Vesselin Dekov (IFREMER)
propose un séminaire intitulé :
Cu-isotope systematics of seafloor hydrothermal systems (résumé ci-joint)
organisé par l’équipe Ressources Minérales de GeoRessources et le Student Chapter SGA de Nancy
Vendredi 13 février 2015
13h-13h45
salle de conférences GeoRessources, Faculté des Sciences et Technologies - Vandoeuvre-les-Nancy - entrée 3B.
Résumé
This presentation focuses on the Cu-isotope composition of vent fluids, chimney sulfides and host rocks from several seafloor hydrothermal fields in the Manus back-arc basin. We found that temperature, magmatic degassing and boiling at sub- and super-critical conditions control the Cu concentrations of the vent fluids. Cu-isotope composition of the vent fluids (measured for the first time) shows spatial and temporal variations and a tendency for positive fractionation relative to the bulk Earth reservoir. Several processes contribute to the Cu-isotope variability in the vent fluids: (1) magmatic volatile input; (2) sub-seafloor precipitation; (3) sub-seafloor re-dissolution; (4) boiling and phase separation.
Heterogeneity of the source rocks and seawater ingress in the upflow zones of the hydrothermal systems are not factors influencing the Cuisotope signature of the venting fluids. A redox-induced distillation fractionation model is used to explain δ65Cu variation across the chimney wall. It is based on the fact that the reduction of Cu2+ in the vent fluid results in precipitation of Cu+-sulfides which preferentially incorporate 63Cu. During the gradual Cu precipitation through the porous chimney wall the hydrothermal fluid becomes progressively depleted in 63Cu and passively enriched in 65Cu. δ65Cusulfide gradually increases towards the chimney exterior. It decreases sharply in the external chimney oxidation layer as a result of the alteration of primary sulfides at ambient seawater conditions and preferential leaching of 65Cu. Two heavy Cu-isotope fluid fronts meet in this external alteration layer: a hydrothermal fluid depleted in 63Cu due to sulfide precipitation in the interior and seawater-based alteration fluid enriched in 65Cu due to sulfide dissolution at the exterior. Oxidative dissolution of seafloor hydrothermal sulfides and (possibly) hydrothermal fluids provide a heavy Cu-isotope flux to the ocean. It is combined with the passive enrichment of seawater in heavy Cu due to fractionation (preferential 63Cu sorption to the seafloor Fe-Mn-oxyhydroxide deposits) and balances the light Cu-isotope input from the continents (via rivers and wind).