Chlorine stable isotopic composition of basement fluids of the eastern flank of the Juan de Fuca Ridge (ODP Leg 168) | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS


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  Chlorine stable isotopic composition of basement fluids of the eastern flank of the Juan de Fuca Ridge (ODP Leg 168)

Type de publication:

Journal Article


Earth and Planetary Science Letters, Volume 260, Ticket 1-2, p.10-22 (2007)






<p>This paper presents chlorine stable isotope compositions (delta Cl-37) of sediment pore waters collected by squeezing sediment cores from the sediment-basement interface along an East-West transect through the eastern flank of the Juan de Fuca Ridge (ODP Leg 168). These "near basement fluids" (NBF) are generally thought to be representative of low-temperature fluids circulating in the off-axis basaltic crust. The delta(CI)-C-37 value of the fluid directly sampled from a flow at the base of Site 1026(WSTP1026) is also reported. NBF display delta Cl-37 values between -2.09 parts per thousand. and -0.12 parts per thousand. relative to the Standard Mean Ocean Chloride (SMOC defined as 0 parts per thousand) and small variations in chlorinity (similar to 4%). These data contrast with the homogeneity of delta Cl-37 values associated with highly variable chlorinities observed in high-temperature on-axis fluids [M. Bonifacie, J.L. Charlou, N. Jendrzejewski, P. Agrinier, J.P. Donval, Chlorine isotopic compositions of ridge axis high temperature hydrothermal vent fluids, Chem. Geol. 22](2005) 279-288.]. The NBF delta(CI)-C-37 values show a general decreasing trend with distance from the ridge-axis except for two fluids. When plotted against 6180 values, the delta Cl-37 of the NBF show two different trends. This paper discusses the possible contributions on NBF delta Cl-37 values of fluid-mixing, water-rock interactions and transport processes (diffusion, ion membrane filtration) that can occur in the igneous basement. However, as none of these processes can fully explain the observed delta Cl-37 variations, the potential effect of the sediment cover is also investigated. At site 1026, the interstitial pore fluid displays a delta Cl-37 signature significantly lower than that of the fluid discharge sample (-1.90 parts per thousand and -0.28 parts per thousand, respectively). This difference, demonstrated here cannot be an artifact of the sampling method, rather indicates the influence of the sediment cover on NBF delta Cl-37 values. The potential contributions of physical processes associated with tran sport/comp action (e.g., diffusion, ion membrane filtration, adsorption, ion exchange) on NBF delta Cl-37 values are qualitatively discussed here but require additional studies for further insights. However, this study indicates that "near basement fluids" (NBF) are not, at least for Cl isotopic compositions, necessarily as representative of fluids circulating in the basaltic crust as initially thought. These results add new constraints on Cl geodynamics and show that Cl-isotopes fractionate during low-temperature circulation of fluids in off-axis and off-margin flow contexts, but not to the extent observed for active margins. Fluids circulating at low-temperature in the magmatic and/or the sedimentary part of the oceanic crust might have played a major role on the delta Cl-37 evolution of seawater over geologic time.</p>


Earth Planet. Sci. Lett.Bonifacie, Magali Monnin, Christophe Jendrzejewski, Nathalie Agrinier, Pierre Javoy, Marc 54 ELSEVIER SCIENCE BV