169 Prospectus


Leg 169 is the second leg of a planned two leg program to investigate the geological, geophysical, geochemical, and biological processes at sediment-covered spreading centers. The leg will address a broad range of scientific problems, but the major emphasis of the leg is to investigate the genesis of massive sulfide deposits. The highly successful Leg 139 drilling also addressed a wide range of problems, but was primarily focused on establishing the broad scale constraints on hydrothermal circulation of seawater through the upper oceanic crust at a sediment covered spreading center. Among the many accomplishments of Leg 139, a few highlights are listed below. Leg 139 represented the first significant penetration and recovery of zero age oceanic crust and established that the uppermost part of the oceanic crust formed at sediment-covered spreading centers is composed not of extrusive basalts, but of interlayered basaltic sills and metasediments. This leg safely drilled into an active hydrothermal upflow zone, within 10's of meters of hydrothermal vents, and successfully recovered core and geophysical logs from rock at temperatures near 300#&176;C. We established that the composition of the venting fluids was consistent with hydrothermal mineral assemblages in metasediments and metabasaltic rocks recovered from a hydrothermal reservoir zone in the upper oceanic crust. Drilling on the flanks of the ridge established that relatively unaltered seawater is recharging into oceanic basement in the area near one of the rift-bounding normal faults. The leg established the presence of an extensive seafloor ore deposit by penetrating 94 meters of massive sulfide. A major step towards the establishment of seafloor observatories was taken by instrumentation of sealed borehole using the CORK system. Geophysical logging, downhole experimentation, and borehole instrumentation provided the first direct constraints on the physical properties of an active submarine hydrothermal system (Davis, Mottl, Fisher, et al., 1992). We look forward with great excitement to the opportunity to extend and refine the scientific insight gained on Leg 139 during the second leg of sedimented ridges drilling on Leg 169.

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