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

Abstract

This paper presents chlorine stable isotope compositions (δ 37Cl) 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 δ 37Cl value of the fluid directly sampled from a flow at the base of Site 1026 (WSTP1026) is also reported. NBF display δ 37Cl values between − 2.09‰ and − 0.12‰ relative to the Standard Mean Ocean Chloride (SMOC defined as 0‰) and small variations in chlorinity (∼ 4%). These data contrast with the homogeneity of δ 37Cl 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. 221(2005) 279–288.]. The NBF δ 37Cl values show a general decreasing trend with distance from the ridge-axis except for two fluids. When plotted against δ 18O values, the δ 37Cl of the NBF show two different trends. This paper discusses the possible contributions on NBF δ 37Cl 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 δ 37Cl variations, the potential effect of the sediment cover is also investigated. At site 1026, the interstitial pore fluid displays a δ 37Cl signature significantly lower than that of the fluid discharge sample (− 1.90‰ and − 0.28‰, respectively). This difference, demonstrated here cannot be an artifact of the sampling method, rather indicates the influence of the sediment cover on NBF δ 37Cl values. The potential contributions of physical processes associated with transport/compaction (e.g., diffusion, ion membrane filtration, adsorption, ion exchange) on NBF δ 37Cl 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 δ 37Cl evolution of seawater over geologic time.