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Mantle convection plays a key role in magmatism and volcanism on Earth. The final distribution of deep mantle material upwelled into the asthenosphere cannot be clearly tracked using seismic imaging techniques. Where mid-ocean ridges and plumes interact, the along-ridge variations in plume-affected basalts constrain the spatial extent of the plume-related flow in the asthenosphere. These variations are helpful for revealing convection throughout the mantle from the core-mantle boundary (CMB) to the bottom of the lithosphere. In this study, regional geophysical data, as well as the results of geochemical independent component analysis of radiogenic Sr–Nd-Pb isotopes of South Mid-Atlantic Ridge (SMAR) basalts, were used to analyze the distribution characteristics of the plume-affected asthenosphere beneath the South Atlantic Ocean. We determined that the ridge scope of the Ascension plume-influenced SMAR segments is bounded by the Ascension transform fracture (~ 7.5°S) to the north and the Bode Verde transform fracture (~ 11.1°S) to the south, while the Saint Helena plume-contaminated SMAR segments are bounded by the Cardno transform fracture (~ 14.2°S) to the north and the Trinidade transform fracture (~ 20.8°S) to the south. Furthermore, we determined that the melt extraction process taking place between the mantle plume and ridge system may weaken the plume-related geochemical signals of these plume-affected MORBs. Our results suggest that the distribution of plume-related asthenosphere under the South Atlantic is influenced by large transform faults that block the propagation of the plumes along the bottom of the lithosphere, as well as the propagation of plume-affected materials along the ridge system. [ABSTRACT FROM AUTHOR]

We present new major and trace element and Sr-Nd-Pb isotope data for basalts from the 25.3–27.7°S segments of the South Mid-Atlantic Ridge (SMAR). A fractional crystallization model shows that olivine, plagioclase, and clinopyroxene crystals crystallized in the magmatic chamber underlying the SMAR 27.1°S and 27.7°S segments, while the SMAR 25.3°S segment basaltic lavas are dominated by fractional crystallization of olivine and plagioclase. Their Ce/Yb, (Tb/Yb) N , and Fe/Mn ratios suggest a mantle source lithology dominated by spinel-lherzolite. The linear correlations between Mg# and TiO 2 , Ba, and Zr/Nb indicate that SMAR basalts in this study have primary mantle melts with similar chemical compositions and partial melting extents. The along-ridge variations in the fractionation-corrected incompatible element (Ba, Zr/Nb, (La/Sm) N) and Sr-Nd-Pb isotope compositions of the SMAR 20–40°S basalts show that Tristan mantle plume materials exist in the SMAR asthenosphere and gradually decrease from the southern SMAR 35°S segment in the vicinity of Tristan Island to the northern SMAR 25.3–27.7°S segments. The mixing models of the fractionation-corrected Ba contents, (La/Sm) N ratios and radiogenic Sr-Nd-Pb isotope ratios show that variable degrees of contamination by the Tristan mantle plume contribute to the heterogeneous SMAR 20–40°S mantle source. The proportion of Tristan plume-related materials retained in the SMAR 20–40°S asthenosphere is <10%, which gradually decreases from south to north. By integrating the evolution of the South Atlantic and the historical interaction between the SMAR system and the Tristan mantle plume, we demonstrate that the systematic variations in the geochemical compositions of the SMAR 25.3–27.7°S basalts are induced by the presence of the Tristan mantle plume materials in the southern SMAR asthenosphere. [Display omitted] [ABSTRACT FROM AUTHOR]

The spatial scope of the Saint Helena hotspot influence on SMAR system is from SMAR 14.2°S to 20.8°S segments bounded by Cardno T.F. and Trinidade T.F. The sublithospheric flow channels between Saint Helena plume and the northern segments (e.g., segment 2) could be continuous, while those at the southern segments (e.g., segments 4–5) change to be more schlieren-like. Possible flow directions of the Saint Helena hotspot materials from Cardno seamount toward the SMAR system are inferred. The ends of the inferred flows directions are at the SMAR segments 2–5, where the basalts have stronger Saint Helena plume-related geochemical signals, and the ridge axes keep shallower axial depth and lower mantle Bouguer gravity anomaly relative to the other segments. Following the possible directions, the lateral footprint of the asthenospheric mantle influenced by the Saint Helena plume has grown to several hundreds of kilometers over the past 80 Ma. • New geochemical and Sr-Nd-Pb-Hf isotopic data for the SMAR basalts are presented. • Saint Helena hotspot is still affecting the SMAR system nowadays. • The spatial scope of Saint Helena influence on SMAR segments is between 14.2°S and 20.8°S. • Saint Helena plume-influenced SMAR segments are bounded by Cardno and Trinidade fracture zone. • Asthenospheric material flow paths from Saint Helena hotspot toward the SMAR axes are inferred. Ridge-hotspot interaction plays a significant role during the whole geological evolution history of Atlantic Ocean. Whether the ongoing spreading south Mid-Atlantic ridge (SMAR) is affected by those mantle plumes gradually migrating away from the ridge or not, need to be further explored. Here we investigate the chemical components of basalts from the SMAR 13.2°S–24.2°S segments, which contain the Saint Helena plume-like chemical traces that have enriched incompatible element ratios (e.g., K 2 O/TiO 2 , Nb/Zr, (La/Sm) N) and higher 87Sr/86Sr, 206Pb/204Pb and lower 143Nd/144Nd and 176Hf/177Hf relative to those plume-free SMAR MORBs. These, combining with geophysical data and off-axis seamounts data, show that the spatial scope of Saint Helena hotspot influence on SMAR system is bounded by the Cardno fracture zone (∼14.2°S) to the north and the Trinidade fracture zone (∼20.8°S) to the south. Furthermore, this study infers possible flow directions of the Saint Helena hotspot materials from Cardno seamount toward the SMAR system, and the ends of these flows directions are at the SMAR 14.2°S–20.8°S segments, where the basalts have stronger Saint Helena plume-related geochemical signals, and the ridge axes keep shallower axis depth and lower mantle Bouguer gravity anomaly relative to the other segments. Finally, we develop a sketch-like model to depict material flow styles from Saint Helena plume to the actively spreading SMAR regions along a sloping rheologically boundary layer at the lithospheric bottom. [ABSTRACT FROM AUTHOR]