169 Prospectus


As the second leg of a planned two leg program, Leg 169 will address a broad range of scientific problems, but the major emphasis of the leg is to investigate the genesis of massive sulfide deposits. This project in the northeast Pacific ocean basin will be focused in two ideal laboratories for investigating sediment-hosted massive sulfide deposits, Middle Valley at the northern end of the Juan de Fuca Ridge and Escanaba Trough at the southern end of the Gorda Ridge. The four primary topics that encompass the drilling strategy of this leg are 1) mechanism of formation of massive sulfide deposits at sediment-covered ridges, 2) tectonics of sedimented rifts and controls on fluid flow, 3) sedimentation history and diagenesis at sedimented rifts, and 4) extent and importance of bacterial activity in these environments. We will address these problems by drilling deposits of differing maturity, and by drilling a series of holes across deposits at each of these sites. By examining the sedimentary record of hydrothermal products adjacent to the deposits, we will attempt to constrain the timing and duration of hydrothermal activity. Sampling of the alteration zones beneath the deposits will constrain the sources of metals in the deposits and geochemical reactions that control mineralization. Opening sealed, instrumented boreholes in the Middle Valley hydrothermal field will allow the first subsurface sampling of hydrothermal fluids from an ODP borehole. Additionally, we intend to perform the first active hole-to-hole hydrologic experiment designed to constrain the physical and hydrologic properties that control hydrothermal flow on the scale of an entire vent field.

We plan to unseal, log, sample borehole fluid, and reseal with an instrumented CORK two holes originally sealed and instrumented on the first of this two leg program (Leg 139). One of these holes (Hole 857D) will be deepened prior to sealing and instrumentation. This prospectus details two primary sites (one each in Middle Valley and Escanaba Trough) for deep penetration through sediment-hosted massive sulfide deposits and eventually through the underlying alteration zone. We propose one primary and multiple alternate sites for sampling the principal center of hydrothermal activity in Middle Valley, the Dead Dog vent field. A primary sedimentary reference site in Escanaba Trough is targeted, and several alternate sites that will be drilled as time and recovery dictate are presented. Drilling at Middle Valley and Escanaba Trough will enhance the broad scale constraints on hydrothermal circulation through the upper oceanic crust at a sediment-covered spreading centers developed subsequent to Leg 139. The presence of instrumented boreholes allows active experimentation as well as continued monitoring and sampling of hydrothermal fluids at ridge crests. Our multidisciplinary approach, encompassing integrated geological, geophysical, geochemical, hydrological, and biological investigations ensures the most productive yet economical exploration and utilization of these exceptional natural laboratories.

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