the oxygen potential was probably rather high, whereas many natural sediments are reduced. Changes in the proportions of H2O, CO2 and reduced volatile species (CH4, H2, etc.) may affect both the composition and the properties of the melt/fluid phase. Experiments will be carried out at 7-12 GPa and ~800-1400°C using a multianvil press. This pressure range is of special interest because a number of reactions occur that control the stability of hydrous phases in pelitic systems including decomposition of phengite and lawsonite, dehydration of topaz and formation of high-pressure phase EGG. Starting materials will be synthetic mixtures corresponding to average sediment compositions [Global Subducted Sediment (GLOSS)] with variations in H2O, CO2, the K/Na ratio, and the addition of graphite to achieve reducing conditions. The bulk compositions will also be doped with a range of trace elements (including REE, LILE and HFSE).