Origine et circulation des fluides dans les sédiments des marges. Contribution de l'hélium et du méthane dans la compréhension des processus. Étude de 2 zones actives

Fluid migration is a very important process in marine sediments on margins. Studying fluids is fundamental to constrain the energy potential of margins. Methane is the predominant gas in marine sediments, deriving primarily from the degradation of organic matter. However methane could be also generated through the serpentinization of mantle rocks underlying the sedimentary cover in some specific geodynamical settings. One way to test this hypothesis is to use helium isotopes, which are known to be powerful tracers of the fluid-mantle interaction. To this purpose, a new method has been developed for the sampling and the quantitative extraction of dissolved helium from sediment pore-waters. During the Zairov2 and Vicking cruises, samples were collected in two cold seep areas: the Regab pockmark on the Congo-Angola margin, and the Hakon Mosby mud volcano on the Norwegian margin. Helium isotopes in pore-fluids reveal no mantle signature, which indicates the absence of any methane generated by serpentinization in these areas. Nevertheless, taken together with methane and major ion contents in pore-waters, helium isotope profiles obtained with this new analytical method give significant new results. At the Regab pockmark, helium appears to be a more sensitive tracer of water advection than temperature, allowing the determination of advection velocities and the quantification of expelled water rate. At the Hakon Mosby mud volcano, the results confirm a concentric zonation of advection and a deep origin for the volcano fluids.