HELIUM-ISOTOPES IN ICELANDIC GEOTHERMAL SYSTEMS .1. HE-3, GAS CHEMISTRY, AND C-13 RELATIONS

Gas samples from seventeen high-temperature and twenty-two low-temperature geothermal systems have been analyzed for chemistry and He-3/He-4 ratios. Within the Neo-Volcanic Zone the He-3/He-4 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 R(A) in central Iceland. Outside of the rift zones a mantle helium component also dominates with the highest He-3/He-4 ratios found in waters circulating through 9-My-old crust in Northwest Iceland (up to 29 R(A)). The minimum Icelandic He-3/He-4 ratio (excluding a methane seep east of the rift) is 8.5 R(A) at Kverkfjoll, 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 Kverkfjoll 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 RA) and deep-mantle plume He with R/R(A) > 29. High-temperature systems have CO2/He-3 ratios of 10(9) to 10(10) that encompass the range found in MORB (1-3 X 10(9)). However, the CO2/He-3 values have been subjected to postmagmatic effects that alter and obscure the original magmatic CO2/He-3 ratios. Delta(C-13) in the fluid-phase CO2 is well defined at -3.8 parts per thousand in the high-CO2 fluids (up to 1 mol/kg fluid), very similar to MORB values. CH4/He-3 ratios vary widely, from 3 X 10(4) to 10(8). Most high-temperature systems from southwestern and northern Iceland have CH4/He-3 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.