Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic - A He-Pb-Nd-Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich Ridges

He-Pb-Nd-Sr isotope systematics in basalts from the Mid-Atlantic Ridge from 65 to 78 deg N are reported for mapping the zone of influence of the Jan Mayen and Iceland mantle plumes in the Arctic. The geographical variation and the two distinct trends observed in the He-Pb-Nd-Sr isotope space clearly indicate that the boundary between the zone of influence of the 'low He-3/He-4' Jan Mayen plume and the 'high He-3/He-4' Iceland mantle plume is in the vicinity of the Spar fracture zone (FZ). The Jan Mayen plume extends preferentially northward, probably because of decreasing spreading rate and the cascading of the buoyant mantle plume across the Jan Mayen FZ due to the large change of level of the base of the lithosphere caused by the large age offset. Incompatible parent-daughter element concentrations and their ratios for the basalt population from the Jan Mayen plume are highly coherent with the Pb-Nd-Sr isotope ratios and show essentially the same geographical pattern in spite of large variations in the mean degree of fusion and extent of fractional crystallization. In contrast, over the southern Kolbeinsey Ridge, unusual decouplings are observed between He-Pb-Nd-Sr isotopic systematics and incompatible element variations. These decouplings are explained by a modified version of the dispersion model of the Iceland plume by Mertz et al. (1991), based on more limited isotopic data, and the fluid dynamic models of Ito et al. (1999). (Author)