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Thermally driven fluid circulation through oceanic crust profoundly influences the physical, chemical, and biological evolution of the lithosphere 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 m.y.) is more than three times that at the axis (e.g., Parsons and Sclater, 1977; Stein and Stein, 1994), and the ridge-flank mass flux is at least 10 times as large (Mottl and Wheat, 1994). Ridge-flank circulation generates enormous solute fluxes; alters sediments and basement rocks; supports a vast subseafloor biosphere; and influences thermal, mechanical, and chemical processes as plates are subducted (e.g., Alt, 1995; Cowen et al., 2003; Elderfield and Schultz, 1996). These processes cross-cut all three primary themes motivating the Initial Science Plan for the Integrated Ocean Drilling Program (IODP). It is appropriate that the first expedition of IODP launches a new class of experiments designed to resolve the fundamental nature of fluid pathways in the crust and the dynamic influences of fluid circulation on Earth evolution.

Despite the importance of ridge-flank hydrothermal processes, 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 fluid circulation, alteration, and geomicrobial processes; or quantitative relations between seismic and hydrologic properties. IODP Expedition 301 explored these properties, processes, and relations and will help 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.

IODP Expedition 301 benefits from operational and scientific achievements of the Ocean Drilling Program (ODP), particularly ODP Leg 168, which focused on hydrothermal processes within uppermost basement rocks and sediments along an age transect across a young ridge flank (Davis, Fisher, Firth, et al., 1997). Leg 168 emphasized the fundamental physics and chemistry of ridge-flank hydrothermal circulation, and the associated alteration of sediments and shallow basement, through sampling, downhole measurements, and post-drilling observations within the upper tens of meters of basement. Although it was extremely successful (or perhaps because it was so successful), ODP Leg 168 raised many new questions about hydrothermal properties and processes within ridge flanks. IODP Expedition 301 focuses on the eastern end of the Leg 168 drilling transect, leveraging and extending results from ODP Leg 168 in three primary ways:

• Through deeper drilling, coring, and downhole measurements within basement;
• By expanding the multidisciplinary mixture of ridge-flank research in a single area, including microbiological and biogeochemical analyses of sediments and basement rocks; and
• By establishing a three-dimensional network of borehole observatories, to be used in long-term, crustal-scale experiments.

IODP Expedition 301 comprises the first part of a two-expedition program; it is important to understand the complete experimental plan to place Expedition 301 planning, operations, and results in context. A second IODP expedition to this area (to be scheduled) will add two additional boreholes and observatories to the three-dimensional network and will initiate a series of controlled, multidisciplinary, cross-hole experiments. The complete program of drilling, sampling, measurements, observatory installation, and experiments was divided into two expeditions for several operational and scientific reasons, as described below, but the first expedition was also designed to generate high-quality samples and data, to initiate a new phase of passive monitoring within ridge-flank basement rocks, and to address outstanding hydrogeologic and related questions.

The primary goals of Expedition 301 included replacement of two borehole observatory systems established at Sites 1026 and 1027 during Leg 168 and establishment of two new observatories extending up to 400 m into basement at Site U1301 (Second Ridge) (Figs. F1, F2). This effort dovetails with plans under way to develop a cabled network of seafloor observatories across the Juan de Fuca Plate, which should facilitate active and passive monitoring experiments for the next 10–20+ y. Other Expedition 301 goals included coring, sampling, and short-term downhole measurements in basement and limited collection of high-quality advanced piston corer (APC) sediment cores at Site U1301. Secondary objectives included drilling, coring, and sampling one or more holes in a region of known hydrothermal seepage at First Ridge (Figs. F1, F2), where sediment thins above a buried basement high, and drilling, coring, and sampling a much thicker sediment section to the east, Deep Ridge (Figs. F1, F2), where basement temperatures and alteration should be more extreme. We achieved all of our primary goals during Expedition 301, with the exception of replacing the borehole observatory in Hole 1027C (Table T1). This observatory can be replaced during the follow-up expedition.

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