Plume-ridge interactions of the Discovery and Shona mantle plumes with the southern mid-Atlantic ridge (40°-55°S)

We report on 66 Pb, Sr, and Nd isotope analyses of basalts dredged along the MidAtlantic Ridge (MAR) from 40 degrees to 55 degrees S. The results strongly indicate interaction and mixing between the off-ridge Discovery and ridge-centered Shona mantle plumes and the ambient asthenosphere beneath the MAR. In addition, the Bouvet mantle plume appears to be feeding the southernmost portion of the MAR as suggested earlier by le Roex et al. [1987]. The Discovery and Shona plumes have enriched mantle and high-mu (mu=U-238/Pb-204) affinities, respectively. Their proximity to one another suggests a genetic relationship, probably associated with subducted altered oceanic crust recycled through the mantle with some sediment (Discovery) or without sediment (Shona). The Discovery Ridge Anomaly exhibits Pb, Sr, and Nd isotopic discontinuities resulting from southward preferential plume flow across the Agulhas transform beginning similar to 13 Ma. The presence of a component with unusually low Pb-206/Pb-204 accompanied by high Sr-87/Sr-86 and low Pb-208/Pb-204 and Nd-143/Nd-144 in the Discovery Ridge Anomaly and to a lesser extent in the Shona Ridge Anomaly indicates three-component mixing between the ambient asthenosphere, the Discovery and Shona plumes, and this low-mu (LOMU) component which possibly represents subcontinental lithospheric mantle material. We also note that in Pb, Sr, and Nd isotopic space, ocean island basalts from the Tristan, Gough, and Discovery family of plumes could be interpreted as resulting from binary mixing between a generic plume component similar to Bouvet or the "C" component [Hanan and Graham, 1994] and the LOMU component, which progressively increases southward. The LOMU component seems to be a characteristic feature of the South Atlantic and Indian Ocean mantles and is thought to reside passively in the shallow mantle because of delamination of subcontinental lithospheric mantle following the breakup of Gondwana.