Seawater recharge along an eastern bounding fault in Middle Valley, northern Juan de Fuca Ridge

A transect of four boreholes was drilled at ODP Site 855 in Middle Valley, a sedimented spreading center on the northern Juan de Fuca Ridge, near an eastern bounding (normal) fault. Systematic variations in pore water chemical profiles along this transect are consistent with recharge of seawater into basaltic crust along the exposed fault, followed by flow of formation fluid laterally within upper basaltic basement. Chemical data are interpreted using an advection-diffusion model, resulting in a seawater flux into the seafloor of similar to 9 m(3) yr(-1) per meter of fault, with a specific discharge of >= 100 m yr(-1). Calculations suggest that flow is confined to a thin interval within upper basaltic basement. Integrating over the 17-km length of exposed fault results in a recharge flux that is only 3% of the present day, high-temperature hydrothermal discharge in Middle Valley. Most of the seawater recharge occurs elsewhere through more transmissive pathways.