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