ABSTRACT

We have measured chemical compositions (CO₂, CH₄, N₂, O₂, H₂S, H₂, He, Ne and Ar), and isotopic compositions of light elements (δ¹³C, δ¹⁵N and δD) and of rare gases (He, Ne, Ar, Kr, and Xe) in seven gaseous and four fluid samples collected in Icelandic geothermal areas. Major chemical constituents of the gas samples are CO₂, N₂, H₂ and H₂S. Isotopic Compositions of Ne, Kr and Xe are indistinguishable from those of the atmosphere within the experimental accuracy of this study. There are slightly lower ⁴⁰Ar/ ³⁶Ar ratios than in the air for samples with extremely high ³He/⁴He ratios. No positive evidence for excess ¹²⁹Xe is found. The measured ³He/⁴He ratios in the samples can be explained in terms of mixing among three end members: MORB-type He with a ³He/⁴He ratio of 1.1 × 10^-5, plume-type He of 5.0 × 10^-5, and atmospheric He of 1.4 × 10^-6. Samples obtained from the northeastern part of the island contain typical MORB-type He, whereas, significant contributions (up to 33%) of plume-type He are apparent in samples from the southern region. Based on the ¹³C/¹²C ratios, CO₂ in Icelandic gases is considered to be directly derived from magma, whereas CH₄ may be formed by reaction of reduced carbon with water in the source magma. There is an inverse correlation between the ³He/⁴He ratios and δ¹⁵N values in Icelandic gases, suggesting co-genetic origin of He and N₂.