169 Prospectus


Water sampling
Observations from the Alvin submersible demonstrated that Hole 858G is now a producing hole; the CORK seals have apparently failed and hydrothermal fluid is venting from inside the reentry cone. Sampling undiluted hydrothermal fluid in the borehole below the level of casing represents one of the goals of this expedition, and will require particular care to minimize the potential for mixing with cold sea water. We expect the temperature of this fluid to be in excess of 200°C, and perhaps as high as 280°C. Temperatures this high exceed the operational limits of either the WSTP (~120°C) or the Fisseler water sampler (~85°C). The only tool available to collect an uncontaminated, high temperature water sample from the borehole is the Los Alamos dewared high temperature water sampler.

Thin section preparation
Accurate descriptions of the mineralogy present in massive sulfide and sulfide mineral-bearing samples requires polished thin sections for reflected light petrography. Particular care must be taken when these sections are prepared to avoid preferentially plucking phases from the thin section billets and to preserve textural relationships between minerals with high polishing hardness (pyrite, arsenopyrite) and low polishing hardness (galena, silver-rich sulfosalt minerals, clay minerals in the gangue material). Production of high quality polished thin sections of material containing minerals with differing polishing properties is an art. In practice, this requires careful attention to detail and slow polishing with little vertical pressure applied to the sections. Preparation of high quality polished sections of sulfides, therefore, takes considerably more time than required for standard thin section preparation. This may result in the necessity for the Shipboard Laboratory Specialist assigned to the thin section laboratory to be dedicated to thin section preparation during operations at some sites. In order to effect this, members of the Shipboard Scientific Party may be requested to help with core handling to allow the Shipboard Laboratory Specialist the time required for sample preparation.

Geochemical analyses
Nonroutine shipboard analyses will include atomic absorption analyses of sulfide rock and sediment samples for Cu, Pb, Zn, Fe, and Ag. This will require digestion of the samples with Aqua Regia. We also expect to analyze sulfide rock and sediment samples for bulk compositions by X-ray fluorescence. Adequate standards will need to be obtained to cover the expected range of compositions of these mineralized samples. Calibration early in the leg will require an allotment of time from the Shipboard X-ray Laboratory Specialists.

Bacteriological sampling
In addition to sampling biota that may be present on the recovered CORKs and thermistor strings, sample collection from recovered core must be designed to minimize the chance of contamination. This aseptic sampling must be done as soon as possible after recovery of the cores and may include A) taking whole round intervals, and/or B) sampling plugs of material removed from the center of the core on the catwalk.

Pore-water sampling
Thermal and chemical gradients in the active hydrothermal areas are very steep. In order to characterize these steep gradients, it will be necessary to take whole round samples for pore fluid compositions from closely spaced intervals in the uppermost portions of some holes. For the highest temperature sites, sampling frequency may exceed one pore fluid sample per section.

Core and sample handling
Sulfide minerals, in particular pyrrhotite, are subject to rapid oxidation. Core containing significant amounts of sulfide will be stored in D-tubes sealed in tight air bags that have been flushed with N2 gas to minimize degradation of the cores. Moisture accelerates the oxidation of sulfide-rich cores, so efforts should be made to store the cores with as little contained moisture as is practical. Similarly, samples collected for post-cruise shorebased studies should be dried and sealed prior to shipping.

Transfer of seismic data at sea
A new seismic survey in the area of operations will be run just prior to arrival of the JOIDES Resolution on site. The chief scientist of the survey operation has offered to hold the RV Sonne on location until our arrival, so that these data can be transferred to the Resolution for use in specific site location as required.

Launching and recovery of instruments
A unique experiment in the operational strategy for this expedition is the measurement of a transient pressure pulse measured at one CORKed hole, when a nearby hole is unsealed and deepened. In addition to measurements in the borehole, we hope to deploy an array of three Pop-Up Pore Pressure Instruments between the holes. PUPPI's are pressure sensing probes, to be deployed either over the side or from the moonpool of the JOIDES Resolution. The design of the probe is such that it is a free-fall, programmable memory tool, with a probe that penetrates 3 m into the seafloor sediment. The device records differential pore pressure between two ports on the probe. When measurements are complete, a coded acoustic signal severs the data logger from the probe, and the data logger floats to the sea surface for recovery. The transient overpressure caused by circulating cold seawater into an unCORKed borehole should be reflected in the pore pressure data recorded by this PUPPI array. Installation and recovery of the PUPPI's will be an operation similar to deployment and recovery of site transponder beacons, where after release on dGPS coordinates, the PUPPI's are tracked until emplaced, and recovered by either hook or Zodiac after acoustic release prior to leaving Middle Valley.

In the event pre-cruise evaluations, and or operations during the cruise, suggest that the permiability of the sediments is so low as to hinder recording of the expected pressure pulse. The deployment strategy outlined above may be revised, and we may elect to deploy the PUPPI's adjacent to the APC Holes DD1-DD4 in the Dead Dog area.

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