Origin of trace gases in submarine hydrothermal vents of the Kolbeinsey Ridge, north Iceland

Two hydrothermal fields of the Kolbeinsey Ridge area, north of Iceland, show vent gas characteristics which can be related to the subsurface conditions. Helium isotopes (R/R-air = 9.8, 10.9) indicate a mantle-derived origin and can be considered as a mixture of MORE helium and a deep-mantle plume helium component. The carbon isotope composition of CO2 ranges between -2.4 and -7.8 parts per thousand. The less negative delta(13)C-CO2 values were-found at Grimsey. The data from Grimsey are very similar to those previously published and regarded as being characteristic for the Icelandic magmatic source. However, small amounts of biogenic CO2 and/or subsurface calcite precipitation are responsible for the lighter isotope values of CO2 from Kolbeinsey. CH4/He-3 ratios which are higher than in MORB indicate an additional (sedimentary) methane source for Kolbeinsey and Grimsey hydrothermal gases. The presence of higher hydrocarbons up to butane, together with the carbon isotope values of methane (delta(13)C = -26.1 to -39.8 parts per thousand) suggest a probably high-mature organic source within thick sediments of the Tjornes Fracture Zone and smaller depressions on the west side of the Kolbeinsey Ridge crest. Geochemical characteristics of hydrocarbons present in KR hydrothermal fluids are, however, typical for a mixed (thermogenic and high-temperature hydrothermal, e.g. EPR-type) origin. Moreover, it is likely that secondary processes such as bacterial oxidation and thermal cracking determined the geochemical characteristics of the gases. (C) 1999 Elsevier Science B.V. All rights reserved.