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

• Published in: Chemical Geology
• Main Authors: Kuhn, Thomas, Herzig, Peter, Hannington, Mark D., Garbe-Schönberg, C.-Dieter, Stoffers, Peter
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2003
• Subjects: Kolbeinsey; Kolbeinsey Ridge; Tjörnes; Grimsey; Iceland
• Online Access: https://oceanrep.geomar.de/id/eprint/6834/; https://oceanrep.geomar.de/id/eprint/6834/1/1-s2.0-S0009254103002079-main.pdf; http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V5Y-49WG22C-1&_user=2387607&_handle=V-WA-A-W-AB-MsSAYZW-UUA-U-AAWEYUAEWA-AAWDBYWDWA-BCUWDWWWY-AB-U&_fmt=summary&_coverDate=12%2F15%2F2003&_rdoc=2&_orig=browse&_srch=%23toc%235799%232003%2399797999; https://doi.org/10.1016/S0009-2541(03)00207-9

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.