Influence of the Sierra Leone mantle plume on the equatorial Mid‐Atlantic Ridge: A Nd‐Sr‐Pb isotopic study

Abstract

We report on a Pb‐Nd‐Sr isotope and rare earth study of Mid‐Atlantic Ridge (MAR) basalt glasses collected across the equatorial fracture zones from 7°S to 5°N (65 stations). The 1600‐km‐long profile reveals two mixing zones in the mantle that are isotopically distinct but cover the same range of (La/Sm)n ratios (0.3–2), with a gradational boundary between the Romanche and the Chain fracture zones. The potential mantle temperature profile inferred from Na2O content is also quite distinct. The north zone is dominated by a major, La/Sm and HIMU type Pb isotope anomaly centered at 1.7°N±300 km, which is flanked by two zones mildly radiogenic in Pb but depleted in light REE. A kinematic and evolutionary model describing the dispersion and interaction of the Sierra Leone plume with the asthenosphere and the MAR in the last 75 m.y. is proposed for this zone, which includes St. Paul and St. Peter's Rocks. In contrast, over the south zone the isotope/geochemical profiles are well correlated at all length scales and opposite in sign from the inferred potential mantle temperature profile and mean percent fusion. Broad negative gradients are observed between the Romanche and the Charcot fracture zones, superimposed by spikelike anomalies at the intersection with the eastern part of the Romanche and Chain transform faults, where cold plate edge effects prevail. The heterogeneous mantle model of Sleep [1984] and Langmuir and Bender [1984] is applicable to this zone, that is the volatile and radiogenic Pb‐rich lumps are preferentially melted during mantle decompression and passively sampled. The lumps may reflect the early dispersion of the St. Helena or Ascension mantle plumes under a thick lithosphere, followed by redistribution due to intense shearing, continental lithosphere delamination, and secondary mantle convection. The presence of a depleted asthenosphere unpolluted by plumes along the 400‐km‐long MAR segment between the Charcot and Ascension fracture zones is also apparent in the data.