Abstract

3He/4He ratios in olivine phenocrysts have been measured in 58 Ma basaltic lavas from the Hold with Hope region of northeast Greenland. These lavas constitute a part of the North Atlantic volcanic province. The samples exhibit a wide range of 3He/4He ratios between 1.7Ra and 21Ra (Ra =the atmospheric 3He/4He ratio =1.386 × 10−6), which is comparable to the previously recorded range for Quaternary Icelandic basalts. These He isotope variations may partly result from crustal assimilation, but 3He/4He ratios higher than those of mid-ocean-ridge basalts support early participation of plume-head material at the onset of volcanic activity in the North Atlantic volcanic province. As with the Deccan-Réunion plume and the Ethiopia-Afar plume, this long-term contribution of 3He-rich material is fully consistent with models that ascribe a deep (i.e., deeper than the convective mantle that drives plate tectonics) origin for plume-source material, but contradicts those that postulate a shallow-mantle origin for continental flood basalts and large igneous provinces.

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