Abstract
We present elemental and isotopic (Sr–Nd–Pb–Hf–Os–He) data on primitive alkalic lavas from the Prinsen af Wales Bjerge, East Greenland. Stratigraphical, compositional and 40Ar–39Ar data indicate that this inland alkalic activity was contemporaneous with the upper parts of the main tholeiitic plateau basalts and also post-dated them. The alkalic rocks show a marked crustal influence, indicating establishment of new magmatic plumbing systems distinct from the long-lived coastal systems that fed the relatively uncontaminated plateau basalts. The least contaminated lavas have high 3He/4He isotope ratios (R/RA 12·4–18·5), sub-chondritic 187Os/188Osi (0·120–0·126), low εNdi (∼+4) and εHfi (∼+6) that plot below the ‘Nd–Hf mantle array’, and trace element characteristics similar to HIMU ocean island basalt (OIB). The uncontaminated magma is inferred to have more radiogenic 206Pb/204Pb values (>19·2) than the plateau basalts and Icelandic basalts, and thus represents a possible ‘enriched’ component to explain the compositional variations within the plateau basalts. One model to explain these compositional features is preferential melting of recycled material within the plume upwelling beneath the thick lithospheric cap, with 3He contributed from volatile-rich fluids from elsewhere in the Icelandic plume. The exact nature of the recycled component is not yet resolved, although Hf isotope compositions rule out any significant role for recycled pelagic sediment, and the low 187Os/188Os limits the participation of recycled basaltic material and argues instead for a contribution from the mantle section of the recycled slab.