Chlorine stable isotopic composition of basement fluids of the eastern flank of the Juan de Fuca Ridge (ODP Leg 168) Magali Bonifacie a, ⁎ , Christophe Monnin b , Nathalie Jendrzejewski a , Pierre Agrinier a , Marc Javoy a a Laboratoire de Géochimie des Isotopes Stables, Equipe de Physico-Chimie des Fluides Géologiques, UMR7154, Institut de Physique du Globe de Paris–Université Paris VII, 2 place Jussieu, T54-64 E1, 75251 Paris Cedex 05, France b Laboratoire Mécanismes de Transfert en Géologie, CNRS–Université Paul Sabatier, 14 avenue Edouard Belin, 31400 Toulouse, France Received 27 July 2006; received in revised form 4 April 2007; accepted 6 May 2007 Editor: H. Elderfield Available online 10 May 2007 Abstract This paper presents chlorine stable isotope compositions (δ 37 Cl) 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 37 Cl value of the fluid directly sampled from a flow at the base of Site 1026 (WSTP1026) is also reported. NBF display δ 37 Cl 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 37 Cl 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 37 Cl values show a general decreasing trend with distance from the ridge-axis except for two fluids. When plotted against δ 18 O values, the δ 37 Cl of the NBF show two different trends. This paper discusses the possible contributions on NBF δ 37 Cl 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 37 Cl variations, the potential effect of the sediment cover is also investigated. At site 1026, the interstitial pore fluid displays a δ 37 Cl 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 37 Cl values. The potential contributions of physical processes associated with transport/compaction (e.g., diffusion, ion membrane filtration, adsorption, ion exchange) on NBF δ 37 Cl 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- Earth and Planetary Science Letters 260 (2007) 10 – 22 www.elsevier.com/locate/epsl ⁎ Corresponding author. Present address: Division of Geology and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard. M/C100-23, Pasadena, CA 91125, USA. E-mail address: bonifaci@gps.caltech.edu (M. Bonifacie). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.05.011