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doi:10.2204/iodp.proc.301.203.2008

Introduction

During Ocean Drilling Program (ODP) Leg 168, 10 sites were drilled across the eastern flank of the Juan de Fuca Ridge to examine conditions of fluid-rock interaction in three distinct hydrothermal regimes extending over a ~120 km linear transect perpendicular to the spreading ridge (Davis, Fisher, Firth, et al., 1997). The main aim of Integrated Ocean Drilling Program (IODP) Expedition 301 to the eastern flank of the Juan de Fuca Ridge was to evaluate the formation-scale hydrogeologic properties within oceanic crust, determine how fluid pathways are distributed within an active hydrothermal system, and elucidate relations between fluid circulation, crustal stratigraphy, alteration, microbiology, and seismic properties (see the "Expedition 301 summary" chapter).

In this report, we present data for Pb isotopic analysis of representative sediment samples, which are squeezed residues that remained after taking pore water, collected from IODP Site U1301 (Fig. F1). Study of lead isotopes is important for understanding seafloor hydrothermal processes, including leaching of oceanic crust during high-temperature alteration, and the source of metals in hydrothermal deposits. The relatively impermeable sediment cover over the spreading center traps hydrothermal fluids and allows chemical interaction between fluids and sediments. For instance, sediment-hosted massive sulfide deposits, currently forming at the sediment-filled Middle Valley segment of the northern Juan de Fuca Ridge, have Pb isotopic compositions intermediate between Juan de Fuca basalts and Middle Valley turbiditic sediments (Cousens et al., 2002), indicating that hydrothermal fluids have interacted with the sediment pile.

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