On- and off-axis chemical heterogeneities along the South Atlantic Mid-Ocean-Ridge (5–11°S): Shallow or deep recycling of ocean crust and/or intraplate volcanism?

Abstract

The Mid-Atlantic Ridge between the Ascension and Bode Verde Fracture Zones exhibits anomalous crustal thickness and geochemical compositions, which could reflect the presence of either small, enriched heterogeneities in the upper mantle or a weak, diffuse mantle plume. We report new trace element (106 samples) and Sr, Nd and Pb (double spike) isotope data from 72 ridge axis samples and 9 off-axis seamount samples between 5 and 11°S, as well as U–Th–Ra disequilibria data for the seamounts. The U-series data constrain the age of one sample from Seamount D, furthest (120 km) east of the shallowest part of the ridge, to be < 10,000 yrs old and the samples from the other three seamounts closer to the ridge to be younger than 240,000 yrs. As can be most clearly discerned on a diagram of 208Pb/ 206Pb vs. 143Nd/ 144Nd, at least four distinct components are required to explain the geochemical variations along the ridge: 1) a common depleted (D-MORB-like) component in samples near and north of the Ascension Fracture Zone (4.8–7.6°S), representing the most depleted compositions sampled thus far along the mid-Atlantic ridge, 2) an enriched component upwelling beneath Ascension Island and the northern A1 ridge segment (segment numbers increase from A1 to A4 going south from the Ascension Fracture Zone), 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, which extend from Ascension Island and the A3 seamounts respectively to the depleted D-MORB component, interpreted to reflect local ambient mantle. 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.