Between 5° and 11°S, the Mid-Atlantic Ridge displays
anomalous crustal thickness and geochemical compositions,
thought to be related to either small scale upper mantle
heterogeneities or a weak, diffuse mantle plume. We report
new high precision trace element and Sr, Nd and Pb (DS)
isotope data for 72 ridge axis samples and 9 off-axis seamount
samples along with U–Th–Ra disequilibria data for off axis
seamounts at c. 9.7°S. At least four distinct components are
needed to explain the geochemical variations along the ridge:
1) a common depleted (D-MORB-like) component near and
north of 4.8–7.6°S, 2) an enriched component upwelling
beneath Ascension Island and the northern A1 ridge segment
(segment numbers ascend from north to south), 3) an enriched
component upwelling beneath the A2 ridge segment, and 4) an
enriched component upwelling beneath the line of seamounts
east of the A3 segment and the A3 and A4 segments. The A1
and the A3+A4 segment lavas form well-defined mixing
arrays from Ascension Island and the A3 seamounts
respectively to the depleted D-MORB component. We
propose that the enriched components represent different
packages of subducted ocean crust and/or ocean island basalt
(OIB) type volcanic islands and seamounts that have either
been recycled through 1) the shallow mantle, upwelling
passively beneath the ridge system or 2) the deep mantle via
an actively upwelling heterogeneous mantle plume that
interacts with the ridge system
