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, sho w vent gas characteristics which can be related to the subsurface condition s. Helium isotopes (R/R-air = 9.8, 10.9) indicate a mantle-derived origin a nd can be considered as a mixture of MORE helium and a deep-mantle plume he lium component. The carbon isotope composition of CO2 ranges between -2.4 a nd -7.8 parts per thousand. The less negative delta(13)C-CO2 values were-fo und at Grimsey. The data from Grimsey are very similar to those previously published and regarded as being characteristic for the Icelandic magmatic s ource. However, small amounts of biogenic CO2 and/or subsurface calcite pre cipitation are responsible for the lighter isotope values of CO2 from Kolbe insey. 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 carbo n 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 t he Tjornes Fracture Zone and smaller depressions on the west side of the Ko lbeinsey Ridge crest. Geochemical characteristics of hydrocarbons present i n KR hydrothermal fluids are, however, typical for a mixed (thermogenic and high-temperature hydrothermal, e.g. EPR-type) origin. Moreover, it is like ly that secondary processes such as bacterial oxidation and thermal crackin g determined the geochemical characteristics of the gases. (C) 1999 Elsevie r Science B.V. All rights reserved.