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Thermally driven fluid circulation through oceanic lithosphere profoundly influences the physical, chemical, and biological evolution of the crust and ocean. Although much work over the last 30 years has focused on hot springs along mid-ocean ridges, global advective heat loss from ridge flanks (crust older than 1 Ma) is more than three times that at the axis (Parsons and Sclater, 1977; Stein and Stein, 1992) and the ridge-flank mass flux is at least ten times as large (Elderfield and Schultz, 1996; Mottl and Wheat, 1994). Ridge-flank circulation generates enormous solute fluxes, profoundly alters basement rocks, supports a vast subseafloor biosphere, and continues right to the trench, influencing the thermal, mechanical, and chemical state of subducting plates (e.g., Alt, 1995; Ranero et al., 2003). These processes crosscut all three primary themes motivating the Initial Science Plan for the IODP.

Despite the importance of fluid-rock interaction in the crust, little is known about the distribution of hydrologic properties; the extent to which crustal compartments are well connected or isolated (laterally and with depth); linkages between ridge-flank circulation, alteration, and geomicrobial processes; or quantitative relations between seismic and hydrologic properties. IODP Expedition 301 comprises the first part of a two-expedition experiment to explore these processes and relations and to address topics of fundamental interest to a broad community of hydrogeologists working in heterogeneous water-rock systems: the nature and significance of scaling phenomena and the applicability of equivalent porous-medium representations of discrete fracture-flow processes. Expedition 301 benefits from operational and scientific achievements from 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 across a young ridge flank. The primary goals of Expedition 301 include replacement of long-term observatories established in two reentry holes during Leg 168 and establishment of two new observatories, creating a three-dimensional observational network in upper oceanic basement. These observatories will be used to passively monitor thermal and pressure conditions in basement and to collect long-term chemical and microbiological samples. During a later expedition, researchers will use these observatories for a series of multidisciplinary crustal-scale experiments. Other primary goals of Expedition 301 include coring, sampling, and short-term downhole measurements. Secondary objectives include drilling, coring, and sampling one or more holes in a region of known hydrothermal seepage, where sediment thins above a buried basement ridge, and drilling, coring, and sampling a much thicker sediment section to the east, where basement temperatures and alteration should be more extreme.

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