Mantle heterogeneity beneath the southern Mid-Atlantic Ridge: trace element evidence for contamination of ambient asthenospheric mantle

We report new trace element data for an extensive suite of quench basalt glasses dredged from the southern Mid-Atlantic Ridge (MAR) between 40 degree S and 52.5 degree S. Ratios between highly incompatible trace elements are strongly correlated and indicate a systematic distribution of incompatible element enriched mid-ocean ridge basalt (MORB) (E-type: Zr/Nb = 5.9-19, Y/Nb = 0.9-8.4, (La/Sm) sub(n) = 1.0-2.9) and incompatible element depleted MORB (N-type: Zr/Nb = 30-69, Y/Nb = 11-29, (La/Sm) sub(n) = 0.48-0.79) along this section of the southern MAR. A notable feature of N-type MORB from the region is the higher than usual Ba/Nb (4-9), La/Nb (1.2-2.4) and primitive mantle normalised K/Nb ratios (> 1). Ba/Nb ratios in E-type MORB samples from 47.5 to 49 degree S are especially elevated (> 10). The occurrence and geographic distribution of E-type MORB along this section of the southern MAR can be correlated with the ridge-centred Shona and off-axis Discovery mantle plumes. In conjunction with published isotope data for a subset of the same sample suite, a model is developed whereby prior to the breakup of Gondwana and the opening of the South Atlantic Ocean, the underlying asthenospheric mantle was locally contaminated by fluids/melts rising from the major Mesozoic subduction zone along the south-southwest boundary of Gondwana, leaving a subduction zone geochemical imprint (elevated (K/Nb) sub(n) and super(87)Sr/ super(86)Sr ratios, decreased super(143)Nd/ super(144)Nd ratios). Subsequent impingement of three major mantle plume heads (Tristan/Gough, Discovery, Shona) resulted in heating and thermal erosion of the lowermost subcontinental lithosphere and dispersal into the convecting asthenospheric mantle. With the opening of the ocean basin, continued plume upwelling led to plume-ridge interactions and mixing between geochemically enriched mantle derived from the Shona and Discovery mantle plumes, material derived from delamination of the subcontinental lithosphere, and mildly subduction zone contaminated depleted asthenospheric mantle.