Other
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.4221p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 12, pp.4221-4228
Other
7
Scientific paper
Gas samples from seventeen high-temperature and twenty-two low-temperature geothermal systems have been analyzed for chemistry and 3 He / 4 He ratios. Within the Neo-Volcanic Zone the 3 He / 4 He ratios show a consistent regional pattern: 14-19 times the atmospheric ratio ( R A ) in the southwest, 8-11 R A in the north, and 17-26 A in central Iceland. Outside of the rift zones a mantle helium component also dominates with the highest 3 He / 4 He ratios found in waters circulating through 9-Myold crust in Northwest Iceland (up to 29 R A ). The minimum Icelandic 3 He / 4 He ratio (excluding a methane seep east of the rift) is 8.5 R A at Kverkf joll, in central Iceland at the southern end of the narrow Northern Rift Zone; throughout the NRZ the ratios vary only from 8.5 to 10.7 R A . The Kverkf joll ratio is precisely the mean MORB ratio: (8 ± 1) R A . Thus, the mantle helium emerging at Iceland is a simple mixture of two components: MORB He (8 R A ) and deep-mantle plume He with R / R A : 29. High-temperature systems have CO 2 / 3 He ratios of 10 9 to 10 10 that encompass the range found in MORB (1-3 × 10 9 ). However, the CO 2 / 3 He values have been subjected to postmagmatic effects that alter and obscure the original magmatic CO 2 / 3 He ratios. ( 13 C) in the fluid-phase CO 2 is well defined at -3.8%. in the high-CO 2 fluids (up to 1 mol/kg fluid), very similar to MORB values. CH 4 / 3 He ratios vary widely, from 3 × 10 4 to 10 8 . Most high-temperature systems from southwestern and northern Iceland have CH 4 / 3 He ratios less than 10 6 , while those from central Iceland have consistently higher ratios of the order of 10 7 . Local conditions and possible proximity to an organic source of methane can have a strong effect on this ratio.
Arnórsson Stefán
Craig Harmon
Poreda Robert J.
Welhan J. A.
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