The Grimsey field: A shallow-marine hydrothermal system north of Iceland

摘要

The Grimsey hydrothermal field is a shallow-marine, sediment-hosted hydrothermal system situated in the Tjornes Fracture Zone, which represents the transition from northern Iceland to the southern Kolbeinsey Ridge. Hydrothermal activity occurring in a 300 m x 1000 m large area at a water depth of 400 m is characterized by high-temperature (250℃), clear hydrothermal fluids venting from anhydrite chimneys and coalesced anhydrite-rich mounds. Disseminated pyrite-marcasite precipitates have been found in sediments but no massive sulfidcs occur. Most of the venting fluids show signs of subcritical phase separation indicating that the whole field is underlain by a large boiling zone. Trace metal content of the sulfides is very low, except for mercury (up to 26 ppm). The lack of extensive Mg metasomatism is the most striking feature of the sediments underlying the anhydrite-rich mounds. Mg-containing minerals (kerolite/stevensite) are only present in the anhydrite chimneys where they line fluid channelways. Subcritical phase separation and the presence of a metal-rich brine at depth is suggested for the Grimsey hydrothermal field. The vapour-dominated phase obviously reaches the seafloor carrying excess mercury which is fixed in pyrite. With these characteristics, the Grimsey hydrothermal field is different from other known sediment-hosted hydrothermal systems most likely as a consequence of its shallow water depth.