CARBON CYCLING IN SUBDUCTION ZONES: THE IMPORTANCE OF METASOMATISM

Subduction zones play a key role in the long-term carbon cycle as they transfer carbon from the Earth’s surface to the deep Earth. In the downgoing slab, carbon is present as solid reduced organic (graphitic carbons and diamond) and oxidized inorganic (carbonates) phases, as well as in fluids. The respective stability of these phases during subduction, particularly carbonates, is the subject of intense research, but we have a poor knowledge on the possible exchanges between the C reservoirs and on the influence of local redox and chemical gradients on the stability of C-bearing phases during subduction. Alpine Corsica (France) provides a unique window on subduction processes with no retrograde alteration as attested by the widespread presence of lawsonite eclogites and blueschists. Two examples of metasomatism involving C-bearing phases at lithological contacts will be discussed based on detailed field, petrological and geochemical observations. At a first locality where impure marbles are at direct contact with serpentinites, complete calcite reduction is observed generating graphite and secondary wollastonite. Conversely, at a second locality, graphite is completely leached from the matrix in metasediments at direct contact with a gabbro, and only remains as inclusions in secondary lawsonite in the reaction zone. Both localities illustrate metasomatic processes during subduction affecting C-bearing phases in metasediments at contact with mafic or ultramafic lithologies. The global implications of such metasomatic reactions on the long-term carbon cycle will be discussed.