Chemical and isotopic compositions of gases in geothermal fluids in Iceland.

Abstract

(Received October 24, 1984: Accepted February 22, 1985 We have measured chemical compositions (C02, CH4, N2, 02, H2S, H2, He, Ne and Ar), and isotopic compositions of light elements (S13C, S 15N and 5D) 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 C02, 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 X 10-5, plume-type He of 5.0 X 10-5, and atmospheric He of 1.4 X 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 '3C/ `C 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 815N values in Icelandic gases, suggesting co-genetic origin of He and N2.

Hydrothermal activity on the summit of Loihi Seamount, Hawaii.

“…Helium was analyzed on seawater clamp-sealed into 40m1 copper tubings immediately after it was brought on board. For detailed analytical procedures, refer to Sano et al (1985). The 'He/'He ratio was expressed in R/RA, where R and RA (= 1.4 X 10-6) are the isotopic ratio of sample and atmospheric helium, respectively.…”

Sources, degassing, and contamination of CO2, H2O, He, Ne, and Ar in basaltic glasses from Kolbeinsey Ridge, North Atlantic

“…Prior work on Iceland has established that magmatic He and Ne (Mamyrin et al, 1972;Condomines et al, 1983;Kurz et al, 1985;Sano et al, 1985;Hilton et al, 1990;Burnard et al, 1994;Breddam et al, 2000;Dixon et al, 2000;Moreira et al, 2001), and possibly Ar (Trieloff et al, 2000), are isotopically distinct from typical mid-ocean ridge basalt (MORB). Icelandic lavas may also be water-rich compared to the majority of MORB (Nichols et al, 2002).…”

Noble gas signatures of magmatic sources and processes.

“…For example, Icelandic gases show 3He/4He ratios of more than 1 x 10-5, while most of their 40Ar/36Ar ratios are similar to that of the atmosphere (Sano et al 1985). In Japan, the natural and volcanic gases show variable 'He /4 He ratios ex ceeding the atmospheric value, but their 40Ar / 36Ar ratios are also similar to that of the at mosphere (Nagao et al, 1979(Nagao et al, , 1981.…”

“…Hence, the amount of magmatic gases in a rock is con trolled by the cooling rate of the rock and the sur rounding pressure. Nagao et al (1979Nagao et al ( , 1981, Phinney et al (1978), Sano et al (1985), Takaoka (1985), Tolstikhin (1980), Welhan et al (1988).…”