Geochemical segmentation of the Mid-Atlantic Ridge north of Iceland and ridge–hot spot interaction in the North Atlantic

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Abstract

[1] Hafnium, Nd, and Pb isotope data are reported for mid-ocean ridge basalts (MORB) from the Mid-Atlantic Ridge between 50°N and 78°N. Treating Pb isotopes separately from Sr, Nd, and Hf isotopes drastically reduces the number of end-members required to account for mixing properties in the investigated basalt population. Three geochemical end-members account for >99.95% of the observed Pb isotope variability. The isotopic compositions of Pb are somewhat decoupled from those of other isotopic systems due to nonlinear mixing relationships between isotopic compositions of different elements and elemental fractionation by magmatic processes. The first principal component reflects a mixture of two geochemical end-members, DM and the common component C. The third end-member (EM) is enriched and its Th/U ratio substantially higher than that of the first two end-members. The HIMU end-member apparently is missing. Iceland and Jan Mayen divide the northern Mid-Atlantic Ridge into three isotopically distinct segments. The strong first Pb principal component present in Icelandic basalts expands southward into the Reykjanes Ridge, but not northward into the Kolbeinsey Ridge. Similar features are observed for Hf and Nd isotope compositions. By contrast, the second Pb principal component shows a smooth transition between Kolbeinsey MORB and subaerial Iceland data. The distribution of the two Pb principal components present in the basalts around Jan Mayen likewise is contrasted: the first Pb principal component and its Nd and Hf counterparts flow northward into the Mohns Ridge but are arrested southward by the Jan Mayen fracture zone. Basalts north of Jan Mayen display the most radiogenic Hf reported so far for any MORB worldwide. For ∼1000 km along the Mohns and Knipovich Ridges, Hf isotope compositions fail to correlate linearly with ɛNd, but instead define a hyperbolic array between the C component and an end-member unusually depleted in Hf. The latter carries a strong garnet signature and may appear by disequilibrium melting of streaks of subcontinental lithosphere left by continental rifting. The presence of continental crust beneath Jan Mayen is not supported by either Ce/Pb, Nb/U, or Hf/Sm. If Jan Mayen is a hot spot, its material is injected along a major mantle discontinuity separating distinct convective domains.

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