Middle Valley

Bent Hill Area

Hole 856H had been drilled on Leg 139 to a depth of 93.8 m before abandonment due to unstable hole conditions. Our plan was to attempt to deepen this hole through the massive sulfide deposit sampled on Leg 139 and into the underlying alteration zone. We surmised that circulation of oxygenated seawater into the borehole during the last five years may have altered the pyrrhotite/pyrite massive sulfide assemblage to iron oxide/oxyhydroxide, effectively stabilizing the borehole wall. After reentry with a rotary coring bottom hole assembly (our only option as this hole was initiated with the same configuration) we successfully recovered 47 cores to a depth of 500 mbsf with recovery just over 12%. We note, however, that we experienced significantly higher recovery rates (34% in 9 cores) when the core barrel was retrieved prior to full penetration of a joint of drill pipe. Coring operations ended on August 28, when the scientific objectives had been achieved, and time allocated for this operation had expired.

Logging operations began with a temperature run to evaluate which tools would be operable in what was expected to be a particularly hostile (high temperature) environment. A ledge prevented the Bureau de Recherche Geologique et Miniere in Orleans (BRGM) temperature tool from logging to the bottom of the hole, so hole cleaning operations were initiated. Although we had dropped the first bit on the seafloor to allow future deepening of Hole 856H, the hole cleaning operation ended with dropping a bit in the bottom of the hole to avoid pulling the drill string out of an unstable hole. Two logging runs, first with the Triple Combo (natural gamma ray, sonic velocity, neutron porosity, resistivity), followed by the Formation Microscanner string were successfully completed. Internal temperature monitors on these tools, however, indicated it would be imprudent to continue logging operations with the more temperature sensitive geochemical tool, so logging operations were terminated. During one of the pipe trips made for cleaning Hole 856H, the ship was moved in DP mode to the Dead Dog hydrothermal mound to launch two Pop-Up Pore Pressure Instruments (PUPPIs). PUPPIs are pressure sensing probes, which were deployed by dropping them over the side of the JOIDES Resolution. The design of these instruments is such that they are programmable memory tools with a probe that penetrates 3 m into the seafloor sediment. The devices recorded differential pore pressure between two ports on the probes which were embedded in the seafloor. When time for operations at the Dead Dog area was complete, a coded acoustic signal severed the data loggers from the probes, and the data loggers floated to the sea surface for recovery. Installation and recovery of the PUPPIs was an operation similar to deployment and recovery of site transponder beacons, where after release on dGPS coordinates, the PUPPIs were tracked until emplaced, and recovered by hook from the Resolution prior to leaving the Dead Dog area. This release operation was undertaken during a pipe trip to maximize efficient use of ship time.

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