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

Size and geometry of sulfide deposits and hydrothermal alteration zones
Compositional variations within and between deposits
Source of metals in massive sulfides
Constraints on fluid temperatures and compositions that deposited massive sulfide
Composition of fluids in producing boreholes
Timing and duration of hydrothermal activity
Formation of hydrothermal mounds in active vent fields

II. Tectonics of sedimented rifts and controls on fluid flow

Controls on igneous activity at sedimented rifts and the importance of sill emplacement
Permeability and structural controls on hydrothermal circulation
Interrelationship of faulting and fluid flow
Constraints for hydrologic modeling
Factors controlling fluid flow on the scale of individual vent complexes and the importance of subseafloor fluid mixing

III. Sedimentation history and diagenesis at sedimented rifts

Source and deposition rate of sediments
Diagenetic reactions in high heat flow regime
Organic matter alteration and generation of hydrothermal petroleum

IV. Extent and importance of bacterial activity

Role of bacteria in oxidation of organic matter, reduction of sulfate, and precipitation of carbonate
Activity of thermophilic bacteria in sealed boreholes at known temperatures and fluid compositions
Extent of thermophilic and nonthermophilic bacteria in the subsurface and relationship to hydrothermal fluid compositions
Comparison of bacterial populations in active and inactive hydrothermal deposits

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

• Measure temperature gradient and sample hydrothermal fluids in sealed Holes 858G and 857D with minimum disturbance for Hole 858G.
  
• Deepen Hole 857D to recover deeper portions of the sill-sediment complex.
  
• Log Hole 857D to determine changes in physical properties related to hydrothermal alteration, determine structural effects of sill emplacement, and tectonic controls on fluid flow.
  
• Reinstrument and reseal Holes 858G and 857D to determine in situ temperature and pore pressure.
  
• Conduct hole-to-hole experiment on hydraulic conductivity and induced seismicity.
  
• Determine the mechanism of growth of hydrothermal mounds in the active vent fields.
  
• Measure pore fluid composition and constrain the extent of lateral flow and fluid mixing in the shallow subsurface.
  
• Establish the extent and importance of a potential hydrological seal (caprock) in focusing hydrothermal discharge.
  
• Determine the extent of microbiological activity within the sealed boreholes and sediments in an active hydrothermal field.
  
• Study organic matter maturation as a function of the present thermal regime and determine peak paleotemperatures recorded by organic matter.

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

• Investigate vertical extent of massive sulfide.
  
• Determine mineralogical, textural, and compositional zonation of the hydrothermal precipitates.
  
• Determine the relative importance of sedimentary and basaltic sources for the different hydrothermal assemblages.
  
• Document the mechanism of mound growth and maturation and the relative importance of subsurface precipitation and recrystallization versus growth at the surface via chimney formation.
  
• Study the mineralogical and chemical variation related to late stage alteration.
  
• Determine type, zonation, and extent of hydrothermal alteration in the stockwork zone under the sulfide mound.
  
• Determine the extent, composition, and zonation of alteration lateral to the deposit.
  
• Identify mechanisms that focus hydrothermal flow including formation of caprocks, intrusion of sills, and faulting.
  
• Compare the microbiological activity in an active hydrothermal field (Dead Dog) with microbiology in an inactive mound (Bent Hill).
  
• Establish the timing and duration of hydrothermal deposition, oxidation, and redeposition by mass wasting of the deposit as recorded in adjacent sediments.
  
• Compare a deposit with a basalt-dominated geochemical signature (Bent Hill) with a deposit characterized by a sediment-dominated geochemical signature (Escanaba Trough)

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

• Drill a reference section in sediments away from any hydrothermal influence to determine stratigraphy, sediment sources, depositional history, organic geochemistry, and physical properties outside of the thermal effects of vents.
  
• Determine pore water composition to study diagenetic reactions.
  
• All the investigations listed above for the Bent Hill sites

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