Regional distribution of 3He anomalies in the Icelandic crust

Abstract

The distribution of 3He in geothermal fluids of Iceland has been investigated in detail. Samples were selected to: (1) examine relationships between 3 He 4 He in basalts and geothermal waters; (2) assess the influence of in situ produced radiogenic He on the pattern of 3 He 4 He variation; and (3) the influence of regional groundwater flow patterns on the distribution of 3He anomalies. The present results for 64 samples when combined with previously published data demonstrate a close correspondence between He isotope ratios in basalts and geothermal fluids. He isotope ratios ( R = 3 He 4 He exceeding the normal MORB range of 8 ± 1 R a ( R a = air 3 He 4 He ) are widespread in Iceland, and occur both in the volcanic zones and areas of older crust. Mass-balance considerations indicate that radiogenic He produced in the Icelandic crust makes a minor contribution only to the overall He budget but its presence may be reflected by the small number of R R a-values of < 8. Primarily, the range in R R a ratios reflects variability in the He signature of mantle-supplied melts. The presence of high R R a-values in older parts of the Icelandic crust does not appear to arise from dispersal of 3He from the neovolcanic zones by the regional groundwater flow pattern. Rather, contemporary degassing of melts, usually without a surface volcanic expression, appears to be widespread in Iceland even in areas where the crust is up to ⩾ 10 Ma. In these locations the melt fractions must be small and mostly freeze into the Icelandic crust.