Hydrothermal Prospecting and Petrological Sampling in the Lau Basin: Background Data for the Integrated Study Site

In the spring of 2004, Martinez and others (this meeting) carried out an extensive mapping program of the southern Lau Basin spreading centers with high resolution bathymetry and sidescan data. They were also defined the overall distribution of mantle plumes along the ridges using miniature autonomous plume recorders and CTD tow-yos. These important new data were the first step of an integrated RIDGE 2000 program of four cruises to identify the overall tectonic, petrological, hydrothermal and biological characteristics of the spreading centers to permit a well informed selection of targets for more focussed studies in coming years. During September and October of 2004, on board R/V Kilo Moana, we are carrying out the second phase of investigations, with the aim of identifying specific hydrothermal sites and providing regional sampling of lavas from the region. The primary tools of investigation for hydrothermal site discovery are the autonomous benthic explorer (ABE) as well as CTD's and bottom towed cameras. Making use of new CTD data to verify the plumes identified in the Martinez et al cruise, ABE is being used in three different modes. Phase I focusses on the water column at the level of the neutrally buoyant plume. These results enable planning of the more focussed Phase II, closer to the bottom, to provide high resolution bathymetry and characterization of the buoyant plume. Phase III provides photographic coverage of even more limited regions focussed on the vent fields defined in Phase II. Camera tows using TOWCAM are planned to provide the highest resolution images of new hydrothermal sites. This cruise represents the first attempt to use ABE in such a staged and systematic program of regional vent site discovery. Since the ship is free to carry out other investigations during the ABE dives, which can be 24 hours or more in length, high density sampling of the various Lau Spreading Centers will be carried out at the same time as the hydrothermal investigations. A newly designed percussion core and free-fall winch provide the potential of very rapid sample turn around and the best sampling coverage of any back-arc spreading center. Shipboard analyses of rocks collected by dredging and percussion core and of fluids collected during CTD casts in conjunction with a double pass along the ridge system will permit hypothesis testing in real time. Preliminary results based on the multi-disciplinary sea-going program will be reported.