The Grimsey hydrothermal field offshore North Iceland: crustal structure, faulting and related gas venting

The results of a combined geophysical and geochemical research program are presented that focused on Grimsey hydrothermal field (GHF) which is located at 66°35'30″N, 17°39'30″W east of the island of Grimsey in the Tjörnes Fracture Zone. The vent field is situated at the sourthernmost tip of a submarine ridge which is connected to the offshore part of the Theistareykir Fissure Swarm. Reflection seismic recordings were interpreted in conjunction with earthquake catalogue data to detect active fault structures and potential heat sources in the subsurface. An inter-linked fracture network forming a downwards converging system of faults connected to a deep-reaching normal fault is assumed to provide the preferential pathways for gases ( ³He, CO ₂, CH ₄ etc.) migrating from a possible deep-seated gas source (lower crust/upper mantle) to the surface. The location of hydrothermal vents was detected by concentration measurements of dissolved methane in the water column, which coincide with polarity reversals in the seismic seafloor reflection. Both the molecular composition of the gas and the isotopic composition of methane at GHF indicate a predominating abiogenic source (Fischer-Tropsch reactions at 250-500°C) mixed with thermogenic hydrocarbons. New seismic data from the GHF were compared with gas geochemical data which indicate that the thermogenic hydrocarbons are related to (up to 60 m thick) sediments deposited in a basin located east of the ridge.