Origin of fluids and anhydrite precipitation in the sediment-hosted Grimsey hydrothermal field north of Iceland

The sediment-hosted Grimsey hydrothermal field is situated in the Tjörnes fracture zone (TFZ) which represents the transition from northern Iceland to the southern Kolbeinsey Ridge. The TFZ is characterized by a ridge jump of 75 km causing widespread extension of the oceanic crust in this area. Hydrothermal activity occurs in the Grimsey field in a 300 m×1000 m large area at a water depth of 400 m. Active and inactive anhydrite chimneys up to 3 meters high and hydrothermal anhydrite mounds are typical for this field. Clear, metal-depleted, up to 250 °C hydrothermal fluids are venting from the active chimneys. Anhydrite samples collected from the Grimsey field average 21.6 wt.% Ca, 1475 ppm Sr and 3.47 wt.% Mg. The average molar Sr/Ca ratio is 3.3×10−3. Sulfur isotopes of anhydrite have typical seawater values of 22±0.7‰ δ34S, indicating a seawater source for SO42−. Strontium isotopic ratios average 0.70662±0.00033, suggesting the precipitation of anhydrite from a hydrothermal fluid–seawater mixture. The endmember of the venting hydrothermal fluids calculated on a Mg-zero basis contains 59.8 μmol/kg Sr, 13.2 mmol/kg Ca and a 87Sr/86Sr ratio of 0.70634. The average Sr/Ca partition coefficient between the hydrothermal fluids and anhydrite of about 0.67 implies precipitation from a non-evolved fluid. A model for fluid evolution in the Grimsey hydrothermal field suggests mixing of upwelling hydrothermal fluids with shallowly circulating seawater. Before and during mixing, seawater is heated to 200–250 °C which causes anhydrite precipitation and probably the formation of an anhydrite-rich zone beneath the seafloor.