An introduction to the scientific results of Leg 168

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Hydrothermal circulation through the oceanic crust is known to carry a significant portion of the heat from cooling lithospheric plates, to exert a strong influence on the chemistry of the oceans, and to modify the composition of oceanic crust before it is recycled by subduction. Among the objectives of Ocean Drilling Program (ODP) Leg 168 were (1) elucidation of the fundamental physics and fluid chemistry of ridge flank hydrothermal circulation and (2) documentation of consequent alteration of the upper igneous crust and sediments that host the flow in a young ridge flank setting. This was accomplished with a transect of sites on the eastern flank of the northern Juan de Fuca Ridge drilled into crust ranging in age from 0.8 to 3.6 Ma (Fig. 1). In most ocean basins, burial of the igneous crust and the resultant hydrologic isolation of permeable basement takes place over tens of millions of years; on the eastern Juan de Fuca Ridge flank, burial is accelerated as a consequence of the proximity of the ridge to the abundant supply of turbidite sediments shed from the adjacent North American continental margin during the Pleistocene. The arrangement of Leg 168 drill sites along an east-to-west transect spanned a range of crustal ages, sediment thicknesses, and thermal and chemical states in basement (Fig. 1), allowing an assessment of current conditions and how conditions may have changed over time. Sites at the western end of the transect were closest to the spreading axis and the position of sediment/igneous-basement onlap and showed the greatest extent of hydrothermal cooling and the least amount of rock and fluid alteration. In contrast, sites at the eastern end of the transect revealed considerably greater rock and fluid alteration. Penetration into the basement section beneath the sediments was intentionally shallow during Leg 168; primary attention was given to lateral variations in basement fluid composition, formation temperatures, fluid pressures, and alteration. Deeper penetration and experiments were reserved as goals for later drilling. Results based on Leg 168 shipboard studies are summarized in the Leg 168 Initial Reports volume (Shipboard Scientific Party, 1997) and in a short summary article (Davis et al., 1997). Because of recent changes in ODP publications policy, many papers containing results from Leg 168 appear in journals rather than this volume. In the rest of this introduction we highlight results of post-cruise research, with an emphasis on hydrogeologic processes and papers that appear outside this volume.