169 Prospectus


The primary focus of the second leg of drilling is investigation of the mechanisms of formation of massive sulfide deposits at sedimented ridges. Middle Valley and Escanaba Trough (Fig. 1) are ideal laboratories for systematically establishing the origin of these deposits. The individual deposits in these two areas are considerably larger than most deposits discovered thus far on bare-rock ridges and are large enough to be easily targeted and drilled by the JOIDES Resolution. Differences in both the maturity and composition of the massive sulfide deposits at the two ridge segments indicate that comparison of the different deposits should provide more information on the processes controlling massive sulfide generation than could be obtained by more extensive drilling of only one of the deposits. The composition of deposits in Escanaba Trough indicates a dominantly sedimentary source for the metals, whereas those in Middle Valley appear to be intermediate between basaltic and sedimentary sources (Figs. 2, 3). The primary objectives of this leg are to investigate the following areas:

I. Mechanism of formation of massive sulfide deposits at sediment covered ridges

II. Tectonics of sedimented rifts and controls on fluid flow

III. Sedimentation history and diagenesis at sedimented rifts

IV. Extent and importance of bacterial activity

Our drilling strategy involves drilling deposits of differing maturity to investigate the evolution of hydrothermal deposits. Factors that control the location, size, and composition of massive sulfide deposits will be investigated by drilling a series of holes across deposits at each of the sites. Examination of the sedimentary record of hydrothermal products adjacent to the deposits will provide constraints on the timing and duration of hydrothermal activity. Drilling of the alteration zones beneath the deposits will constrain the sources of metals in the deposits and geochemical reactions that control mineralization. The existence of cased and sealed boreholes in the Middle Valley hydrothermal field will allow the first subsurface sampling of hydrothermal fluids from an ODP borehole, and potentially allow us to sample hyperthermophyllic chemolithoauthotrophic bacteria that may have colonized the thermistor strings in the sealed holes. Furthermore, reinstrumentation of these holes will allow active experimentation on induced seismicity in a seafloor hydrothermal system and hole-to-hole hydrologic experimentation designed to constrain for the first time the physical and hydrologic properties that control hydrothermal flow on the scale of an entire vent field. The scientific objectives for the three primary work areas are outlined below and are summarized on a hole-by-hole basis in Table 1. This table correlates the primary objectives with the major results from the sites drilled on Leg 139, and shows which scientific objectives will be addressed by the sites proposed for Leg 169.

I. MIDDLE VALLEY - Dead Dog active hydrothermal field and CORK operation
Priority Targets: Holes 858G, 857D, DD1-4

II. MIDDLE VALLEY: Bent Hill inactive sulfide mound
Priority Targets: Holes BH1, BH2-6, BH8

III - ESCANABA TROUGH: active sulfide mound
Priority Targets: ET7, ET1-4, ET5

To 169 Regional Geology

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