Tracing material contributions from Saint Helena plume to the South Mid-Atlantic ridge system

Abstract 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., K2O/TiO2, 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.