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

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Abstract
He‐Pb‐Nd‐Sr isotope systematics in basalts from the Mid‐Atlantic Ridge (MAR) from 65°N to 78°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 “low3He/4He” Jan Mayen plume and the “high3He/4He” Iceland mantle plume is in the vicinity of the Spar FZ. Modeling indicates that the dispersion of the Jan Mayen plume is not purely radial, but extends preferentially northward, probably because of decreasing spreading rate and the cascading of the buoyant mantle plume across the Jan Mayen fracture zone (FZ) due to the large denivellation (change of level) of the base of the lithosphere caused by the large age offset (∼20 Myr). The incompatible parent and daughter (PD) 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 <F> 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, as well as inferred melting conditions. These decouplings are best explained by a modified version of the dispersion model of the Iceland plume byMertz et al.[1991] which was based on more limited isotopic data, and the fluid dynamic models ofIto et al.[1999]. In addition to binary mixing of the Iceland plume with the depleted asthenosphere a combination of (1) defluidization of the Iceland mantle plume occurring deeper than the dominant zone of dry decompression melting for enhancing He and heat transport along the southern Kolbeinsey Ridge and (2) fractional melting accompanying the northward dispersion and decompression of the Iceland mantle plume is required in order to explain the difference in wavelength between the He gradient and the Pb‐Nd‐Sr isotope gradients observed in mapping the dispersion of the Iceland plume along the MAR. Note that the Tjörnes TZ does not act as a dam against the northward dispersion of the Iceland plume, contrary to what the rare earth element variation, by itself, previously suggested. Finally, over the Knipovich Ridge the large scatter and lack of any systematics between the Pb‐Nd‐Sr isotope and related parent/daughter ratios along this immature, discontinuous, and shear‐dominated ridge, running parallel and close to the Svalbard continental break (74°–79°N), suggest the involvement in the melting of randomly distributed continental mantle lithosphere schlierens present in the depleted upper mantle source of the Knipovich Ridge basalts.