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 δ¹⁸O 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 δ¹⁸O 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.