A high-resolution sampling profile constituted of 52 basalt samples from across the East Pacific Rise (EPR) was investigated. These samples provide a unique opportunity to study the coeval recording of isotopic signals derived from sub-marine eruptions at a fast spreading ridge over a time interval of ∼320 kyrs. Additionally, the study area is marked by the presence of a neighboring seamount chain that has recently caused the ridge to jump twice towards it. Combining previous geochemical studies and bathymetry, we established a first-order chronology between analyzed samples, and have reconstructed the evolution of basalt compositions as the ridge and seamounts advance and finally merge. Our data reveal the existence of two distinct types of isotopic variability within the samples. One that has a low amplitude and frequency and is accounted for by the continuous melting of the ambient mantle, indicating a process with a ∼125 kyr periodicity. The other, of higher amplitude, is discontinuous in time, and likely reflects the seamounts source influence on the ridge. Our results support a two-step hotspot-ridge interaction including a first stage (≥ 600 ka) of regional enrichment of the depleted ridge mantle by hotspot material; and a second, more recent (at least 300 ka) even wherein ambient mantle melts mixed with proximal melts from heterogeneous seamounts sourced nearby. We also propose that the ancient gabbroic component previously identified in this region appeared very recently (< hundreds ya) as erupted lavas are exclusively recorded in a cluster proximal to the axial zone of the ridge.

Plain Language Summary

The Earth’s mantle makes up 80% of the planet’s volume; however, mantle rocks are rare at the surface. At Mid-ocean ridges, the mantle experiences melting and derived melts may reach the Earth's surface as volcanic basalts. These rocks continuously record the geochemical signature of their mantle source region. Very little is known about the evolution of lava composition over time due to the difficulty of sampling off-ridge axis igneous rocks. This study focuses on a specific portion of the East Pacific Rise at ∼16°N, where it interacts with a nearby hotspot. Basaltic rocks were collected with the French submersible Nautile (Ifremer) across the ridge axis during the cruise Parisub (2010, R/V L’Atalante). The sampling profile obtained covers a period of time that allows long term monitoring and assessment of mantle sourced signals and fluctuations in erupted basalts for > 300 kyrs. We have established a relative chronology between these samples, and propose a paleo-reconstruction of the area accounting for both the isotopic variations observed and previously reported geophysical results. Our new results also suggest rapid magma transfers within the crust independent of melting of seamounts and/or ridge sources, including periodic injections of enriched mantle material into the ridge-melting region.