HELIUM ISOTOPE COMPOSITION OF THE EARLY ICELAND MANTLE PLUME INFERREDFROM THE TERTIARY PICRITES OF WEST GREENLAND

Picrites from the 61 million year old Vaigat Formation of the Nuussuaq Peninsula in West Greenland have He-3/He-4 ratios trapped in olivine phenocrysts which range up to 30 times the atmospheric ratio. These hi gh values, measured during gas extraction by crushing in vacuum, are s imilar to the highest magmatic He-3/He-4 ratios found in young terrest rial volcanic rocks. By analogy with young basalts, in which crushing selectively extracts magmatic helium, any significant cosmogenic He-3 appears to be absent in these picrites. Additional evidence for the ab sence of cosmogenic helium is provided by fusion results on the crushe d olivine powders and by a single stepwise crushing experiment, in whi ch only magmatic and radiogenic helium components are resolvable. The West Greenland picrites have Pb, Nd and Sr isotope compositions which overlap those found in picrites from Iceland and in basalts from Loihi Seamount, localities which today also have high He-3/He-4 ratios. Iso topic variations in He, Pb, Nd and Sr for the West Greenland picrites are interpreted to largely result from interaction of the early Icelan d mantle plume with the upper mantle during plume ascent and dispersio n beneath the continental lithosphere. The presence of high 3He/4He ra tios in West Greenland, and the onset of magmatism across the North At lantic Volcanic Province near 62 Ma, supports the hypothesis for very rapid dispersion (>1 m/year) of mantle plume head material during the earliest stages of plume impact, as predicted in recent numerical simu lations of plume behavior during thermal mantle convection with non-Ne wtonian rheology. (C) 1998 Elsevier Science B.V. All rights reserved.