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"During the expedition, HyBIS accomplished nearly 100 hours of dive time and reached a maximum depth of 5,125 meters." acteristic green hue to many of the deposits. High-defini- tion video images and samples of inactive chimneys at the eastern end of the mound show how hydrothermal activity has migrated westward toward the present-day volcanic rift axis. Thus, the ultradeep Beeb vent site probably derives its thermal energy from volcanic heat sources, while faults localize the hydrothermal venting. Second Hydrothermal Vent Site HyBIS dives at the second plume site located and im- aged an active hydrothermal vent field named Von Damm, located at 2,300 meters depth on the western flank of the MCSC on Mount Dent. The EM 120 multibeam bathymetry showed that Mount Dent is an oceanic core complex that exposes lower-crust and upper-mantle rocks. HyBIS filmed and sampled a vigorous high-temperature hydrothermal system forming a 100-meter-diameter cone with spires and chimneys at its summit. Serpentinite and talc breccia, sampled from the active hydrothermal mound, indicated an altered and metamorphosed ultramafic base- ment. Initial analyses show the chimneys are composed predominately of magnesium silicate clays with secondary copper, iron and tin sulfides, and estimated fluid tempera- tures of 200° to 250° C. Conclusions By focusing on technological solutions to meet the expe- dition's specific needs, an ROV that is readily accessible to the deep-ocean scientific community was developed. HyBIS requires minimal mobilization spread and manpower. The vehicle proved its capability when it was used in 2010 to lo- cate, image and sample the deepest hydrothermal vent sites known in the Cayman Trough. During the expedition, Hy- BIS accomplished nearly 100 hours of dive time, surveyed tens of kilometers of deep seafloor and reached a maximum depth of 5,125 meters. It collected high-definition video, hundreds of still images, and sampled rocks, vent fluids and fauna from two hydrothermal sites. Hydrothermal polymetallic sulfide mineralization on the ocean floor is the result of seawater-rock interactions driven by magmatic heat sources typically located at depths of 1 to 3 kilometers below the axial rift. It is predicted that at ultradeep, high-temperature hydrothermal sites seawater is transformed to a supercritical fluid where it has greater host- rock reactivity and base-metal solubility. Consequently, the venting of such fluids may lead to significant enrichments in copper and other base metals in seafloor ore deposits. The initial observations and sampling, via the HyBIS system, of the two contrasting hydrothermal vent sites in the Cayman Trough supports the hypothesis of a strong link between QPS Rolls out the latest FMMidwater See the invisible with the NEW FMMidwater Visualize and edit data that's in between the surface and the seabed. • Intuitive and fast data extraction • Simple user interface for rapid data analysis • Easily locate and extract shoalest point of a wreck • Direct connectivity to QINSy • Ability to write your own plug-ins • Support for all popular multibeam systems www.qps.nl 18 st / SEPTEMBER 2012 www.sea-technology.com