First observations of high-temperature submarine hydrothermal vents and massive anhydrite deposits off the north coast of Iceland

High-temperature (250 degree C) hydrothermal vents and massive anhydrite deposits have been found in a shallow water, sediment-filled graben near 66 degree 36'N in the Tjornes Fracture Zone north of Iceland. The site is located about 30 km offshore, near the small island of Grimsey. The main vent field occurs at a depth of 400 m and consists of about 20 large-diameter (up to 10 m) mounds and 1-3 m chimneys and spires of anhydrite and talc. A north-south alignment of the mounds over a 1-km strike length of the valley floor suggests that their distribution is controlled by a buried fault. Widespread shimmering water and extensive white patches of anhydrite in the sediment between the mounds indicates that the entire 1-km super(2) area occupied by the vents is thermally active. A 2-man research submersible JAGO was used to map the area and to sample vent waters, gases, and chimneys. Actively boiling hydrothermal vents occur on most of the mounds, and extensive two-phase venting indicates that the field is underlain by a large boiling zone (200 x 300 m). The presence of boiling fluids in shallow aquifers beneath the deposits was confirmed by sediment coring. The highest-temperature pore fluids were encountered in talc- and anhydrite-rich sedimentary layers that occur up to 7 m below the mounds. Baked muds underlie the talc and anhydrite layers, and pyrite is common in stockwork-like fractures and veins in the hydrothermally altered sediments. However, massive sulfides (pyrite-marcasite crusts) were found in only one relict mound. Subseafloor boiling has likely affected the metal-carrying capacity of the hydrothermal fluids, and deposition of sulfides may be occurring at greater depth. Although the mounds and chimneys at Grimsey resemble other deposits at sedimented ridges (e.g. Middle Valley, Escanaba Trough, Guaymas Basin), the shallow water setting and extensive boiling of the hydrothermal fluids represent a distinctive new type of seafloor hydrothermal system.