Mantle plume-ridge interactions in the Central North Atlantic: A Nd isotope study of Mid-Atlantic Ridge basalts from 30°N to 50°N

Abstract

We report on Nd isotopes and Sm and Nd contents of 46 fresh Mid-Atlantic Ridge basalts over the Azores swell from 30°N to 50°N. The latitudinal 143Nd/144Nd and Sm/Nd profiles anti-correlate with the 87Sr/86Sr profile previously established [1]. Sharp minima in 143Nd/144Nd, identical in value, are found at the 46°N, 43°N, 39°N, and 35°N short-wavelength geochemical anomalies previously recognized, as well as the 1100 km long gradient south of the Azores platform. A single binary mixing line is observed in 143Nd/144Nd vs. 87Sr/86Sr space, suggesting that the incompatible element-rich mantle sources underlying these four short-wavelength anomalies have a similar long-term origin. However, at a given 143Nd/144Nd (or 87Sr/86Sr), the MORB population from 40.5°N to 50°N is systematically lower in incompatible elements and their ratios, such as La/Sm, Nd/Sm, Cl/F and Rb/Sr, than the 40.5°N-30°N population. The required fractionation must be relatively recent. But the bulk chemistry of these MORB rules out that it was generated directly under the ridge by large varying degrees of partial melting of passive heterogeneities embedded in the depleted asthenosphere. An earlier, but not too distant, metasomatic enrichment or depletion event is required. A small 1–2% melt fraction removal by fractional decompression melting of a wet garnet lherzolite, at its wet solidus, can readily account for the depletion of the 46°N and 43°N relative to the 39°N mantle sources. We speculate that the short-wavelength 46°N, 43°N and 39°N anomalies were caused either by: (1) a family of plumes with the same long-term origin and thermal boundary layer source; or (2) by the detachment of blobs from a single bending plume converging with the MAR axis at 39°N and preferentially discharging southwestward along the spreading axis. In either case, the rising plumes, or the detached blobs, at 46°N and 43°N occurred on the east flank of the MAR below the thick lithosphere. The plume-derived material reaching the MAR axis at these latitudes would be residual in nature (mantle restite), as a result of an early removal in intraplate settings of small melt fractions, directly over the points where the plumes are rising, or over where the blobs detached from the bending plume. The previously proposed model invoking a separate off-ridge plume (Great Meteor) captured by the westward migrating MAR is retained for explaining the 35°N MAR anomaly and the related Great Meteor-Corner Seamount-New England Seamount hotspot track [2–4].