Strontium, neodymium and lead isotope constraints on near-ridge seamount production beneath the South Atlantic

Abstract

STUDIES^1–7 of seamounts near the East Pacific Rise (EPR) have shown that, although most seamount lavas are petrographically and chemically identical to mid-ocean-ridge basalt, they are chemically and isotopically more diverse than those erupted on the rise axis. They are also generally more primitive (higher MgO content) and, in some cases, more depleted in incompatible elements than the axial basalts. This indicates that although near-ridge sea-mounts and the EPR share a common mantle source, there must be significant differences in their magma-supply processes. Sea-mounts are probably built by small batches of melt that rise rapidly to the surface, preserving evidence of heterogeneity in the mantle source region. By contrast, the processes of melting and melt segregation, storage and extrusion along the fast-spreading (9–13 cm yr^–1) EPR are more efficient in mixing and homogenizing basalt compositions^1–7. Here we show that the relationship of strontium, neodymium and lead isotope data between seamounts in the South Atlantic and the nearby axis of the slow-spreading Mid-Atlantic Ridge (MAR) is similar to that seen in the Pacific. This indicates that the processes leading to formation of near-ridge seamounts are similar at a wide range of spreading rates. Differences in the specific isotope signatures of lavas from near-ridge seamounts and axes of the EPR and MAR reflect regional differences in the upper-mantle source of mid-ocean-ridge basalts.