Mantle heterogeneities beneath the South Atlantic: a Nd [sbnd]Sr [sbnd]Pb isotope study along the Mid-Atlantic Ridge (3°S–46°S)

Abstract

We report on 55 Nd and Sr isotope analyses of Mid-Atlantic Ridge (MAR) basalt glasses from 3°S to 46°S, using the same samples on which Pb and He isotope ratios were reported earlier (Hanan et al. [1] and Graham et al. [2]). Eighteen new Pb, Sr, and Nd isotope analyses are also reported on basalt glasses from 17 stations from the same region. 87Sr 86Sr ratios range from 0.70212 to 0.70410 and 143Nd 144Nd from 0.51285 to 0.51331. The along-ridge long wavelength 87Sr 86Sr variation delineated by light-REE depleted basalts increases progressively southward toward Tristan da Cunha. Short wavelength, spike-like, positive 87Sr 86Sr anomalies composed of light-REE enriched basalts are found opposite the Tristan, St. Helena, and Circe (Ascension) hotspots (as for the Pb isotopes). The short and long wavelength 143Nd 144Nd variations anti-correlate with those of 87Sr 86Sr . The 17 new Pb isotope analyses confirm both the short and long wavelength trends previously reported by Hanan et al. [1]. These spatial variations, as well as the variations in Nd Sr Pb isotopic space fully confirm the mantle plume-ridge interaction model and upper mantle mixing conditions beneath the South Atlantic inferred previously on the basis of Pb isotopic data only (Hanan et al. [1]). However, in Nd Sr isotopic subspace the Circe and the St. Helena mixing vector are not distinguishable. The short wavelength Nd Sr Pb He anomalies suggest recent lateral sublithospheric channeled flows from these off-ridge plumes to the migrating MAR axis. The long wavelength variations reflect a broad pollution of the asthenosphere by Pb and Sr radiogenic, isotope-rich, mantle material, which has been partly depleted of incompatible elements relatively recently. This broad pollution may be related to the partial melting and dispersion of the Tristan and St. Helena plume heads into the subcontinental asthenosphere, prior to the opening of the South Atlantic. In this two-stage model, the mixing relations in Nd Sr Pb space further require that the incompatible element depletion and dispersion of the St. Helena plume into the asthenosphere occurred before that of the Tristan plume head. We emphasize that this two-stage model is based purely on isotope systematics. The implied thermal and dynamic aspects of this model remain to be evaluated and tested.