Plume‐ridge interaction in the Easter‐Salas y Gomez seamount chain‐Easter Microplate system: Pb isotope evidence

Abstract

Lead isotope ratios are reported on fresh basalt glasses from 28 seamounts located along the western end of the Easter‐Salas y Gomez seamount chain (ESC), southeast Pacific for the purpose of testing the mantle plume source‐migrating ridge sink (MPS‐MRS) model for the origin of this extensive intraplate neovolcanic seamount chain. Most of these basalts west of 103°W are less than 1.5 m.y. old. Results reveal a 1100‐km‐long westward gradient decreasing in radiogenic Pb from Salas y Gomez to the Easter microplate (EMP). The results confirm the previously established four‐point average trend based on data from Salas y Gomez, Easter Island, the east rift, and the west rift of the Easter microplate [Hanan and Schilling, 1989]. Tight linear arrays in Pb isotope space of the combined EMP and ESC data sets indicate binary mixing between the radiogenic isotope‐rich Salas y Gomez plume and the depleted asthenosphere source is the dominant factor in controlling Pb isotopic variation in the region. Mapping of the plume component mass fraction reveals not only a westward dilution of the plume component from Salas y Gomez to the EMP but also dilution with depleted material across the ESC on both flanks. The mapping is consistent with the MPS‐MRS model which calls for a subhorizontal, sublithospheric plume channel connecting the mantle plume conduit near Salas y Gomez with the East Pacific Rise, where plume flow is confined along a thermal groove. Mixing by entrainment of surrounding depleted material takes place progressively downstream. A numerical thermal lithosphere cooling model reveals a rheological boundary layer (i.e., the base of the lithosphere) shoaling by 1.3° to 3.5° from Salas y Gomez to about 100 km east of the EMP. The slope magnitude is consistent with those suggested by laboratory and numerical fluid dynamic models for hot buoyant plume‐ridge channeled flow to develop and be maintained.