Middle Valley had three holes planned at the Dead Dog active hydrothermal field, and several more planned at the Bent Hill inactive sulfide mound. An attempt was planned to deepen Hole 856H on Bent Hill, where a Drill-In-Casing had been set and the hole cored to 93.8 m during Leg 139. Holes 858G and 857D in the Middle Valley hydrothermal field were cased, and CORKs and instrumented strings were installed on Leg 139. Operations to remove and replace the instrument strings and CORKs in those boreholes were planned to allow subsurface sampling of hydrothermal fluids, allow sampling of hyper-thermophylic bacteria that may have colonized the thermistor strings, and permit determination of in situ temperature and pore pressure. Hole 857D was to be deepened as part of a hole-to-hole CORK experiment where the effects of circulating fluid in one hole might be monitored in another.
- Hole 856H (500.0 m total depth) was the first penetration of an oceanic sulfide mound through the massive sulfides, stockwork mineralization zone, underlying sediment, basalt sills, and upper pillow basalt section.
- The first logs were obtained through an oceanic sulfide mound at Hole 856H.
- Pop Up Pore Pressure Instruments (PUPPIs) were deployed and recovered at Holes 1036B and 1036C.
Depth measurements are in meters. Unless otherwise noted, depths in this report refer to "meters below rig floor" (mbrf), in which drill pipe measurements (DPM) are referenced to the top of the "dual elevator stool" (DES) on the rig floor. The "DPM" depths on Leg 169 were 11.1-11.8 m above sea level (depending on the ship's draft). "Meters below sea floor" (mbsf) refers to the indicated depth below the seafloor, which is based on recovery or apparent seafloor contact for each hole. Unless otherwise noted, all times in this report refer to ship time. The shipboard clocks were maintained on US Western Time Zone = GMT -7 hr and College Station CST =GMT -5 hr. Shipboard times remained the same for the remainder of the leg. Sites were positioned during the leg by using the shipboard differential Global Positioning System (dGPS) to position on GPS coordinates from prior site surveys.
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