Regional distribution of 3He anomalies in the Icelandic crust

doi:10.1016/0009-2541(90)90103-E

Chemical Geology

Volume 88, Issues 1–2, 15 October 1990, Pages 53-67

https://doi.org/10.1016/0009-2541(90)90103-E Get rights and content

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.

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^α: Present address: Freie Universität Berlin, FR Geochemie, Boltzmannstr. 18–20, D-1000 Berlin 33, F.R.G.

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Research paperRegional distribution of 3He anomalies in the Icelandic crust

Abstract

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