Petrogenesis and tectonic significance of basalts from the Mid-Atlantic Ridge 26°S: Evidence from Hf isotopes

It is well-known that the Mid-Atlantic Ridge is a representative of slow-spreading oceanic ridge. In this study, a systematic study of Sr-Nd-Pb-Hf isotopes have been carried out on representative basaltic lavas from the oceanic ridge-axis and seamounts of the 26 degrees S Mid-Atlantic Ridge. Integrated with the published data, the origin of basalts and the properties and evolution of mantle source has been discussed, aiming at providing key evidence for understanding mantle heterogeneity and interaction between mantle plume and oceanic ridge. The analysis results of major-trace elements and Sr-Nd-Pb-Hf isotopes suggest that all the samples show isotopically enriched N-MORB characteristics. In addition, the major-trace elements and isotopes of basaltic lavas from the 26 degrees S Mid-Atlantic Ridge have a large variation, and the isotopes show a good correlation, indicating an origin from melting of depleted asthenosphere with involvement of enriched components. Combined with elemental and isotopic characteristics and Sr-Nd-Hf isotope quantitative modeling, the enriched component might be the residual component of Tristan da Cunha mantle plume, implying the characteristics of EMI-type enriched mantle. The modeling shows that the mantle source of seamount basalt consists of ca. 90% similar to 95% depleted components and 10% similar to 5% enriched components. However, the enriched components of ridge axis basalt are less ( <5%). Additionally, the seamount basalt samples from sites 6 and 7 show highly radiogenic Pb isotopic compositions, which conforms to the boundary condition of the Dupal Pb anomaly. Quantitative calculations show that the anomaly might be related to the participation of EM I-type components, which agrees with the results of isotopic quantitative modeling. The possible genetic mechanism for normal mid-ocean ridge basalts (N-MORBs) with enriched isotopes studied here might be the interation between the distal plume and oceanic ridge. The Tristan da Cunha mantle plume is more than 1000km away from the oceanic ridge. During the migration of the mantle plume to the Mid-Atlantic ridge, the thickness of the lithosphere becomes significantly thinner, which provides a good condition for decompression melting, resulting in depletion of incompatible elements in the residual mantle plume. However, the isotopic composition inherits the enriched characteristics of the source. Subsequently, the residues of mantle plume reached beneath the Mid Atlantic ridge, and participated in decompression melting in the ridge region, and eventually formed N-MORBs with depleted incompatible elements and enriched isotopes in the study area. Therefore, the isotopically enriched N-MORBs studied in this paper might record the distal plume-ridge interaction and the re-melting process of the enriched mantle plume beneath the ocean ridge, providing new petrological and geochemical evidence for understanding mantle heterogeneity. Thus, mantle plume-oceanic ridge interaction might not only play a role in formation of E-MORB, it might also have great significance to understanding the petrogenesis of N-MORBs.