FORMATION OF GRAPHITIC CARBON IN A DECARBONATATION FRONT IN ECLOGITES FROM CORSICA (FRANCE)

The geochemistry of organic carbon in subduction zones may be strongly affected by mineral equilibria. We study here the geochemistry of carbon in siliceous-marbles at the direct contact with serpentinites in the Alpine eclogitic meta-ophiolithic units of northern Corsica (France). We have applied a set of spectroscopic (XANES, RAMAN, EDX) and isotopic technics to provide a description of both the organic and carbonate components of the rocks across a decarbonatation front where the reaction CaCO₃+SiO₂=CaSiO₃+CO₂ takes place.

The continuous reaction zone is composed by a centimeter thick pale nephrite layer at the contact with the serpentinites, followed by a thin wollastonite layer and a 5 to 20 cm thick dark zone composed of wollastonite, carbonaceous material (CM), quartz but no carbonates. There is a sharp (<0.5cm) transition to the overlying original metasediment composed of calcite+quartz which is significantly less rich in CM. Raman spectroscopy shows that CM is much more graphitic in the reaction zone than in the original rock. Significant isotopic differences are observed apart the decarbonatation front with δ¹³C (CM) and δ¹³C (calcite) around -16‰ and 2‰ respectively in the original rock far from the reaction zone, whereas δ¹³C (CM) is around -2‰ in the reaction zone. We interpret the graphitic CM in the reaction zone as formed from the reduction of CO₂ produced by the decarbonatation and due to the diffusion of reducing fluids from the underlying serpentinite unit. Mass transfer calculation supports this hypothesis and shows that a complete reduction of carbonates might have occurred. We show that the combined study of isotopic geochemistry and structure of inorganic and organic carbon in metamorphic rocks might be suited to reveal with great fidelity redox gradients in subduction zones.