Extreme He-3/He-4 ratios in northwest Iceland: constraining the common component in mantle plumes

Olivine and clinopyroxene phenocrysts contained in late Tertiary basalts fr om Selardalur, northwest Iceland, carry volatiles with the highest helium i sotope ratio yet reported for any mantle plume. He-3/He-4 ratios measured o n three different samples and extracted by stepped crushing in vacuo fall c onsistently similar to 37 R-A (R-A = air He-3/He-4) - significantly higher than previously reported values for Iceland or Loihi Seamount (see K.A. Far ley, E. Neroda [Annu. Rev. Earth Planet. Sci. 26 (1998) 189-218]). The Sr, Nd and Ph isotopic composition of the same sample places it towards the cen ter of the mantle tetrahedron of Hart et al. (S.R. Hart, E.H. Hauri, L.A. O schmann, J.A. Whitehead [Science 256 (1992) 517-520]) - in exactly the regi on predicted for the common mantle endmember based on the convergence of a number of pseudo-linear arrays of ocean island basalts worldwide (E.H. Haur i, J.A. Whitehead, S.R. Hart [J. Geophys. Res. 99 (1994) 24275-24300]). Thi s observation implies that Selardalur may represent the best estimate avail able to date of the He-Sr-Nd-Pb isotopic composition of the 5th mantle comp onent common to many mantle plumes. (C) 1999 Elsevier Science B.V. All righ ts reserved.