Résumé
Mantle peridotites have the potential to trap CO (sub 2) as carbonates. This process observed in ophiolites and in oceanic environments provides a long term and safe storage for CO (sub 2) . It occurs as a part of a complex suite of fluid-rock reactions involving silicate dissolution and precipitation of hydrous phases, carbonates and minor phases that may in turn modify the hydrodynamic properties and the reactivity of the reacted rocks. The efficiency and lastingness of the process require the renewal of fluids at the mineral-fluid interface. Fractures are dominant flow paths in exhumed mantle sections. This study aims at better understanding the effect of CO (sub 2) -enriched saline fluids on hydrodynamic and chemical processes through fractured peridotites. Experiments were performed using the reactive percolation bench ICARE Lab 3--Geosciences Montpellier. It allows monitoring the permeability changes during experiments. Effluents are recurrently sampled for analysing cation concentration, pH and alkalinity. Reacted rock samples were characterized by high resolution X-ray microtomography (ESRF ID19, Grenoble, France) and SEM. Experiments consisted in injecting CO (sub 2) -enriched brines (NaCl 0.5 M) at a rate of 6 mL.h (super -1) into artificially fractured cores (9 mm diameter X 20 mm length) of Oman harzburgites at T=170 degrees C and P (sub total) = 25 MPa for up to 2 weeks. Fractures are of few mu m apertures with rough walls. Three sets of experiments were performed at increasing value of [CO (sub 2) ] (0, 0.1 and 1 mol/kg). All experiments showed a decrease in permeability followed by steady state regime that can be caused by a decrease in the roughness of fracture walls (dissolution dominated process), thus favoring fracture closing, or by the precipitation of secondary phases. Maximum enrichments in Mg, Fe and Ca of the effluent fluids occur during the first 2 hours of the experiments whereas Si displays a maximum enrichment at t = 20 h, suggesting extensive dissolution. Maximum enrichments are observed with the highest values of the [CO (sub 2) ]. After one day, effluent fluid concentrations decrease and become constant. By analyzing both the permeability and the outlet fluid concentration one can investigate the coupling processes controlling the transport and the reaction mechanisms that in turn act at maintaining the circulation in the fractures.