Major element heterogeneity in the mantle source of the North Atlantic igneous province

High-MgO (> 8.5 wt%), aphyric lavas erupted at various locations in the Nor th Atlantic igneous province are utilized to characterize the nature of man tle melting during the formation of this province. Based on the observation that the Ni concentration in residual mantle olivine mostly falls in the r ange of 2000-3500 ppm, these high-MgO samples are corrected for olivine fra ctionation until the Ni concentration of equilibrium olivine reaches 3500 p pm, to estimate the composition of primary mantle-derived melt. Estimated p rimary melt compositions suggest that this province is characterized by sig nificant major element source heterogeneity possibly resulting from basalt addition prior to melting. Primary melts for Southwest Iceland and Theistar eykir (North Iceland) are shown to require different source mantle composit ions. Whereas the Theistareykir primary melt may be explained by the meltin g of pyrolitic mantle, the source mantle for Southwest Iceland must be enri ched in iron, having molar Mg/(Mg+Fe), or Mg#, < 0.88, This compositional d ichotomy in Iceland seems to continue to adjacent Mid-Atlantic Ridge segmen ts, i.e, the Kolbeinsey and Reykjanes Ridges. The primary melts for East an d Southeast Greenland also indicate a fertile mantle source, and the estima te of Mg# is the lowest for the East Greenland source mantle ( < 0.87). The inferred spatial extent of source heterogeneity suggests the presence of a long-lived compositional anomaly in this igneous province since the openin g of the North Atlantic. (C) 2000 Elsevier Science B.V. All rights reserved .