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

High-temperature (250 degreesC hydrothermal vents and massive anhydrite dep osits have been found in a shallow water, sediment-filled graben near 66 de grees 36 'N in the Tjornes Fracture Zone north of Iceland. The site is loca ted about 30 km offshore, near the small island of Grimsey. The main vent f ield 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 nor th-south alignment of the mounds over a 1-km strike length of the valley fl oor suggests that their distribution is controlled by a buried fault. Wides pread shimmering water and extensive white patches of anhydrite in the sedi ment between the mounds indicates that the entire 1-km(2) area occupied by the vents is thermally active. A 2-man research submersible JAGO was used t o map the area and to sample vent waters, gases, and chimneys. Actively boi ling hydrothermal vents occur on most of the mounds, and extensive two-phas e venting indicates that the field is underlain by a large boiling zone (20 0 X 300 m). The presence of boiling fluids in shallow aquifers beneath the deposits was confirmed by sediment coring. The highest-temperature pore flu ids were encountered in talc- and anhydrite-rich sedimentary layers that oc cur 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 h ydrothermally altered sediments. However, massive sulfides (pyrite-marcasit e crusts) were found in only one relict mound. Subseafloor boiling has like ly affected the metal-carrying capacity of the hydrothermal fluids, and dep osition 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 a nd extensive boiling of the hydrothermal fluids represent a distinctive new type of seafloor hydrothermal system. (C) 2001 Elsevier Science B.V. All r ights reserved.