Ridgeward flow of compositionally heterogeneous mantle produces near-ridge seamount chains in the South Pacific

Many near-ridge seamounts and seamount chains in the Pacific Ocean have a nonplume origin. Yet, their origin remains to be fully understood. Our new geochemical study on seamount basalts from the Pukapuka Ridge (PPR) finds a large along-ridge lava compositional variation with a gradual decrease in a geochemically enriched component toward the East Pacific Rise (EPR) axis. This spatial geochemical variation is best understood as resulting from decompression melting of compositionally heterogeneous mantle flowing toward the ridge, where the mantle consists of low-solidus materials of metasomatic origin dispersed within a more refractory peridotite matrix. Far from the ridge axis, preferential melting of enriched lower-solidus materials under thicker lithosphere leaves less enriched residues that undergo further decompression melting as they flow beneath thinner lithosphere toward the ridge axis. This process gives rise to the progressively less enriched lavas along the PPR chain toward the EPR. The residual enriched mantle component became embedded in the mantle beneath the southern EPR (13°S−23°S), forming an along-axis compositional dome at the EPR-PPR intersection (∼17°S−19°S). We predict that nonplume seamounts are best expressed as linear chains near and perpendicular to ocean ridges on fast-spreading plates as long as the flowing mantle is sufficiently heterogeneous. This finding explains widespread seamounts of nonplume origin in the Pacific Ocean, and it also explains the geophysical asymmetries in the mantle electromagnetic tomography (MELT) area.