Chemical and isotopic compositions of gases in geothermal fluids in Iceland
1985 年 19 巻 3 号 p. 135-148
詳細
1985 年 19 巻 3 号 p. 135-148
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.
