Hafnium isotopes in basalts from the southern Mid‐Atlantic Ridge from 40°S to 55°S: Discovery and Shona plume–ridge interactions and the role of recycled sediments

Abstract

Using high‐precision multiple collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS), 176Hf/177Hf was measured in 64 mid‐ocean ridge basalts (MORB) from the Mid‐Atlantic Ridge (MAR) between 40°S and 55°S and in one basalt from the Discovery Tablemount. εHf of these basalts range from −2.8 to +16.6. The low end of this range exceeds that previously reported for MORB by 3 epsilon units and is only 5 epsilon units higher than the lowest reported ocean island basalt (OIB) value. Besides showing evidence for large‐scale DUPAL‐like pollution, the South Atlantic depleted upper mantle is also a region of localized plume–ridge interactions. 176Hf/177Hf from this study confirms the interaction of the Discovery and Shona plumes with the MAR as has been proposed previously on the basis of Pb, Sr, and Nd isotope ratios in these basalts. ΔεHf deviations from the global εHf versus εNd mantle array show that Shona anomaly basalts tend to fall below the mantle array while most Discovery and LOMU anomaly basalts fall above the array. 176Hf/177Hf versus 206Pb/204Pb for basalts from the Shona anomaly indicate that the ridge‐centered heterogeneous Shona plume is mixing with the ambient upper mantle, which is locally fairly homogeneous. The Shona anomaly basalts can be modeled as three‐component mixtures of ambient depleted upper mantle (ADM) with a recycled plume component consisting of old oceanic crust (ca. 1.5 Ga) and varying proportions of ancient sediment. The proportion of sediment present in the recycled Shona plume component increases stepwise from north to south along the MAR, but calculated sediment fractions are model dependent. In contrast, according to this three‐component model, ancient sediment is probably not a significant component in the intraplate Discovery plume. Delaminated continental material, either from the subcontinental lithosphere or from the lower crust, may be present as the EM1 or LOMU‐type component in the Discovery plume. In addition, both Shona and Discovery plumes likely contain a primitive rare gas component entrained from the lower mantle in order to account for the high 3He/4He and the solar neon isotope compositions of these MORB.