HELIUM-ISOTOPES IN EARLY TERTIARY BASALTS, NORTHEAST GREENLAND - EVIDENCE FOR 58MA PLUME ACTIVITY IN THE NORTH-ATLANTIC ICELAND VOLCANIC PROVINCE

He-3/He-4 ratios in olivine phenocrysts have been measured in 58 Ma ba saltic lavas from the Hold with Hope region of northeast Greenland. Th ese lavas constitute a part of the North Atlantic volcanic province. T he samples exhibit a wide range of He-3/He-4 ratios between 1.7R(a) an d 21R(a) (R-a = the atmospheric He-3/He-4 ratio = 1.386 x 10(-6)), whi ch is comparable to the previously recorded range for Qua ternary Icel andic basalts. These He isotope variations may partly result from crus tal assimilation, but He-3/He-4 ratios higher than those of mid-ocean- ridge basalts support early participation of plume-head material at th e onset of volcanic activity in the North Atlantic volcanic province. As with the Deccan-Reunion plume and the Ethiopia-Afar plume, this lon g-term contribution of He-3-rich material is fully consistent with mod els that ascribe a deep (i.e, deeper than the convective mantle that d rives plate tectonics) origin for plume-source material, but contradic ts those that postulate a shallow-mantle origin for continental hood b asalts and large igneous provinces.