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

• Hf-Nd isotopes from abyssal peridotites and associated MORB define the dominant mantle component in the studied sector of the Equatorial Mid-Atlantic Ridge (MAR). • Abundant magmatism in this part of the MAR is driven by the upwelling of variably melt-depleted, buoyant mantle with high Hf isotope ratios. • Intrinsic density variations of the passively upwelling, sub-ridge mantle can cause periods of excess of melt production, and thus thicker-than-normal oceanic crust, at the Sierra Leone ridge of elsewhere along the global Mid-Ocean ridge system. The Sierra Leone Ridge is the equatorial portion of the Mid-Atlantic Ridge (MAR) between the St. Paul (0–1°N) and Doldrums (7–9°N) Fracture Zones. At ∼80 Ma, a submarine plateau –the Sierra Leone Rise which is now located off-ridge on the African plate and the Céarà 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 ∼1.7°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°N along the MAR. High Hf isotope ratios are preserved in clinopyroxenes from abyssal peridotites (Ɛ Hf = 12–54), indicating that the mantle beneath the Sierra Leone Ridge underwent extensive melting several 10⁸–10⁹ years before remelting under the present MAR. Most peridotites have high Ɛ Hf , but low Ɛ Nd similar to MORBs, although some extend to unusually low Ɛ Nd of ∼-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 Céarà Rise ∼80 Ma ago, and accounts for its elevated topography.