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

High-MgO (s 8.5 wt%), aphyric lavas erupted at various locations in the North Atlantic igneous province are utilized to characterize the nature of mantle melting during the formation of this province. Based on the observation that the Ni concentration in residual mantle olivine mostly falls in the range of 2000^3500 ppm, these high-MgO samples are corrected for olivine fractionation until the Ni concentration of equilibrium olivine reaches 3500 ppm, to estimate the composition of primary mantle-derived melt. Estimated primary melt compositions suggest that this province is characterized by significant major element source heterogeneity possibly resulting from basalt addition prior to melting. Primary melts for Southwest Iceland and Theistareykir (North Iceland) are shown to require different source mantle compositions. Whereas the Theistareykir primary melt may be explained by the melting of pyrolitic mantle, the source mantle for Southwest Iceland must be enriched in iron, having molar Mg/(Mg+Fe), or Mg#, 6 0.88. This compositional dichotomy in Iceland seems to continue to adjacent Mid-Atlantic Ridge segments, i.e. the Kolbeinsey and Reykjanes Ridges. The primary melts for East and Southeast Greenland also indicate a fertile mantle source, and the estimate of Mg# is the lowest for the East Greenland source mantle (6 0.87). The inferred spatial extent of source heterogeneity suggests the presence of a long-lived compositional anomaly in this igneous province since the opening of the North Atlantic. ß 2000 Elsevier Science B.V. All rights reserved.