Helium, oxygen, strontium and neodymium isotopic relationships in Icelandic volcanics

3He/ 4He ratios have been obtained for basaltic, intermediate and acid volcanic glasses from Iceland. Basaltic glasses exhibit a wide range of 3He/ 4He ratios (4 < R/R a < 24), which is consistent with the previously recorded range for Icelandic geothermal systems. In contrast the glasses with intermediate and acid compositions have 3He/ 4He values close to the atmospheric value ( R a) with the exception of a 13-Ma sample which has R/R a= 0.07. 87Sr/ 86Sr, 143Nd/ 144Nd ratios and δ 18O values are reported for the same samples. 3He/ 4He does not correlate with either 87Sr/ 86Sr or 143Nd/ 144Nd ratio and radiogenic components of He, Sr and Nd have apparently been decoupled. Interaction of Icelandic magmas with hydrothermally altered and older Icelandic crust is the preferred explanation for variable and often low δ 18O values. It is suggested that primary 3He/ 4He ratios may have been modified by incorporation of radiogenic helium developed within the Icelandic crust to impose a larger range of 3He/ 4He ratios on the erupted products than was actually inherited from the mantle beneath Iceland. Intermediate and acid samples have all been severely contaminated by atmospheric helium, most probably at very shallow levels within the crust.