Monterey Bay Aquarium Research Institute

West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
Logbook

August 23, 2002: Day #35


Today’s program was divided into two goals and ultimately two separate dives. On dive 470, Tiburon returned to the Mothra hydrothermal site on the southern Endeavour rift valley. In the drawer of the toolsled was the borehole incubator, intended for the borehole placed on the wall of Giraffe two days ago. At that time, the Giraffe borehole was plugged with an aluminum insert. These hollow metal inserts (Fig. 1) were built for the seismometer coreholes and were never designed for the high temperatures and corrosive environment of the hydrothermal chimney wall. They were the only inserts available, however, and we assumed that it would survive two days. We easily found the insert on the wall of Giraffe (Fig 2) but it was a real tug-of-war to get it out of the hole. Only half of the insert was left after only two days! The rest was dissolved and clogging the end of the hole (Fig. 3).



Figure 2. Insert protruding from the borehole on Giraffe. The marker makes the site much easier to relocate. Unfortunately little of the insert inside the hole was left after being cooked in the hydrothermal fluids for two days.

Figure 1. Hollow aluminum insert built for the corehole seismometer sites. The job of the insert is to keep animals and sediment out of the borehole. One of these was inserted into the borehole in the wall of the hydrothermal chimney and left for two days.


Figure 3. The aluminum insert after the ROV finally pulled it from the wall. Over half of the insert was dissolved and left behind in the corehole. This demonstrates the extraordinarily corrosive environment that instruments must be able to survive to monitor the growth of these seafloor hydrothermal deposits.

Figure 1 shows a spider crab inspecting our PVC "biotubes" that we used to collect fragile rocks. The manipulator can grab handfuls of fresh glass and place it into the tubes without worrying that they will be lost during the ROV recovery.

Two hydraulic lines on the Kraft manipulator arm were damaged during the insertion of the borehole incubator. This forced an early end to dive T470 and precluded collecting any water samples. The remainder of the day was then devoted to geological mapping of the axial valley. The goal of this effort is to determine the relationship between the specific volcanic episodes and the hydrothermal system. New methods of dating seafloor basalts may also provide a temporal model of how and where (on- versus off-axis) the upper portion of oceanic crust is built. To accomplish this in a rigorous manner requires careful sampling of rocks that provide abundant fresh glass or holocrystalline cores, systematic documentation of crosscutting or stratigraphic relationships, and good navigation across the seafloor. Since all of our dives are navigation within the context of the high-resolution EM300 bathymetry, we can place samples their geological relationships in real time. We also used modified ROV push cores that have wax cones on top. The ROV pushes the wax into the brittle surfaces and collects bits of glass that may be otherwise impossible to grab with the manipulator. Another addition to this years benthic drawer is a series of PVC tubes with lids held on with elastic. These permitted us to isolate the most fragile samples in a single container such that multiple fragments could be accumulated and preserved on the ROV. Our traverse took us from Mothra across the entire valley floor to the eastern wall and volcanic high. We then turned north and mapped part of the axial valley between Mothra and the Main Field. These figures show some of the more colorful aspects of rock collection.



Figure 2 shows the most common volcanic morphology within the axial valley. These sheet flows are transitional between highly drained lobate pillows and true massive flows. We found similar glassy transitional flows in all of the deeper spots on the axial floor, sometimes separated by fault blocks of more pillowed flows. The protective octopus was less common, but we did see two during this one dive.

Figure 3 shows the top of a collapse pit or pressure ridge with fractured pahoehoe flows overrun by lobate pillowed flows.

 

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