Ancient, buoyant mantle under the Sierra Leone Ridge in the equatorial Atlantic

The Sierra Leone Ridge is the equatorial portion of the Mid-Atlantic Ridge (MAR) between the St. Paul (0-1 degrees N) and Doldrums (7-9 degrees N) Fracture Zones. At similar to 80 Ma, a submarine plateau -the Sierra Leone Rise which is now located off-ridge on the African plate and the Ceara Rise on the South American plate- formed due to excess magma production. Incompatible element-enriched MORB with high Sr-Pb but low Nd isotope ratios suggests that the high magma production could result from the interaction of a mantle plume with the Sierra Leone Ridge, the so-called Sierra Leone mantle plume that may now be centered at similar to 1.7 degrees N along the MAR. In order to define the nature of the mantle source beneath the Sierra Leone Ridge, we present major-trace element concentrations and radiogenic isotope ratios from abyssal peridotites and MORB from 4 to 7 degrees N along the MAR. High Hf isotope ratios are preserved in clinopyroxenes from abyssal peridotites (epsilon(Hf) = 12-54), indicating that the mantle beneath the Sierra Leone Ridge underwent extensive melting several 10(8)-10(9) years before remelting under the present MAR. Most peridotites have high epsilon(Hf), but low epsilon(Nd) similar to MORBs, although some extend to unusually low epsilon(Nd) of similar to-6. We argue that these peridotites not only melted, but were also re-enriched in compatible elements concurrent to melting in the past, and that such incompatible element re-enriched peridotites are now the main component of the sub-ridge mantle in the Sierra Leone area. Extensive remelting of ancient, incompatible element re-enriched peridotite that is compositionally buoyant, in addition to minor amounts of recycled crust, may therefore have caused the abundant magmatism that characterizes this portion of the MAR since formation of the Sierra Leone and Ceara Rise similar to 80 Ma ago, and accounts for its elevated topography.