IODP

doi:10.2204/iodp.sp.327.2010

Introduction

Fluid flow within the volcanic oceanic crust influences the thermal and chemical state and evolution of oceanic lithosphere and lithospheric fluids; the establishment and maintenance of subseafloor microbial ecosystems; the diagenetic, seismic, and magmatic activity along plate-boundary faults; the creation of ore and hydrate deposits both on and below the seafloor; and the exchange of fluids and solutes across continental margins (e.g., Alt, 1995; Huber et al., 2005; Parsons and Sclater, 1977; Peacock and Wang, 1999). The global hydrothermal fluid mass flux through the upper oceanic crust rivals the global riverine fluid flux to the ocean and effectively cycles the volume of the oceans through the crust once every 105–106 y (Elderfield and Schultz, 1996; Johnson and Pruis, 2003; Mottl, 2003). Most of this flow occurs at relatively low temperatures, far from volcanically active seafloor-spreading centers where new ocean floor is created. This "ridge-flank" circulation can be influenced by off-axis volcanic or tectonic activity but is driven mainly by the rise of lithospheric heat from below the crust. Although the average maximum age at which measurable heat is lost advectively from oceanic lithosphere is 65 Ma (Parsons and Sclater, 1977), many sites remain hydrologically active for tens of millions of years beyond this age, with circulation largely confined to basement rocks that redistribute heat below thick sediments (Fisher and Von Herzen, 2005; Von Herzen, 2004).

Despite the importance of fluid-rock interactions in the crust, little is known about the magnitude and distribution of critical hydrologic properties; the extent to which crustal compartments are well connected or isolated (laterally and with depth); the rates and spatial extent of ridge-flank fluid circulation; or the links between ridge-flank circulation, crustal alteration, and geomicrobial processes. Expedition 327 is a critical part of a long-term experimental program that began nearly two decades ago and that has included several survey, drilling, submersible, and remotely operated vehicle (ROV) expeditions; observatory and laboratory testing, sampling, and monitoring; and modeling of coupled fluid-thermal-chemical-microbial processes. Expedition 327 builds on technical and scientific achievements and lessons learned during Ocean Drilling Program (ODP) Leg 168 (Davis, Fisher, Firth, et al., 1997), which focused on hydrothermal processes within uppermost basement rocks and sediments along an age transect, and IODP Expedition 301 (Fisher, Urabe, Klaus, et al., 2005), which penetrated deeper into the crust at the eastern end of the Leg 168 transect (Fig. F1). Both expeditions installed subseafloor borehole observatories (CORKs) in basement holes to allow borehole conditions to recover to a more natural state after dissipation of disturbances caused by drilling, casing, and other operations; to provide a long-term monitoring and sampling presence for determination of fluid pressure, temperature, composition, and microbiology; and to facilitate the completion of active experiments to resolve crustal hydrogeologic conditions and processes (Fisher et al., 2005). Subsequent ROV and submersible expeditions downloaded data from the Leg 168 and Expedition 301 CORKs and replaced batteries, loggers, and sampling systems at the seafloor and downhole.

The primary goals of Expedition 327 are to (1) drill two new basement holes, core and wireline log one of these holes across a depth range of 100–360 meters subbasement (msb), conduct a 24 h pumping and tracer injection test, and install multilevel CORKs; (2) recover an existing CORK installed in a shallow basement hole during Leg 168, deepen the hole by 40 m, and install a new multilevel CORK with instrumentation; (3) recover and replace an instrument string deployed in one of the Expedition 301 CORKs; and (4) complete remedial cementing of another Expedition 301 CORK that is not sealed at the seafloor.

Later submersible expeditions will use these CORKs as perturbation and monitoring points for single- and cross-hole experiments. Expedition 327 will also include an international education and outreach program intended to develop tools and techniques that facilitate the communication of exciting scientific drilling results to a broad audience, build educational curricula, and create media products (photographic, sound, video, and web based) that help achieve critical outreach goals. Secondary objectives of Expedition 327 include coring at sedimentary sites where recovered material may provide insights into hydrothermal conditions and processes within the underlying basaltic crust.