Ž . Chemical Geology 175 2001 343–360 www.elsevier.comrlocaterchemgeo Fluid–sediment interactions related to hydrothermal circulation in the Eastern Flank of the Juan de Fuca Ridge Martine D. Buatier a, ) , Christophe Monnin b , Gretchen L. Fruh-Green c , ¨ Anne-Marie Karpoff d a UniÕersite des Sciences et Technologies de Lille, UFR des Sciences de la Terre, CNRS r UMR 8577 A Sedimentologie et Geodynamique B , ´ ´ ´ 59655 VilleneuÕe d’Ascq Cedex, France b ( ) Laboratoire de Geochimie UMR 5563 CNRS — UniÕersite Paul Sabatier, 38 rue des Trente Six Ponts, 31400 Toulouse, France ´ c Department Earth Sciences, ETH Zurich, Sonneggstr. 5, CH-8092 Zurich, Switzerland d CNRS r EOST, Centre de Geochimie de la Surface, 1 Rue Blessig, 67084 Strasbourg, France ´ Received 14 September 1999; accepted 31 May 2000 Abstract Ž . Ten sites were drilled in the eastern flank of the Juan de Fuca Ridge North East Pacific along a 100 km-long east–west transect during Leg ODP 168. This study focuses on the mineralogical and chemical study of sediments that overly basaltic basement through which seawater circulates. Silicate authigenesis was observed in the sediment layer just above basement at sites located more than 30 km from the ridge axis. This sediment alteration is particularly abundant at ODP Sites 1031 and 1029 where authigenic formation of Fe–Mg rich smectite and zeolite and the dissolution of biogenic calcite are observed. Comparison of the distribution of the alteration in the basal sediment collected along this transect suggests that diffusional transport of aqueous solutes from the basement into the overlying sediment cannot produce the mineralogical and chemical changes in the basal sediments at Sites 1031 located on a basement topographic high, and at Site 1029 located at about 50 km from the ridge axis on a buried basement area. Vertical advection of basement fluid though the sediment section is required to produce this alteration. These processes are still active at Site 1031, based on systematic variations in pore-water profiles and temperatures obtained from stable isotopic data on calcium carbonates and the nature of authigenic minerals. At Site 1029, there is no present-day advection of basement fluids though the sediment section, suggesting that this is a relic site for fluid flow. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Juan de Fuca; hydrothermal; Clays; Ridge flank; Sediments 1. Introduction Physical and chemical effects of hydrothermal fluid circulation within the upper oceanic crust can ) Corresponding author. Present address: Departement Geo- ´ ´ sciences, UFR Sciences et Technique, Universite de Franche- ´ Comte, 16 Route de Gray, 25030 Besanc ¸on, France. ´ be traced through the study of water–rock interac- tions in the sediments. In particular, the nature and chemical composition of authigenic minerals reflect the nature of the fluids from which they precipitated and are indicative of formation temperatures. Authi- genic phyllosilicates are commonly described in Ž oceanic hydrothermal systems Honnorez et al., 1983; Alt et al., 1986; Laverne et al., 1989; Bourasseau et 0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2541 00 00327-2