CO2 flow during orogenic gravitational collapse: Syntectonic decarbonation and fluid mixing at the ductile-brittle transition (Lavrion, Greece)

The knowledge of CO2 transfer between carbonate reservoirs is a key issue to understanding climate evolution through geologic times. Convergent plate boundaries represent major zones for carbon recycling via burial of carbonates. In this paper, we document syntectonic decarbonation of marble at the ductile-brittle transition during the emplacement of the low-angle mylonitic to cataclastic detachment exposed in the Lavrion peninsula along the western border of the Attico-Cycladic Metamorphic Complex. This process is evidenced by the presence, in a quartz vein boudinaged and transposed in the mylonitic marble, of CO2-rich fluid inclusions (i) dismembered and deformed along quartz subgrain boundaries, (ii) within deformation lamellae and (iii) in planes crosscutting subgrains. Microstructural analysis, stable isotope equilibrium and VX properties show that CO2 results from syntectonic decarbonation under a 70-115 °C·km− 1 thermal gradient during exhumation accommodated by regional NNE-SSW extension. We propose that decarbonation and CO2 release under HT/LP conditions were triggered by the increase in temperature caused by thermal relaxation of the thickened crust further enhanced by the intrusion of a granodiorite pluton during Miocene gravitational collapse of the Hellenic orogenic belt. Subsequent circulation of surface-derived fluid equilibrated with carbonaceous material of organic origin above the ductile-brittle transition led to precipitation of low 13C carbonates. This study points that the production of CO2 by syntectonic decarbonation of marble layers, representing tectonically accreted carbonates along convergent plate boundaries, should be considered in the carbon cycle and might thus impact the climate.